Motors 3ph Jenneson Ch11

CHAPTER 11

THREE-PHASE SYNCHRONOUS MACHINES 11.1 Introduction It was shown in an earlier chapter that an alternator driven at a constant speed produces an alternating voltage at a fixed frequency dependent on the number of poles in the machine. A machine designed to be connected to the supply and run at synchronous speed is referred to as a synchronous machine. The description applies to both

motors and generators. A synchronous condenser is a special application of a synchronous motor. While the synchronous motor has only one generally used name, the synchronous generator is on occasion referred to as an alternator or as an a.c. generator. The term alternator has been used in previous chapters and will be used in this chapter but it should be remembered that other terms are in use. In general, the principles of construction and operation are similar for both alternators and generators, just as there were basic similarities between d.c. motors and generators. While alternators were once seldom seen outside power houses and whole communities were supplied from a central source, there is now an expanding market for smaller sized alternators suitable for the provision of power for portable tools. Today, with the growth in computer control, there is a further need for standby generating plant to ensure a continuity of supply to prevent a loss of data from computer memories. So much information is being stored in computers today that even brief interruptions to the power supplies can have serious consequences on the accuracy and extent of information stored.

In the majority of cases, the rotor has the d.c. winding and the stator the a.c. winding. An alternator with a rotating a.c. winding and a stationary d.c. winding, while suitable for smaller outputs, is not satisfactory for the larger outputs required at power stations. With these machines the output can be in megawatts; a value too large to be handled with brushes and slip rings. Because the terminal voltages range up to 33 kV, the only satisfactory construction is to have the a.c. windings stationary and to supply the rotor with d.c. This arrangement has the following advantages: l. extra winding space for the a.c. windings; 2. easier to insulate for higher voltages; 3. simple, strong rotor construction; 4. lower voltages and currents in the rotating windings; 5. the high current \Vindings have solid connections to the "outside" circuit; 6. better suited to the higher speeds (and smaller number of poles) of turbine drives.

11.2 Alternators

11.2.1 Stator The stator of the three-phase synchronous machine consists of a slotted laminated core into which the stator winding is fitted. The stator winding consists of three separate windings physically displaced from each other by 120° E. Each phase winding has a number of coils connected in series to form a definite number of magnetic poles. A four-pole machine, for example, has four groups of coils per phase or four "pole-phase groups". The ends of the three phase windings are connected in either star or delta to the external circuit. Details of phase windings for a three-phase machine were shown in Chapter l 0 to consist of three identical windings symmetrically distributed around the stator.

The three-phase synchronous machine has two main windings: 1. a three-phase a.c. winding; 2. another winding carrying d.c.

11.2.2 Rotor The alternator rotor can be of two types-low speed and high speed.

209

210

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL TR 11.2.3 Prime movers Low speed Most diesel engines used as prime movers for alternators operate within the speed range of 50 r /min and this necessitates the use of rotors witl pairs of poles. Hydroelectric turbines have water-driven in which operate at low speeds, consequently they all rotors with. many poles. While the diesel-driven alt usually has its shaft in the horizontal plat hydroelectric unit has its shaft in the vertical plar method of construction means that special thrust t have to be fitted to take the end thrust of the 1 component.

Fig. 11.1 Stator for a tour-pole 415 V three-phase 350 kVA alternator DUNLITE GENERATING SET MANUFACTURERS

Low speed (salient pole) This type usually consists ofa "spider" similar to that used in d.c. machines, on which are bolted the field poles and the field coils (see Fig. l l .2(a)). Physical constraints limit the use of this type of rotor to low-speed machines. High speed (cylindrical) The cylindrical rotor was developed to meet the needs of higher-speed prime movers. To counteract centrifugal forces its diameter must be small in comparison to its length (see Fig. l l.2(b)).

High speed Turbine prime movers, whether steam or gas, efficiently at speeds in the vicinity of 3000 r / n alternator driven by a turbine and producing a fn of 50 Hz at 3000 r /min must consist of only two In Chapter 6 the relationship between frequency and the number of poles was shown tc

~ ~ By transposition

120/

n=--

p

where n = r/min f = frequency in hertz p = number of poles For a large-diameter rotor of twenty-four 50 Hz

n =

120

X

50

24

= 250 r/min

For a turbine-type rotor of two poles at 50 f

n =

120

x 2

50

= 3000 r/min

(b)

Fig. 11.2 Main types of alternator rotors: (a) low speed-salient pole, (b) high speed-cylindrical

1

THREE-PHASE SYNCHRONOUS MACHINES

211

Example 11.1 At what speed would the governor of a twelve-pole dieseldriven alternator have to be set to enable a frequency of 60 Hz to be generated?

n = 120{ p

120 x 60 = 600 r/min 12 An alternator in this speed range will have a large diameter and have a comparatively short axial length. With turbines, the extra expense and auxiliary machinery needed restricts their use to larger sizes. Higher outputs mean that the length of the alternator must be increased and the increase in length causes complications in cooling. 11.2.4 Alternator cooling Low speed With engine-driven or hydroelectric alternators, there is no great difficulty in providing adequate ventilation because of the characteristically large diameter and short axial length. In addition to the large surface area available for direct radiation of heat, there is a fanning action due to the rotation of the fields; an action which can be increased by the addition of fan blades if necessary. When the axial length is short, the heat developed in the imbedded windings is quickly conducted to the ends, where the fanning action can dissipate it. As the machine size becomes larger, it is often necessary to provide ventilation ducts within the core to provide paths through which cooling air can flow. High speed The provision of adequate cooling facilities is a problem in high-speed machines of large capacity if the operating temperature of the windings is to be kept within safe limits. The surface area available for cooling in a highspeed machine is less than that in a low-speed machine of the same capacity.

The diameter of the rotor must be small enough to keep the surface speed down to a safe value, so for large capacities the length of the machine must be considerable. This long axial length causes difficulty in cooling the central portion of the core, because the heat generated cannot be conducted away quickly enough to limit the temperature rise in the core to a value that will protect the windings and the insulation. These considerations gave rise to the necessity for completely enclosing the alternator, and allowing the use of forced ventilation to carry away the heat produced. Where cooling air is used, it must be filtered to keep it clean and sometimes washed by passing it through a spray chamber to prevent a build-up of dust within the machine. Washing the air has the added advantage of cooling it, and so further reducing the temperature of the alternator and allowing the rating of the machine to be increased. To increase alternator ratings still more, hydrogen gas is used instead of air because ofits greater ability to absorb heat. The machine is completely enclosed and the hydrogen is blown through the alternator and then ~ through a heat-exchanger before being cycled through the (" ~ alternator again. The total exclusion of air from a fully Jsealed machine is necessary to prevent an explosive 'mixture from forming. 3 These cooling methods require considerable powe 1o::: and auxiliary equipment, so the output from th \0 alternator must be increased an appreciable amount fo \;, the method to be economically feasible. Accordingly. it is '. "£,. only used on very high-capacity machines. ~ ~ _,, 11.2.5 Excitation The usual method for d.c. excitation of the rotor \Vindings is for each machine to have its own d.c. generator called an "exciter" (refer to Fig. 11.3). The exciter can be belt-driven or geared down from the synchronous machine. but the usual practice is for the exciter to be directly coupled to the rotor shaft. The exciter armature rotates within the influence of the exciter field, causing a d.c. voltage to be generated in

1----------------1

Exciter

Alternator

, - - - - - - - - - --1 I

I

I I

t

I I

Exciter field

I

Stator windings

Brush gear and slip rings

Rotor field

I I

I

L ___________ J

L

I I I I I

I I I _________________ J

Fig. 11.3 Basic alternator circuit

~

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL TR

212

the armature. The exciter output is fed into the field where V, = generated voltage per phase (r.m.s.) windings of the synchronous machine. By adjusting the IP = flux per pole in webers rheostat in the exciter field circuit, the strength of the f = frequency in hertz magnetic field in the rotor can be varied. N = number of turns per phase kd = a constant, dependent on winding di With very large alternators the d.c. excitation requirements are substantial. This means that the d.c. ti on kp = a constant, dependent on coil pitch generators have to be large also; so large that they may not be able to self-excite. Because of this, the generator may need an exciter of its own-one that is able to self-excite Example 11.2 and provide power for the field of the main d.c. generator Calculate the line voltage of a 50 Hz star-cor which in turn supplies the rotor field of the alternator. alternator given the following details: Some alternators use "brushless" excitation in which IP = 0.67 Wb/pole the exciter armature has been replaced by a three-phase Kd = 0.85 winding which rotates within the influence of a d.c. KP = 0.98 magnetic field, causing a three-phase voltage to be N = 36 turns/phase generated in the exciter. This three-phase exciter output is fed through a full-wave bridge rectifier, mounted on the V, = 4.44 IPJN KdKp end of the exciter and converted to d.c. = 4.44 x 0.67 x 50 x 36 x 0.85 The resulting d.c. is in turn fed into the rotor windings = 4460 v of the synchronous machine. By varying the current through the exciter field, the rotor field is varied and so Then V, = yJ x Vp governs the value of the generated voltage (see Fig. 11.4). = I. 732 x 4460 = 7725 v 11.2.6 Generated voltage The value of the generated a.c. voltage depends on the 11.2. 7 Effect of load on alternator voltage strength of the rotor flux and the speed at which it cuts the An alternator can be considered to consist c windings. Because the speed must be constant (and is components in series: linked to the frequency required), the sole remaining I. an a.c. generating source; factor determining the value of the generated voltage is the 2. a resistor-representing iron and copper losse~ strength of the rotor flux. 3. an inductor-representing the inductance For an alternator the generated voltage is found from: windings and magnetic leakage. Any load placed on the alternator must be assum v, = 4.44 IPJNkdkp I in series with these components as shown in Fjgu Three-phase bridge

rectifier

Three-phase exciter

windings

+ Rotor

Stator

field

windings

Exciter

field

1.

Rotating components

.1 Fig. 11.4 Brushless excitation

213

THREE-PHASE SYNCHRONOUS MACHINES

,---- l I I

I

I I

I I I I I

R

I I I

I Alternator

Load

z

I I L

I

L_

__ J

I

generated voltage v,. For a load with a lagging power factor, however, the magnetic effect of the stator currents opposes that of the rotor, resulting in a weakened rotor field and reducing the output voltage further than did the resistive load (see Fig. l l .6(b)). As before, IR is in phase with the load current!. /Xis at 90°E to JR so placing /Z at a different angle to the previous case. In a similar manner, V, is equal to the phasor sum of the output voltage and /Z. For a load with a leading power factor, the flux caused by the stator currents assists that of the rotor resulting in an increased output voltage (see Fig. l l.6(c)). The characteristics of the three types of loads are shown in Figure 11.7.

Fig. 11.5 Equivalent circuit of an alternator

11.2.8 Voltage regulation The series impedance of the resistance and inductance provides a drop in voltage before the generated voltage can reach the connected load. Additionally the load current in the a.c. windings produces

an

armature

reaction which also affects the output voltage. With a unity power factor load the armature reaction merely distorts the main field and the effect on voltage is minimal, the voltage drop in the main being due to the series impedance. Figure l I.6(a) shows that the resistive voltage drop IR is in phase with the load current I and the voltage drop due to the reactance IX is at 90°E to the IR drop. These two values combine to form a voltage drop IZ due to the impedance of the alternator windings. The phasor sum of the output voltage and /Z gives the

v,_ IZ

: IX

v. IR

(a) Unity power factor

An alternator is required to give a prescribed terminal

voltage at full load. The difference in output between no load and full load is a measure of its voltage regulation. The difference is compared to the full-load value in a similar manner to that for d.c. machines. . Voltage regulat10n = [VNLVFL VFL

X

100 ] %

Example 11.3 A three-phase star-connected alternator has an output

voltage of 3300 V at full load with unity power factor. When the load is removed and the excitation is unchanged the voltage rises to 3350 V. Find the percentage regulation. Change in voltage = 3350 - 3300 = 50 V . 50 x 100 Regulal!on = 3300 = I.5% at unity power factor. Note The regulation must also be referred to the load power factor because these figures at any other power factor would be different.

v, IZ_./.:

_.·IX

V

-E---.--7· ·.: JR

(b) Lagging power factor

Leading power Output voltage

rE=::::::::==:::~~;~-=~;

factor

Unity power factor

Lagging power factor

(c) Leading power factor

Fig. 11.6 Phasors for various power factor loads on an alternator

Load current Fig. 11.7 Effect of power factor on output voltage of an alternator

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL TRI

214

11.2.9 Alternator ratings An alternator is rated according to three basic factors: I. frequency; 2. voltage; 3. current. The first fixes the speed at which the alternator must be driven; the second states the designed output voltage; and the third is the full-load current output. The last two factors help establish the volt-ampere rating, usually expressed in kV A. The power factor of any load placed on the alternator is beyond the control of the manufacturer and because it could vary considerably, the alternator rating cannot be given in kilowatts. Example 11.4

Find the power loading in kilowatts of a three-phase, 415 V, 50 Hz alternator rated at 150 kV A at 0.8 power factor, if the load has a power factor of: (a) 0.8; (b) 0.6.

Machine is rated at 150 kVA and 0.8 power factor, then at this load: power output = 150 x 0.8 = 120 kW At 0.6 power factor, power output = 150

x 0.6

= 90 kW

In both cases, the current flowing will be the full-load current value, which should not be exceeded because of cooling problems within the windings. At 0.8 power factor, P = i.e. 120 000 =

:. I=

VJ VIA. ;j3

x 415 x Ix 0.8

V3

120000 = 208 A x 415 x 0.8 ~

Purchase price The overall cost for smaller units may be lower, terms of cost per kVA they are more expensb operate at lower efficiencies. As the size of tt increases, the cost per kV A reduces while the op1

efficiency increases. Type of prime mover The economy of the prime mover in terms of effi has a bearing on its selection. This in turn is affec the type of service it will encounter. For exarr steam turbine has a good economy throughout its load range. However, it is expensive, large, and n long time to get the unit on load from cold. An i1 combustion engine has poor efficiency at light load, much cheaper to buy initially. For some loads it is c to buy several smaller alternators than one larg' Problems of paralleling the units then have considered (see sect. 11.4). The cost and availab fuel must always be a consideration. While disti more expensive initially, as is the diesel engine its1 fuel cost per hour is less while maintenance costs higher than those for a petrol engine. The petrol er cheaper to buy, the fuel is readily available, and the suited to smaller units used purely for portable supplies on intermittent duties. In the long term th( engine runs better on full loads than the petrol engir petrol engine is more tolerant of dirty fuel than th< engine and does not need specialised skil maintenance purposes.

Starting methods These are governed by the intended use of the geni unit. The quicker the changeover to auxiliary pm more expensive is the starting method. The ct method involves merely starting the unit manually is realised that the main power supply has failed. f

1

This is the full-load current rating for each phase winding of this particular alternator and it applies irrespective of the load power factor or of the load power.

11.3 Emergency power supplies and portable alternators The factors affecting the buying and running of alternators can be many and varied. They range from buying a small portable unit at the best possible price to careful planning for the most suitable unit for a particular purpose. It is not enough to simply select an alternator with respect to the load it has to supply; the choice should be affected by many other considerations. Some of these factors are listed below and their order of importance is governed by the actual use intended for the alternator.

self~contained portable power supply. 7 alternator rated at 6 kVA is driven by a pet engine. The size and weight of the unit is. that it can be carried to any site where po· required. DEPT OF A'

Fig. 11.8 A

215

THREE-PHASE SYNCHRONOUS MACHINES

expensive method involves the use of a changeover contactor which drops out when the main supply fails. In turn this connects a starting motor to the engine and after the alternator has got up to speed connects it to the load. At the top end of the scale is the so-called "no-break"set. The alternator with a heavy flywheel is run as a synchronous motor, being separated from the prime-mover by an electrically operated clutch. When a mains failure

occurs the clutch is released connecting the alternator and the flywheel to the engine. The engine is quickly run up to speed and the alternator reverts to its intended purpose. The changeover period can be short enough to ensure continuity of operation of essential equipment. The method is very expensive with high operating costs.

Load sizes and alternator capacities Smaller generating plant is usually intended for standby purposes for short periods. It usually has only one load connected to it at a time, such as a portable tool or a small lighting load. With middle- and larger-sized alternators consideration has to be given to the possible connection of intermittent larger loads, such as the starting currents of motors. The unit then has to have the electrical capacity and engine power to maintain both the output voltage and frequency during these current surges to avoid interruptions to other equipment connected to the same supply. Operation of alternators With the exception of some manually operated equipment, most operations today are beyond the control of the operator. Where some degree of manipulation is available there are two important factors that should always be considered-voltage and frequency. In most cases the voltage is governed by automatic voltage regulators while the frequency is controlled by the engine governor. The order of operation is to set the speed first, which in turn sets the frequency, and then adjust the voltage of the unit. To do this in the reverse order is to alter the voltage each time the speed is altered.

11.4 Parallel operation of alternators -synchronising Most commercial power stations are designed to have a number of alternators operating in parallel, supplying a common load at constant voltage. Because alternator efficiency is maximum near its full-load capacity, it is more economical to have each machine delivering its approximate rated output. During the early hours of the morning, for example, when there is a light load, it may be necessary to have only one machine connected to the line, delivering its rated output. As the load varies during the 24-hour period, so the number of machines connected in parallel is determined. Before a three-phase alternator can be connected in parallel with another three-phase supply, the followin2 conditions must be fulfilled: 1. The output waveform of each supply must be identical. This is determined by the design features of the alternators. It is standard practice to generate a sinusoidal waveform supply. 2. The phase sequence or' rotation of each supply must be the same and this ensures that the e.m.fs of each supply reach their maximum values in the same sequence; for example, R, W, B. The phase sequence is determined by the method of connection of the alternator phase windings to the terminals of the machine. This check is carried out during the commissioning process after the initial installation, or following a major maintenance overhaul, and it is not necessary to do it each time the machine is connected in parallel with others. 3. The alternator and supply voltages must be the same. 4. The alternator and supply voltages must also be in phase. 5. The alternator and supply frequencies must be identical. The last three conditions are explained in Chapter 10. The value of the voltages, their phase relationship and their frequencies can be adjusted by the operator. The

Fig.11.9 An alternative form of "no~break

unit". The unit shown has a 1470 r.p.m. motor· driving an alternator at all times. The generated frequency is 49 Hz so the unit cannot be used to supply frequency sensitive equipment. The photograph shows the alternator, driving motor, flywheel, clutch and diesel engine. When a power failure occurs, the flywheel keeps the alternator rotating tor 7 seconds allowing the diesel engine to start. The electrically operated clutch then connects the engine to the alternator. The interruption of power to the load is in the order of 0.5 cycle. DEPT OF AVIATION

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL TRi

216 d.c. Alternator on load

,--

Synchronising lamps

Incoming alternator

d.c.

--~\\-'---------+~._-----Three-phase

distributions

Fig. 11.10 "Three dark" lamp method for synchronising alternators

1111

1111

voltage of the incoming alternator is adjusted by varying the field excitation, and the frequency is determined by the speed of the prime mover. To ensure that the alternator and supply voltages are in phase with each other before connecting them in parallel to the load, some method of indicating the phase relationship is required. Smaller-sized alternators can be synchronised with lamps, but for larger machines a more exact method is required.

11.4.1 Synchronising alternators with incandescent lamps "Three dark" method Voltages for synchronising purposes can be checked by connecting a voltmeter to each machine in turn, but this does not give any indication of polarities or phase relationships. Incandescent lamps can be used to indicate this and the circuit is shown in Figure 11.10. The voltage rating of the lamps needs to be twice the alternator phase voltage, and the simplest way to achieve this is to connect two lamps of equal wattage in series. The lamps can be observed as three pairs oflamps or three can be covered, leaving only three visible (as shown in the box in the diagram). If the alternator is properly connected, the three lamps should all become bright and dim simultaneously. If they brighten and dim in sequence, it means that the phase rotation of the alternator is opposite to that on the distribution system, so the phase rotation of the incoming alternator must be reversed.

The lamps flicker at a rate equal to the diffen frequency between that of the incoming alternator' busbars leading to the distribution system. 1 alternator frequency approaches that of the busb• rate of flickering slows down; when the two freq1 are equal, the flickering stops. When the lamps ' (dark), the connecting switch can be closed and t machines will remain synchronised. When all the are dark, there is no potential across the lamps, ind that the two voltages are in phase with each other The disadvantage of this connection is that th< can be dark even with a "small" voltage across ther smaller alternators the two a.c. sources can sync! themselves ifthe difference is not too great, but wit! alternators the mechanical and electrical forces ere: a phase displacement between the two sources ca1 considerable damage.

"Two bright, one dark" method The circuit for this method is shown in Figure 11. can be seen to be similar to that of the previous except that the connections for two of the Ian crossed. Again two lamps are in series and it is t cover up three lamps, leaving only three visible (as by the dotted lines). To use this circuit it is essential to check thj rotation by the "three dark" method first. established that the phase rotation is correct i lamps reconnected, it will be found that the lamps! and bright in sequence. By noting the order of bri! it becomes a reference in determining whetl incoming alternator is fast or slow.

THREE-PHASE SYNCHRONOUS MACHINES d.c. Alternator on load

217

Synchronising lamps

,--1

Incoming alternator

d.c.

I

I

L-------~-+-----_.. Three-phase distribution system

Fig. 11.11 "Two bright, one dark" lamp method for synchronising alternators

Synchronism occurs when the lower lamp in Figure 11.11 is dark and the other two are of equal brilliance. Then the switch can be safely closed. The significance of the correct lamp being dark lies in the fact that it is connected between two similar phases. When these two phases are synchronised, the voltage difference between them is zero. This cannot apply to the other lamps since they are connected across dissimilar phases. The "two bright, one dark" method gives greater

a revolution of the flywheel but varies according to ihe positions of the pistons. Even with a heavy flywheel, the

accuracy-both in determining the relative speeds and

variation in torque can result in changes in the induced

frequencies, as well as showing fairly accurately the instant for synchronising.

currents to flow between alternators in parallel, resulting

11.4.2 Synchronising alternators with a synchroscope A synchroscope is an instrument that indicates both phase relationships and relative speed for an incoming alternator. There are variations between manufacturers for the operating principles, but in general a synchroscope consists of a two-phase stator connected to the incoming alternator with the rotor wound with a polarising coil and connected to the supply source. Some models use rotating vanes with no actual electrical connection to the rotor. The synchroscope front panel is shown in Figure 11. 12 and the connections are shown in Figure 11.13. If there is any difference between the frequencies of the supply and the incoming alternator, a pointer attached to the rotor of the synchroscope will rotate at a speed proportional to this difference. Its direction of rotation indicates whether the incoming machine is running fast or slow (i.e. above or below synchronous speed). At

synchronism the pointer will remain stationary, but it must be brought to an indicated position on the scale

before the main switch of the incoming alternator is closed.

11.5 Hunting in alternators The driving torque of a diesel engine is not uniform during

voltage. These voltage pulses can cause circulating

Fig.11.12 Portable synchroscope contained in a polished wooden box. At the moment of synchronisation the pointer should be stationary over the vertical line. A. J. WILLIAMS

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL TRI

218 d.c.

d.c. Incoming alternator

Alternator on load Synchroscope

'-+---------4-+------ Three-phase distributions

Fig. 11.13 Synchroscope connection for synchronising alternators

1!11·

in mechanical oscillations. With turbines, the pulsing or polarity: one field is that of the rotating stator a hunting is usually due to fluctuations in the governor other that of the rotor.

settings with changes in load. Remedies for hunting involve the use of heavy flywheels and special windings in 11111

the pole faces. These windings are discussed in more detail in the next section.

11.6 Synchronous motors 11.6.1 Construction Stator The stator of a synchronous motor has a three-phase winding, as described in a previous chapter, and is of the same type as that in an alternator.

When this winding is energised with a.c. it produces a magnetic flux, which rotates at a speed called the synchronous speed. It is the same speed at which the synchronous motor would have to be driven to generate

an a.c. voltage at line frequency. The speed can be derived from the same formula used for alternators in section 11.2.3. Rotor Although of similar construction to the alternator rotor, it is usually made with salient poles. When excited with d.c. it produces alternate north and south magnetic poles,

which are attracted to those produced in the stator. 11.6.2 Operating principle A synchronous motor works on the principle of magnetic attraction between two magnetic fields of opposite

A synchronous motor has torque only at synch speed, so special steps have to be taken to get the up to speed and synchronised with the supply. Tl magnetic fields are then rotating at the same spe1 lock in with each other. A later section in this c discusses starting methods for synchronous motors

11.6.3 Effect of load on a synchronous mo When a synchronous motor runs on no load, the I positions of stator and rotor poles coincide as sh Figure l l.14(a). When a load is applied, the rotor must still cont rotate at synchronous speed but due to the rel action of the load, the rotor pole lags behind th< pole. Their relative positions are displaced by the: (called the "torque" or "load" angle), as shown in l l.14(b). The greater the load applied, the lar: torque angle. The magnetic coupling between each stator an pole distorts according to the load applied. If the I the motor becomes excessive, the magnetic C< breaks and the rotor slows down until it stops. When the motor is rotating at synchronous with a fixed d.c. excitation in the rotor windings, tt flux cuts the stator windings, inducing a voltage phase winding and opposing the applied voltage law). The phase relationship between this induced and the applied voltage depends upon the

219

THREE-PHASE SYNCHRONOUS MACHINES Rotation of stator Torque angle

I

\

Rotor

\.

·~ I

(a) No load

(b) Loaded Fig. 11.14 Relative positions of stator and rotor magnetic fields in a synchronous motor

positions of each stator and rotor pole, which in turn depend upon the load applied to the motor. Neglecting motor losses, on no load the torque angle is zero, and so the induced voltage V, and the applied voltage V are equal and opposite. The resultant voltage VR across the windings is zero, and so the current drawn from the supply is also zero. This is illustrated by the phasors in Figure l l .15(a). When a light load is applied to the motor, the torque angle a increases, and the induced voltage Vg in the stator windings is now ( 180 - a) 0 E out of phase with the applied voltage V, as shown in Figure l l. l 5(b ). These two voltages combine to produce an effective voltage v. across the stator windings, which is sufficient to draw a current I from the supply. Because of the relatively high inductance of the stator windings, the line current /in each winding lags each resultant voltage v. by nearly 90°E. This causes the line current I to lag the applied voltage by cl>. As the load is increased, so the torque angle is increased. This causes an increase in the resultant voltage VR across each stator winding, as seen in Figure l l.15(c). Because of the increase in the value of VR• the line current I increases, and the phase angle ¢ between the applied voltage V and the line current I also increases.

v,

For a fixed rotor winding excitation, an increase in load on a synchronous motor will therefore cause an increase in current drawn from the supply.. with a poorer power factor. 11.6.4 Effect of varying field excitation If the load applied to a synchronous motor is constant, the power input to the motor is also constant. When the rotor field excitation is varied, the induced voltage in each stator winding is also altered. The phasor diagram in Figure l l.16(a) represents the conditions for a given load at unity power factor. The power input per phase is Vl1. If the rotor field excitation is decreased, the induced voltage Ve decreases, as shown in Figure l l. l 6(b ). This causes the line current h to lag the applied voltage Vby <1>2. Since the load, and so the power input, is constant, the power component of /2 must remain the same as Ii in Figure l l.16(a). The line current Ii must increase to accommodate the lagging power factor. A reduction in the d .c. field excitation therefore causes an increase in line current, and a lagging power factor. If the d.c. excitation is increased, the induced voltage V,increases as shown in Figure l l.16(c). The line current h will therefore lead the applied voltage V by ci>J, and will

v

(a) No load

(b) Light load

Fig. 11.15 Effect of load on line current with constant excitation

(c) Heavier load

220

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL TR, Where large amounts of power are being dist1 and power factor correction is needed, specially d< synchronous motors are run without any load con1 Under these circumstances the overexcited synch motor is called a synchronous condenser.

Voltage control

v An important application is in the control of volt:

I

transmission lines. Synchronous motors are inst::: suitable positions along the line and their exc adjusted as desired to cause them to draw lagi leading currents in order to raise or lower the \I When synchronous motors are installed unde1 conditions, there is a tendency to greater stab voltage on the transmission line.

(a) Unity power factor

I] (b) Lagging power factor vg

I

Low-speed drives A synchronous motor has good efficiency and speeds its higher initial cost is adequately compens the comparatively lower running cost. At low spe1 induction motor has a decreasing efficiency, wl synchronous motor retains its high efficiency.

,,

v

-+-----:,

',, '

'-

' V

(c) Leading power factor

Fig. 11.16 The effect of varying the d.c. excitation

also be greater than Ii in Figure l l.16(a) because the power component is the same, due to the load remaining constant. An increase in d.c. excitation therefore causes an increase in line current and a leading power factor. This characteristic of the synchronous motor, where its power factor can be altered by varying its d.c. excitation, gives rise to its main application in industrypower factor correction.

11.6.5 Applications of synchronous motors Power factor correctiOn If a synchronous motor has sufficient d.c. excitation to cause it to draw a leading current from the supply, the effect is one of power factor correction for other loads within an installation. A motor running with a leading power factor is called overexcited, and is often designed to run as a synchronous motor driving a load and correcting overall power factor at the same time. The driven load selected is usually one in demand throughout the installation (e.g. air compressors, hydraulic systems or frequency changers for portable tools). An added advantage can be an economical incentive offered by supply authorities for ensuring a certain minimum value power factor in an installation. For example, the charge per kWh may be reduced if the power factor does not drop below 0.75 or some similar figure.

11.6.6 Hunting in synchronous motors A change in load on a synchronous motor causes a in the value of the torque angle (Fig. 11.14). In! the inertia of the rotor prevents an instant chang1 new conditions. with the result that the rotor shi ihe point- of equilibrium and then has to correc While the rotor and the rotating field in the stator rotating at a synchronous average speed, the ch load on the rotor causes this periodic swing aro point of equilibrium. This surging or hunting ca undesirable fluctuation in line current to the mot The usual method for damping these surges i damper winding, called an amortisseur wine consists of copper bars embedded in the pole fac< rotor and shorted out at each end (Fig. 11. I"

/Dampe• rQ .. o

-

-

-

IT:. I

--s

.

loo o

--

-

--11 •• ·1

Fig.11.17 Salient pole with amortisseur wind

THREE-PHASE SYNCHRONOUS MACHINES

surging causes an induced voltage in the copper bars. This results in a magnetic field being created and opposing the

221

connected to the supply. It is an expensive method. particularly if high starting torques are required.

surging effect. Often the shorting-out bars are extended around the Induction motor starting rotor. resulting in a squirrel cage-type rotor winding A reduced line voltage is applied to the stator windings about the salient poles. While damping any tendency of and the d.c. winding on the rotor is short-circuited. With the rotor to hunt, they can also assist the motor in starting. the aid of the amortisseur winding, the complete machine behaves as an induction motor as it accelerates up to a 11.6. 7 Starting methods for synchronous motors speed slightly below synchronism. At an appropriate time the short is removed from the rotor winding. d.c. is Auxiliary motors applied and the full line voltage applied to the stator Some synch'ronous motors are equipped with a special winding. Because the speed is only slightly less than motor designed for use during the starting period only. synchronous speed, the rotor field is able to lock in with The auxiliary motor runs the synchronous motor up to the stator field and accelerate to synchronism. speed, at which stage it is first synchronised and then

Exercises 11.1

What advantages are there in using the rotating d.c. field-type construction for synchronous machines? 11.2 What are the constructional differences between low-speed and high-speed alternators? 11.3 Explain why a low-speed synchronous machine has a large salient pole-type rotor. 11.4 What is the purpose of the "exciter"? 11.5

11.6 11. 7

How many poles must a synchronous machine have to operate at 250 r/min and a frequency of 50 Hz? How does the power factor of the load affect the output voltage of an alternator? State five conditions that must be satisfied before an alternator can be synchronised with an existing supply.

11.8

What is meant by the term "phase sequence" when applied to three-phase synchronous alternators? 11.9 In what way does the principle of operation of a synchronous motor differ from that of an induction motor? 11.10 Why is a synchronous motor not selfstarting? 11.11 State two characteristics that are applicable only to a synchronous motor. 11.12 Explain how an increase in the load applied to a synchronous motor affects the line current and power factor. 11.13 How can the power factor of a synchronous motor be changed? 11.14 What are some applications for synchronous motors?

CHAPTER12

THREE-PHASE INDUCTION MOTOR! 12.1 Introduction The majority of a.c. motors used in industry are of the induction type. They are rugged and have a high degree of reliability. A three-phase induction n1otor consists of a laminated stator with three identical windings placed symmetrically in slots within it. The rotor is also laminated, and usually has single-turn conductors placed within its slots and short-circuited at the ends. To achieve special characteristics, conventional windings are sometimes used instead. The motor derives its name from the fact that the currents flowing in the rotor are induced and not drawn directly from the supply.

12.2 Construction 12.2.1 Stator The laminated stator core is made up from sheet steel punchings with slots on the inner surface. The windings consist of three identical windings, laid out in the same fashion as the alternator and synchronous motor. In motors of higher power ratings the stator slots are of the open type to allow the insertion of pre-shaped and insulated coils, but in smaller sizes the slots are partially closed to reduce the air gap as much as possible.

Fig. 12.1 The component parts of a 415 V, 3.7 kW, four-pole three-phase induction motor. This particular motor is of the totally enclosed type and is intended for direct coupling to its load as shown by the flanged construction of the endshield at POPE ELECTRIC MOTORS the upper left.

222

The stator core is held in the motor frame whi serves to carry the bearings holding the rotor, to the coils and to provide a means whereby the wholl mounted (see Fig. 12.1 ). The motor frame takes various forms, depen1 the conditions under which the motor will aper; open-type frame allo\vs free ventilation to take drip-proof frame has a closed upper half, while a ventilation through the lower half; a totally enclm prevents the exchange of air between the inside outside of the frame.

12.2.2 Motor enclosures The conditions governing the actual installatio induction motor are normally beyond the contrc motor manufacturer. As a result the motor is factured in various enclosures. A motor dr compressor for a refrigerated display cabir example, may operate under such clean and d ditions that the motor enclosure need only pr mounting for the bearings and a means for fi:;. motor in a horizontal plane. At the same time closure provides mechanical protection against ac spillage and enables cooling air to circulate freely· the motor windings. Compare this situation with a water turbin used for irrigation purposes. In most cases the t mounted vertically at the bore head and is g protection from the weather. The motor neeC totally enclosed to prevent the entry of water and is by means of heat transfer through the motor 1 The air sealed within the motor housing is circulat internal fan, so transferring the heat generatec windings to the housing. This heat is then transl the atmosphere by a second fan circulating free a the outside of the motor housing. For detailed information on electric motor s reference should be made to Australian Stan1 1359 on the requirements for rotating e machines. It is an extensive standard with many and often calls up other standards that may be to particular sections. Electrical rotating macl now classified by two letters followed by four n

223

THREE-PHASE INDUCTION MOTORS

I

II \I II II \I II Fig. 12.2 Squirrel-cage rotor for an induction motor POPE ELECTRIC MOTORS

111111 111111

111111

This classification number is different for such categories as cooling, mounting and protection. Fig. 12.3 Wound rotor for an induction motor

12.2.3 Rotor

Squirrel-cage rotor The rotor of a three-phase motor consists of a shaft with bearings, laminated iron core, and rotor conductors. The most common type of construction is that with rotor bars in the lamination slots rather than a winding. The rotor bars, short-circuited at each end by a solid ring, are often made of copper strip welded to copper rings, but for small

to medium size motors they may be cast in one piece out of aluminium. Usually included in the rotor casting is a series of vanes for creating air movement. Figure I 2.2 shows these vanes standing out from each shorting ring. The photograph also shows skewed conductors in the rotor. The main purpose for slanting the conductors in the rotor is to ensure a smooth steady acceleration during starting. Varying the physical design features of the bars affects the motor performance. Embedding them deeper into the rotor, for example, increases their inductance and gives a lower starting current but at the same time creates a lower pull-out torque. This type of rotor is then restricted to loads requiring low-starting torques such as centrifugal pumps. The rotor windings, if assembled without the laminations, resemble a metal cage giving rise to the often-used name of "squirrel-cage" rotors although the standards refer to them simply as "cage" rotors.

Wound rotor The wound rotor is fitted with insulated windings, similar to the stator winding and having the same number of

poles. Usually the rotor winding has three phases, connected internally in star, and terminating at three slip rings. A typical wound rotor is shown in Figure 12.3. The slip rings are connected by means of brushes to a star-connected variable resistance, as in Figure 12.4. This rotor rheostat provides the means of increasing the resistance of the rotor circuit during starting, thereby producing a high starting torque at a low starting current. As the speed increases, the external resistance is gradually reduced, lowering the rotor circuit resistance as the rotor reactance decreases. Under operating conditions, the variations in rotor circuit resistance provide a means of cbntrolling the speed of the motor-an increase in resistance produces a reduction in speed. This also produces a loss in efficiency due to the 12 R losses in the rheostat. The wound-rotor motor is more expensive than the squirrel-cage motor due to the cost of manufacture of the wound rotor. It also has a higher starting torque and lower starting current, but poorer running characteristics than the squirrel-cage motor.

12.3 Operating principles 12.3.1 Rotating magnetic fields For its operation a three-phase induction motor is dependent on a rotating magnetic field being established by the a.c. windings. The three separate windings are

Three-phase supply

Stator

Rotor

Slip rings

Fig. 12.4 Circuit for a wound-rotor induction motor

Rheostat

224

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL TF A

+

A

..-

/

s,

~

/

•··

/

.. 7 ..I

··.

I ..

I I

I \

I

·.

\,.. ,;::

/ •/

ccr

....

/

/

... ....

..

...·

(b) Three phases

(a) One phase

Fig. 12.5 Polarities and connections in a two-pole, three-phase motor

installed in the stator at 120° E intervals to each other and provide a fixed number of poles for each phase. This is shown diagrammatically in Figure l 2.5(a) for one phase of a two-pole machine. Figure 12.5(b) shows the three phases in relationship to each other giving a total of six poles. Phase A is drawn as a solid line, phase Bas a dotted line and phase Casa dashed line. Note that this sequence is carried through for the explanation and applies to the

for example, alternates in direction in the diagran not rotate in any way. lt simply varies in stren

direction in the vertical plane. Similarly a pulsatir also established by the other two phases giving a three magnetic fluxes which combine into one r

flux. This flux rotates at synchronous speed. At r

current waveforms, the magnetic fields and the phasors.

'•

Assun1ption

In the following explanation for the production of a rotating field one assumption has been made as a

reference, that winding ends A, B, C when connected to a positive source of voltage makes the adjacent iron core a north magnetic pole. From this it will follow that the opposite poles become south magnetic poles. These details are also shown in Figure 12.5(a). If the current flow is reversed then the magnetic poles are also reversed. With the three windings connected in star by joining ends Ai, B1, C, together, and the ends A, B and C connected to a three-phase supply, the phase currents IA, !Band !care 120 °E out of phase with each other. These are shown in Figure 12.6. Because each current is

alternating, each pair of poles sets up a magnetic flux that continually changes from one polarity to the other. Note that although the flux set up by A phase in Figure l 2.5(b),

le

---.

\

·;

/

\

I ·..

\

\

I

I

I

I

0°

:

\

240°

120~

I I

\ .

\

... \

I

' 2

/

3

4

..... 5

6

7

Fig. 12.6 Waveform diagram showing three-phase< at 120° E to each other (for reference num text)

225

THREE-PHASE INDUCTION MOTORS

c

····........ . .

/

- - , - , - •-••-~- R ,/

B

···...

.......... B

Flg.12.7 The resultant flux produced by currents flowing at position 1 in Figure 12.6

position I in Figure 12.6, the current IA is zero and no flux is produced by the winding A-A1. Current Is is negative and so will produce a south pole at Band a north pole at B1. Current leis positive and so will produce a north pole at C and a south pole at C1. Because currents /oand le are equal the two magnetic fields are equal in strength. The direction of these fields are shown in Figure 12. 7. In the accompanying phasor diagram the addition of these two fields is shown giving a resultant instantaneous field 4>n. At position 2 in Figure 12.6, IA is positive, Io is still negative while leis zero. This produces a north pole at A, a south pole at B, and nothing at C. These are shown in Figure 12.8 together with the phasor diagram showing the addition of the phasors to give the resultant instantaneous magnetic field. Since all coils have an equal number of turns, the relative strengths of the magnetic fields can be gauged by measuring the vertical heights of the current waveforms at the positions indicated by the reference number. In this instance the direction of the resultant magnetic field has shifted 60° E clockwise from that in

position I. If drawn to scale it can also be shown that the length of the resultant has remained constant, indicating that the field strength has remained constant. At position 3 (Fig. 12.6), IA is positive, producing a north pole at A and a south pole at A,, 18 is zero, and le is negative, producing a south pole at C and a north pole at C 1. These fields are drawn out in Figure 12.9 together with their phasors. The resultant field has rotated a further 60°E in a clockwise direction. (There is a 60°E difference between all the numbered positions in Fig. 12.6.) For each of the numbered positions the resultant field rotates a further 60°E in a clockwise direction. For one complete cycle of current (360°E) the resultant magnetic field rotates 360°E.

Fig. 12.8 The resultant flux produced at position 2 in

Fig. 12.9 The resultant flux produced at position 3 in

Figure 12.6

12.3.2 Rate of rotation BycomparingFigures 12.6, 12.7, 12.8and 12.9itcanbe seen that for the time intervals of 60° E between the positions l, 2 and 3 the resultant field rotates an equal

Figure 12.6

226

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL Tl .---------{_) R

amount around the stator. For a complete cycle of a.c., a two-pole field rotates one complete revolution around the stator. The synchronous speed of the magnetic field in revolutions per minute can be determined from the frequency of the supply.

~c,

Example 12.1

A two-pole machine is connected to a 50 Hz supply. Find the speed at which the magnetic field rotates around the stator. 50 Hz = 50 cycles per second speed of rotation = 50 revolutions per second = 50 x 60 revolutions per minute = 3000 r/min. With a four-pole machine, 360°E represents one-half of a full revolution of the stator field, and the speed of rotation of the field is consequently halved. Similarly, the speed of field rotation for a six-pole machine is reduced to one-third that of a two-pole machine. In each case the speed is usually expressed in revolutions per minute, whereas the frequency is in hertz (cycles per second). The speed in revolutions per minute can be found from the following formula: .-------~

120{ ns1•11 = - " . p

c

...... ow I I

L _ - - - - - -------0 B (a) RWB sequence

... ·····

c

B,

' ' \Ow \

...

:\

where

nsyn =

..................................../ bs

number of revolutions per minute

.f = frequency in Hz p = number of poles

(b) RBW sequence

The speed n of the rotating magnetic field is called the synchronous speed of the motor. The synchronous speeds of common sizes of motors at a frequency of 50 Hz are given in Table 12.1. The formula above is identical to that shown in Chapter 6. Table 12.1 Speed of the rotating field in an induction motor for various number of poles Poles

2

4

6

8

1O

12

600

500

Synchronous

speed (r/min)

3000 1500 1000 750

12.3.3 Direction of rotation and reversal The direction of rotation of a rotating field depends on the phase sequence of the three currents flowing through the windings. In Figure 12.1 O(a) the three supply lines R. W and Bare connected to terminals A, Band C of the motor. The resultant magnetic field rotates in a clockwise direction. In Figure 12. 1O(b) the supply lines to phases B and C have been changed over and. using the procedure from the previous section. it can be shown that the rotation of the magnetic field is reversed. That is. the direction of rotation of the field. can be controlled by interchanging any two supply lines to the motor. In section 12.4.1 on torque it is shown that the rotation of a three-phase induction 111otor is in the same direction as that of the rotating field.

Fig. 12.10 Phase sequence and field rotatil

12.4 Induction and its effects When the stator windings of a three-phase i motor are energised from a three-phase s1 magnetic field is produced, rotating at sync speed. This rotating magnetic field crosses the ai1 cuts the rotor conductors, inducing a voltage (magnetic field, conductors and relative motior the rotor circuit is complete (through end rings it of the squirrel-cage rotor, or external resistance ii of the wound rotor), the induced voltages ca currents to flow in the rotor conductors.

12.4.1 Torque Figure 12.11 (a) represents a part of the stator an of an induction motor with the stator flux rot2 clockwise direction as indicated. When these line cut the rotor conductors from left to right, the movement between the stator flux and the re ductor is from right to left. By applying Flemin hand rule (sect. 6.1. l) the direction of inducec flow in the conductor is towards the reader. D comparatively high rotor currents flowing, a Jar established around the conductor as sho\vn i 12.1 l(b). The stator and rotor fluxes react withe; as shown in Figure 12.11 (c) to form a resultant f resultant field tends to straighten itself out, a1

-

227

THREE-PHASE INDUCTION MOTORS

LJ

N

N

Rotor conductor

-

~

Thrust

Rotor flux

.

'

(a) Stator flux

{b) Rotor flux

(c) Resultant flux

Fig. 12.11 Production of torque in an induction motor

process causes a force to be exerted on the rotor conductor trying to force it to the right and out of the stator magnetic field. A similar force is exerted on all the rotor conductors as the field rotates, and if sufficient force is created the rotor will commence rotating in the same direction as the rotating magnetic field. Provided it is free to rotate the rotor will accelerate until it approaches synchronous speed. This rotating force, called the torque of the motor, is the result of the interaction of the two fluxes. The stator flux remains fairly constant, but the rotor flux varies with the rotor current, which is determined by such factors as the impedance, the induced voltage and the relative speed of the rotor conductors. 12.4.2 Slip To produce torque, there must be a rotor flux caused by current flowing through the rotor conductors. If the rotor could run at synchronous speed, there would be no relative motion between the stator flux and rotor conductors. Consequently, there would be no induced voltage, no rotor current, no rotor flux, no torque developed, and so the rotor would slow down. An induction motor therefore cannot run at synchronous speed. With the rotor runningjust below synchronous speed, relative motion exists and sufficient torque is developed to keep the rotor turning. The difference between the synchronous speed of the rotating field and the actual speed of the rotor is calfed the slip speed. It is commonly expressed as a percentage of the synchronous speed. Example 12.2

Determine the slip of a four~pole induction motor running at 1440 r/min when connected to a 50 Hz supply.

n,,. =

120{ p

slip speed

=

=

120 x 50 = 1500r/min 4 1500 - 1440 = 60 r/min

1 ~~

1500

percentage slip =

4%

44

0x

100

The formula for determining percentage slip is:

s% = where s%

nsyn - n nsyn

x 100

= percentage slip

= synchronous speed n = rotor speed

nsyn

At standstill (i.e. when starting) the slip is !00%, whereas if the motor could run at synchronous speed, the slip would be zero.

12.4.3 Rotor frequency When the rotor of a two-pole motor is at standstill and the stator is connected to a 50 Hz supply, each rotor conductor is cut by a north pole and a south pole at a rate of 50 times per second. At standstill, the frequency of the rotor voltage (rotor frequency) is the same as the frequency of the supply (stator frequency).

As the rotor speeds up to half the synchronous speed (!500 r/min), the rotor conductors are cut by only onehalf as many north and south poles per second as at standstill, and so the rotor frequency is one-half the supply frequency (i.e. 25 Hz). If the rotor revolved at synchronous speed, the rotor frequency would be zero. The rotor frequency depends upon the differences in the speeds of the stator flux and the rotor (i.e. the slip of the motor), as shown in Figure 12.12. The rotor frequency can be calculated using the following formula:

~ ~ where fr = rotor frequency in Hz s = slip percentage f = supply frequency in Hz

228

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL TF Supply frequency ..................

Rotor frequency

/Rotor stationary 0

Slip

100°/o

Fig. 12.12 Relation between rotor frequency and slip

Example 12.3

Determine the rotor frequency of a two-pole, 50 Hz induction motor if the rotor speed is 2850 r/min.

s =

nsyn -

n x 100

nSJ'll

3000 - 2850 3000 x 100

f,,

= 5%

s.[

=TOO 5 x 50

= loO = 2.5

Hz

As the rotor frequency varies, so does the rotor inductive reactance, and this affects the starting and running characteristics of the motor.

12.5 Operating characteristics 12.5.1 Squirrel-~age motors When power is first applied to a stationary motor, the stator windings act as transformer primary windings with the resultant magnetic field rotating at synchronous speed. The rotor then behaves as a shorted secondary winding causing a high circulating current in the rotor bars and a high starting current in the stator windings. As the rotor accelerates in the direction of the rotating field,

the difference between its speed and the rotating m field becomes less and the generated voltage cam rotor circulating currents also becomes less. This reduces the stator current. The typical relationship between the stator and the rotor speed is shown in Figure 12.13( initial circulating current in the rotor is affected frequency of the supply, the resistance of the rot and the inductance of the rotor circuit-that is, the limiting factor is the impedance of the rotor circu the usual type of power transformer, the frequ the supply is the line frequency, but in this c frequency commences at line frequency and decreases as the motor speed increases. As a consi the torque created can change as the speed chan. Figure l 2. l 3(b) for the typical relationships I speed and torque. For small values of slip the t assumed to be proportional to the slip. As the mo increases the torque increases and the speed de until the torque reaches a maximum value ca breakdown torque. If the motor is loaded bey' point, the torque and the speed both decrease motor quickly comes to a standstill. An overa for starting torque is in the region of 1.5 times t torque, while the breakdown torque is usually abc the rated torque. Australian Standard 1359.41 minimum requirements for these torque val· provides a table for a range of motor sizes. The resistance of the rotor conductors constant at power line frequencies for all 1 purposes, while the inductive reactance decreas( rotor speed increases. As a guide, torque re maximum when the rotor resistance in ohms is the rotor reactance in ohms. Since the resistance then the breakdown torque can only be al relationship to the motor speed by altering the inc of the rotor. In turn this affects the starting Australian Standard 1359.41 allows for only t• types of rotor-normal and high torque-and a types are necessarily by prior arrangements 1nanufacturer. For the high-torque motor the breakdown torque remains around twice rate<

Breakdown torque Current ( 0/11)

Torque(%)

Locked

)._1or torque

Rated speed

Rated torque

100 Rated current

100

0

n (a) Current/speed curve

fl syn

0

n {b) Torque/speed curve

Fig. 12.13 Operating characteristics tor a squirrel-cage induction motor

n,

229

THREE-PHASE INDUCTION MOTORS

300 300 Torque %

200 Torque %

100

Rated torque -------------

200

r---Rated torque

100

Oc__ _ _ _ _ _ _ _

~-l-

Speed

Speed

n~yn

Fig.12.17 High resistance rotor bars

Fig. 12.14 Low starting torque rotor bars

300 Rated

speed 200

Torque %

100

Rated torque

o~------------e~ Speed nsyn

Fig.12.15 Standard rotor bars

300

Torque

200

%

100

_____

Rated ------.,speed

Rated torque

--------------

o~----------'+Speed

bars of greater cross-section where part is imbedded deeper into the rotor magnetic circuit. Starting torque is still about 150% of running torque but the starting current is reduced to about five times the running current. It is suitable for use with equipment of low starting inertia such as fans, blowers and some types of machinery. Figure 12.16 gives one example of a rotor with two sets of rotor bars. The inner set is shown with half as many bars as the outer set and includes an optional air gap. Depending on performance requirements there may be different shaped bars, no air gap or a full set of bars in the cage. Starting torque is high-here it is 225% of rated torque-and starting current is about five times rated running current. Applications are air compressors, crushers, refrigerator compressor motors or reciprocating force pumps. A typical example of high resistance rotor bars with low starting current requirements is shown in Figure 12. l 7. With this construction the starting torque can be increased to about 275% with fairly low starting currents. It is at the expense of a lower rated speed (i.e. increased slip). Typical uses are flywheel mounted machinery such as presses and punches. It is excellent with hoists where the maximum load occurs at the start of the lift. The details above apply particularly to copper rotor bars. If aluminium is used for the rotor bars then the cross-sectional area of the bars must be increased to allow for the metal's higher resistivity. The shape may also be changed to incorporate desired starting and running characteristics. Figure 12. l 8 shows a "tear-drop" shaped cast aluminium bar. In practice the shape may also be inverted to alter the characteristics to suit a particular purpose.

nsyn

Flg.12.16 Double cage rotor tor high starting torque

while the starting torque is increased to approximately 2.5 times rated torque. Figure 12.14 shows typically shaped rotor bars where the starting current is about 6-7 times rated current and the motor has a starting torque of approximately 150% of rated running torque. Its use is restricted to very low starting torque requirements. Figure 12.15 shows rotor

Fig. 12.18 Cast aluminium rotor bars

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL T

230

12.5.2 Wound-rotor motors The introduction of resistance into the rotor circuit of an induction motor produces three effects: I. rotor current is reduced, resulting in less stator current; 2. starting torque is increased, because rotor and stator magnetic fields are more in phase with each other; 3. slip speed is increased. An adjustable resistor is used external to the rotor, which is wound with comparatively low resistance windings. The value of the external resistance can be adjusted as required and as the motor accelerates, the value is gradually reduced until all the resistance is out of the rotor circuit and the motor behaves as an ordinary induction motor. The torque-speed characteristic of a typical threestage wound-rotor motor is shown in Figure 12.19. When all the resistance is in the rotor circuit, the starting current is low and the starting torque is high as shown by curve a. If this resistance is left in, the full-load torque would occur at approximately 25% slip, resulting in extremely poor speed regulation. If one stage of the resistance in the rotor circuit is shorted out, the operating characteristics are modified as shown by curve b. If all the Cxternal resistance in the rotor circuit is shorted out the operating characteristic is shown by curve c.

d Rated speed

r' ······ ... Aated_!_o~~

100

The normal starting procedure is to start th with all the resistance in the rotor circuit. As th speeds up the resistance is reduced and tht increases in speed, but maintains a high torque. the starting procedure, the torque-speed curve is c by the thicker curve d.

12.5.3 Operating parameters By comparing Figures 12.13 and 12.19, it can be' full-load torque occurs at a greater slip in a wow motor than a squirrel-cage motor. This is due to 1 resistance of the windings in the wound rotor. On no load, the stator current ofany inductic is largely a magnetising current, with a smal component required to supply the no-load Accordingly, the power factor of an induction n no load is very low. The no-load current is relatb when compared with a transformer because of reluctance of the magnetic circuit, due to the between the stator and rotor. The stator flux remains fairly constant from n full load, and so the magnetising current is al constant. In Figure l 2.20(a), the no-load stator c lags the supply voltage by cf> degrees. As a load is applied to the motor, a load cun required to accommodate that load. This load ct lags the supply voltage slightly, due to the effo stator and rotor reactance. The two current corr Io and /' 1 combine to give the total stator current load. The phase angle decreases from cf> to q,, power factor of the induction motor increases as on the motor increases. Figure 12.21 gives representative characterist for some parameters of a three-phase induction 1 shows the speed decreasing and the slip increasi: load on the motor is increased. It also shows current increasing and the power factor improvi same time .

a ___ _

12.6 Motor starting methods 0

n

Flg.12.19 Operating characteristic for a wound-rotor motor

Induction motors are subject to the same limitations as d.c. motors. In Chapter 7 it was sh a d.c. motor needed a series resistor to limit f starting current and, as the motor accelera resistance was gradually reduced until it was no

---(a) No-load conditions

(b) Loaded conditions

Fig. 12.20 Current phasors for an induction motor

THREE-PHASE INDUCTION MOTORS 100%

231

L, 0-----j

L'D-----1

Lau-----1 Fig. 12.22 Basic power circuit for primary resistance motor

st·arting

resistance, the greater is the voltage drop across each

Power output

Fig. 12.21 Typical parameters for a three-phase induction motor showing how they alter as the load varies

circuit. Limiting the current drawn by the motor during starting is directly applicable to both a.c. and d.c. motors, and is achieved by reducing the voltage across the motor windings. The only exception to this is the wound-rotor

resistor and the less the voltage at the motor. Because of this lower voltage, the starting current is reduced. As the rotor accelerates, the resistance is reduced in steps until full voltage is applied across the motor terminals. This method of starting greatly reduces the starting torque of the motor because it is proportional to the square of the applied voltage.

i.e.

It must be appreciated that this expression shows the starting torque is reduced four times if the applied voltage motor. is halved. Such a reduction in torque may prevent a motor While ad .c. motor can only use series resistance, there 1 from starting against even a small load. are alternative current-limiting methods for a.c. motors.

Advantages of current limiting I. Less mechanical forces exerted on windings. 2. Less mechanical shock forces on motor frame, shaft and transmission. 3. Steadier acceleration with connected loads. 4. Reduction of line voltage drop. 5. Less disturbance to the supply system. Disadvantages of current limiting I. Reduction in starting torque. 2. Extra cost of starting equipment. 3. Increased maintenance requirements. 4. More con1plex equipn1ent.

12,6,1 Direct-on-line starting Cage motors may be started with the full supply voltage

connected across the stator windings. This method is usually referred to as direct-on-line or D.O.L. starting. The large starting current can cause excessive voltage drop in the supply lines and disturbances to the supply voltage. For this reason, the supply authorities usually limit the starting current of motors. The D.0.L. method of starting is usually restricted to smaller-sized motors.

12,6,3 Star-delta starting Another way to reduce the starting current by reducing the voltage applied to each winding is the star-delta method. The motor is started in the star connection and when it has gained sufficient speed, it is quickly changed over to the delta connection. The starting torque is considerably reduced with this method, which is usually applicable to motors being started on light loads, the main load being applied after the motor has reached full speed and is connected in delta. The two ends of each phase winding of the motor are brought out to the stator terminals. During the starting sequence in star, the voltage across each phase is 1/ V3 or 58% of the line voltage. As a result, the torque is reduced 2 to ( 1/ VJ) or 1/3 of its normal running value. The line and phase currents in star are equal, but when the windings are connected in delta this condition no longer applies. The phase voltage is increased VJ times or 173% over the star connections, consequently the phase current is increased by the same ratio. In addition the line current is now equal to V3. Iphase, or three times the line current value for the star connection. These values are illustrated in Figure 12.23 for a winding impedance of 24 ohms.

12,6.2 Primary resistance starting An effective method for reducing the starting current of an

induction motor is to add resistance in series with the supply lines (see Fig. 12.22). The higher the value of

12.6.4 Autotransformer starting Autotransformer starters are the most popular of any reduced voltage type. The voltage applied across the

232

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL 1 Torque and input power 113 of "run" condition

10 A

30 A

10 Ai

Start (star-connected)

Run (delta-c

Fig. 12.23 Comparison of star and delta starting

stator windings can be reduced to a percentage of the supply voltage by using a star-connected autotransformer. During starting, the primary of a step-down autotransformer is connected to the supply, and the secondary is connected to the stator windings of the induction motor, as shown in Figure 12.24. By providing a range of tappings in the transformer windings (e.g. 60%, 70%and 80%), it is possible to have a choice of voltages (and currents) for starting purposes. When the motor is up to speed, the stator windings are connected across the full supply voltage, and the autotransformer is open-circuited. The use of a transformer makes it possible to reduce the line input current at a greater rate than that at which the torque is reduced. Transformers are discussed in greater detail in Chapter 15, but briefly: input voltage x input current = output voltage x output current i.e.

v, x r,

=

v, x r, (neglecting all losses)

During starting, a reduced voltage ( V,) is applied to the motor, so reducing the starting current(!,). Because of transformer action, however, the input current (/1) is reduced still further. It can be illustrated by the following example.

Example 12.4

A 415 V, three-phase induction motor draws 160 connected D.O.L. If an autotransformer starter, motor connected to the 70% tapping, is used to motor, determine: (a) the voltage applied to the motor during start (b) the starting current taken by the motor; (c) the starting current drawn from the supply.

,,

,, V1 = 415 v

Vi

=

70°/o of V1

Fig. 12.25 Circuit diagram for example 12.·

(a) For 70% tapping: motor voltage = 70% of input voltage =70%of415V = 0.7 x 415 = 290.5 v

0 Fig. 12.24 Starting connections with a three-phase, star-connected autotransformer

233

THREE-PHASE INDUCTION MOTORS

(b) For 70% tapping: motor current = 70% of D.O.L. starting current = 70%of 160A = 0.7 x 160 = 112A (c) V,J,

:. J,

V2l2

V,h = 290.5 x 112 v, 415 = 78.4 A Fig. 12.26 Starting connections for an induction motor

Note that the motor voltage has been reduced by 70% to 290 V. Because T o:: V2 , the torque will have been reduced to (0.7) 2 =0.49 of the D.O.L. value. While the motor current has only been reduced to 70%, the input current has been reduced to 0.49 or 49% of the D.0.L. value. The significance of these figures can be seen when compared to those of the same motor when the primary resistance starting method is used. Motor impedance between lines at standstill

v 415 z =I= 160 =

2.59 !1 (D.0.L.)

The line input current from example 12.4 is 78.4 A (autotransformer). For primary resistance starting with this same value of current the applied voltage would be reduced to: V

= !Z = 78.4 x 2.59 =203.lV

For the same input current for both methods (78.4 A), the relative torque values would be: D.O.L.

415)' ( ill x 100 = !00%

autotransformer

( 2!~55 )' x

primary resistance

203 ( 415· 1 )' x 100 = 24"' 70

100 = 49%

That is, for the same input current the autotransformer starter enables the motor to develop twice as much starting torque as the primary resistance method. With three autotransformers used as in Figure I 2.24 it is usual, when changing over to full voltage, to opencircuit the star-point and momentarily supply the motor through part of the transformer windings in series. These parts of the windings are then shorted out, effectively taking them out of the circuit. This method is called the Korndorfer method of starting. Autotransformers are more often used in the open-delta circuit where only two windings are used as in Figure 12.26. It is a cheaper method and while the circuit is unbalanced during the starting sequence, it is balanced as soon as the motor is in the running connection.

using an open-delta autotransformer

12.6.5 Secondary resistance starling This method of starting can only be used with woundrotor motors. Full line voltage is applied to the stator windings and the starting current is limited by connecting external resistance across the rotor terminals, as shown in Figure 12.27. As ihe motor's speed increases, the resistance is gradually removed from the circuit until at full speed all the resistance is shorted out. 12.6.6 Other types of motor starters Liquid Instead of using resistors as a form of current limiting, one trend is to use liquid containers with two electrodes and a chemical electrolyte. Although this is often more compact than the resistor type there needs to be some form of increased maintenance to check on the state of the electrolyte. The liquid is a simple replacement for the resistor and is often a mixture of water and salt, with occasionally other chemicals.

Solid state Starters with this type of construction are initially expensive, but generally incorporate some form of operator-adjustable starting current control. In addition, many systems incorporate a variable frequency generator for speed control. Most models work on a principle of converting the alternating current to direct current and then generating voltages and frequencies to suit. See also sections 12.9.2 and 17.9.2.

12.7 Typical pushbutton-operated starter circuits 12.7.1 Circuit protection Electrical starting circuits are protected against faulty operation with fuses, circuit breakers and contactors. L,

0-----,'

L2

0----;

Lo

o------', Fig. 12.27 Basic power circuit for secondary resistance motor starting

234

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL -

Fuses A fuse is designed to become an open circuit once a certain

value of current is exceeded. This does not mean that a fuse will "blow" immediately its rating is reached. The fuse element can carry small overloads for a period of time, depending on the amount of the excess current and the rate at which the heat being generated can be dissipated. This characteristic makes a fuse suitable for motor

L, L, La

protection circuits, where the fuse must be able to handle

the starting currents.

*

Circuit breakers Circuit breakers, on the other hand, can be designed to operate with only small overloads and steps may have to be taken to slow the action down and enable motors to be started. Some circuit breakers operate on a magnetic attraction principle, others on a thermal element; most operate with both magnetic and thermal elements.

Contactors Both fuses and circuit breakers are designed to protect electrical circuits against excessive currents, but serve no useful purpose in the event of power failures. As a means of protection contactors are used. When a power failure or low-voltage situation occurs the contactor drops out, so switching the equipment off until power is restored. As an added protection against faulty starting sequences, most motor starters are automatic once the initial pushbutton operation has been made. The following examples show pushbutton-operated circuits for each of the five means of starting three-phase induction motors. The circuits are shown with fuse protection and thermal overload current protection as possibly the most common protection methods encountered. Individual manufacturers have their own preferences for starter circuits and these may vary in detail from one firm to another and from one model to another.

K1.1

K1.2

E

K1.3

Oil-

Power circuit

I

Control ci.

Fig. 12.28 Contactor circuit tor D.0.L. starting

L, L, La

12.7.2 D.O.L. contactor starter circuit (Fig. 12.28)

Circuit operation 1. Pressing the start button completes a circuit from L 3 through the normally closed stop button to coil KI, and the overload to L2 • 2. Main contactor coil Kl then closes and applies full line voltage directly to the motor via contactor contacts Kl.I, Kl.2 and Kl.3. 3. Contact Kl.4 bridges out the start button contacts so that, on the release of the start button, the contactor remains in the operational state-Le. the control circuit is latched in the "on" position. Pressing the stop button disables the latching circuit and allows the main contactor to revert to the "off" state.

K1.1

12.7.3 Primary resistance contactor starter circuit (Fig. 12.29)

Circuit operation 1. Pressing the start button completes a circuit from L 3

through the normally closed stop button to coil KI, and the overload to L2.

Power circuit

Control ci1

Fig. 12.29 Contactor circuit for primary resistance s

235

THREE-PHASE INDUCTION MOTORS 2. The main contactor KI operates. Contact Kl .4 closes and bridges out the start button contacts, so that on the release of the start button the Kl contactor circuit remains latched.

3. Contacts Kl.I, Kl.2 and Kl.3 close, and a reduced line voltage is applied to the motor through the resistors in series with each line to the motor. The starting current is limited by the resistors to a value below that of D.O.L. starting. 4. Delayed action contact K 1.5 operates after a predeter-

mined delay and completes the circuit for coil K2. Its operation causes contacts K2. l, K2.2 and K2.3 to close and allow full line voltage to be applied to the motor.

5. Pressing the stop button de-energises all coils and allows the starter to revert to the "off' state.

2. When K2 operates, it causes the "ends" of the three windings to be joined in "star" via contacts K2. l, K2.2 and K2.3. 3. Simultaneously, coil K3 is open-circuited by K2.5. This is the delta connecting coil and must be isolated when the star connection is in operation. Similarly, when the delta connection is in operation the star connection must be isolated, a method called "electrical interlocking". As a precaution, the star and delta connecting contactors are often mechanically interlocked in addition to the electrical interlocking

provided by contacts K2.5 and K3.4. 4. When K2.4 closes, a voltage is applied to the timer K4 and to coil Kl. This allows Kl.4 to close and bridge the start button. 5. Contacts Kl.I, Kl.2 and Kl.3 close and apply a voltage to the "starts" of the motor windings.

6. The voltage applied across the windings is only a proportion of full line voltage (0.58), and starting

12.7.4 Star-delta contactor starter circuit (Fig. 12.30)

current is reduced accordingly.

7. When the time delay period has elapsed, contact K4. l

Circuit operation I. Pressing the start button completes a circuit from L3

opens and forces contactor K2 to disconnect the star connection. The dropping-out action of K2 completes

through the normally closed stop button and two normally closed contactor contacts (K4.l and K3.4) to coil K2, and the overload contact t to L2.

the circuit of coil K3 through K2.5, and it is then activated. This open-circuits the interlocking contact

K3.4 and also switches off the timer K4 via K3.5.

L,

L,

E

E

K3.1

I

K3.2

K3.3

I

1<1.4

K4.1

~ K3.4

K2/5

K2.5

K3.5

K3/5

Control circuit Fig. 12.30 Con/actor circuit for star-delta starting

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL -

236

8. Contacts K3.l, K3.2 and K3.3 close and complete the delta connection, allowing full line voltage to be applied to the motor. 9. Pressing the stop button de-energises all coils and allows the starter to revert to the "off' state.

12.7.5 Autotransformer contactor starter circuit (Fig. 12.31)

Circuit operation 1. Pressing the start button completes a circuit from L3

through the normally closed stop button, a normally closed delay contact Kl.5, electrical interlock K3.3, coil K2, and the normally closed thermal overload contact t to L 2. 2. When K2 is activated, it closes the contacts K2. l and K2.2 connecting the ends of the autotransformers to line L 2 in an open delta configuration. 3. The operation of K2 simultaneously closes contact K2.3 and opens contact K2.4-the electrical interlock to prevent K3 operating while K2 is active. 4. K2.3 supplies power to coil Kl, which is also activated. Contacts Kl.I, Kl.2, Kl.3 and Kl.4 close. Full line voltage is connected to the autotransformers and a reduced line voltage is supplied to the motor via the transformer tapping. Contact Kl.4 ensures that a

voltage is available to the control circuit whe button is released. 5. The delayed opening contact Kl.5 opens afi determined time lapse and forces K2 to op the delta connection. Contact K2.4 then clo and coil K3 is activated. 6. Contacts K3.l and K3.2 close, and full lin is applied to the motor through the Kl cc series with two lines. The electrical interloc K3.3 opens and isolates coil K2. 7. Pressing the stop button de-energises all allows the starter to revert to the "off'' state. 12.7.6 Secondary resistance contactor start (Fig. 12.32)

Circuit operation l. Pressing the start button completes a circui1

through the normally closed stop button, coi the thermal overload contact t to L2 • Coil ~ is in parallel with coil Kl, is activated at time as Kl but only operates after a pred< time delay. 2. Contact K 1.4 bridges out the start button cc that on the release of the start button the , remains in the operational state-Le. the cont1

is latched in the "on" position.

L, L, La

I I K1 .1

K1.2

K1.3

K3.1

IE I IE I K32

K1.4

K2.3

1

~ Power circuit

I I I I I I

Control circuit

Fig. 12.31 Contactor circuit for autotransformer starting

237

THREE-PHASE INDUCTION MOTORS

L, L, L,

K1.4

E

K2.1

R

R

R

R

R

R

K1/4

Power circuit

K3.1

K4/2

Control circuit

Fig. 12.32 Contactor circuit for secondary resistance starting

3. Contacts Kl.I, Kl.2 and Kl.3 close and apply full line voltage to the stator terminals of the motor. The rotor has two resistors in series with each winding and, as the ends are connected in star, current flows in the rotor windings and the motor is able to generate torque and commence turning. 4. After the delay time, K2 operates and closes contact K2.l. This causes K4 to be activated along with K3, a second time delay relay. 5. Contacts K4.l and K4.2 then close and reduce the amount of resistance connected across the slip rings. This action enables the motor to attain a higher speed. 6. After a further time delay, coil K3 operates and closes contact K3.l. Coil K5 is then activated and closes contacts K5 .I and K5 .2. This action removes the remainder of the resistance in the rotor circuit and the motor is in its normal running mode. 7. Pressing the stop button de-energises all coils and allows the starter to revert to the "off' state.

12.7.7 Part winding starters Another method of motor starting gradually gaining a measure of acceptance is the part winding motor starting method. As with other methods the primary intention is to reduce starting current and/or torque of a motor. With larger motors the initial starting torque can transmit high and damaging shock values to transmission components, and some type of reduced torque starting becomes essential. The phase windings of the stator are divided into parallel sections each of the requisite number of poles and each capable of withstanding full line voltage. Parts of the stator winding are energised and as the motor gains speed more sections of the stator winding are energised. Normal control and power components are used to provide the necessary switching. As much of the motor's windings remain connected to the line in a closed transition sequence, current surges are kept to a minimum.

238

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL l

Table 12.2 Relative characteristics of various starting methods

Starting

Stator

method

voltage at

Starting current

start

(

Direct-on-

line voltage

0

/o/FL)

Starting torque {O/o TFL}

700°/o

No. of

starting steps

Current surge during transition stages

n.a.

150o/o

line

Types of

Example loads

Genera

comme

loads suited light ine~ia loads

centrifugal

fans

poor sta torque

motorgenerator

starting

pumps; lathes

starting greate1

load to Primary reduced resistance

300o/o

40o/o

2+

no

almost no

Star-delta

200°/o

33o/o

2

yes

load light loads

reduced

units Autotrans-

300o/o

reduced

80o/o

2

yes

substantial hydraulic proportion pumps; of full load conveyors

former

Secondary line voltage

1 OOo/o

1 OOo/o

2+

no

high inertia shock loads; presses; loads

resistance

shears Note: The figures quoted in this table must be considered as a general

113 full I torque starting

slightly than fu torque rotor ref adjuste

start (T

~uide

only. Many variables can be encountered-types an, stators and rotors, applications, loads and starters being only some oft e factors.

L,

In Figure 12.33 each phase winding has been divided into two sections, each section sequentially being connected to the line voltage. Starting torque is down to approximately 45% and starting current is about 65% of normal D.0.L. starting. Because of current imbalance during the starting sequence, the motors tend to be noisy at this time, which restricts application of the methoc! to cases in which a motor requires occasional starting before running for long periods.

(a) Start (stage 1)

12.8 Motor output power

L,

In an induction motor, the magnetic fields of the stator and rotor interact to cause a force to be developed on the rotor conductors. This force, acting at a distance from the shaft equal to the radius of the mean circle of the rotor conductors, develops a torque or turning force at the shaft of the motor. Torque is measured in newton-metres (Nm) and is calculated by the product of force (newtons) and radius (metres).

T= F.r

(b) Transition (stage 2)

L,

L,

Example 12.5 A motor exerts a force of 360 newtons at the rim of a pulley with a diameter of 0.5 metre. Calculate the torque developed by the motor.

T= F.r

(c) Run (stage 3)

= 360 x .Q2 = 90 Nm 2

=

L,

Fig. 12.33 Part winding starting

L,

239

THREE-PHASE INDUCTION MOTORS Example 12.6

12.9 Speed control of induction motors

If the n1otor in example 12.5 was fitted with a 0.3 m diameter pulley, calculate the force exerted at the rim of the pulley.

motor was shown to be:

F

= .I_

nsyn =

r

90 - 0.15 = 600N When considering the mechanical output of the induction motor, it is necessary to determine the power

produced. power

= rate of doing work = work done per second (watts)

work done = force exerted

distance

x

distance moved

= 2rr.r for 1 revolution (where r = radius in metres) = 2 rrr X

power

In section 12.3.2 the synchronous speed of an induction

n

for 1 second

60 (wheren = r/min) = force x distance 2rrrn 2rrn =FX(;0=6QXFr

But torque ( 7) = Fr

60

where n is in r/min. Example 12.7 A 415 V, three-phase, 50 Hz, four-pole induction motor has a full-load speed of 1440 r/min. Calculate the power produced by the motor if it develops a torque of 100 Nm.

p = 2rr.n. T 60 2 x .,,. x 1440 60 15 079 watts 15.1 kW

p

That is, the speed depends on both the frequency and the nu1nber of magnetic poles in the machine. The synchronous speed is that of the rotating magnetic field, while the actual speed must always be less than this by the amount of slip necessary to allow the motor to develop the required amount of torque. To change the speed of an induction motor by an appreciable amount, other than by loading it to alter the slip speed, either the frequency of the supply or the number of poles in the windings must be changed. Under normal circumstances the speed of the induction motor must then be considered as fixed. If any application requires that the

speed be adjustable, then special and expensive equipment must be used. (Refer to sect. 17.9.)

12.9.1 Wound-rotor motors The degree of slip in a wound-rotor motor may be

changed comparatively easily by varying the amount of external resistance in the rotor circuit. This provides dubious results in that the speed changes every time the

load changes, efficiency can be as low as 40% because

2rrnT P =--watts

i.e.

120(

of the /ZR losses in the resistors and the available torque

can be appreciably reduced. In addition, heavy-duty resistors have to be provided owing to the fact that the normal resistors are rated for starting purposes only (i.e. a short duty cycle). Combinations of wound-rotor motors connected in various ways have attempted to overcome these disadvantages-called cascading and concatenation-but the methods tend to be cumbersome and expensive. Special motors have been developed with

better results, but the costs are still so high that the direct current machine becomes competitive even after allowing

for the provision of a d.c. supply.

x 100

Example 12.8

If the above motor draws 31 A from the supply, and the power factor is 0.86, determine the efficiency of the motor.

P;. = VJVIA. _ output T/ - input = 15 079 x IOO VJVIA. 15 079 x 100 = ,jj x 415 x 31 x 0.86 = 78.7%

12.9.2 Squirrel-cage motors This type of motor under normal operating circumstances

is considered as a fixed speed machine with only very small variations in speed from no load to full load. Reducing the supply voltage has negligible effect on the speed but reduces the amount of available torque and eventually the machine stalls. Special connection arrangements of the windings 1hc1nsclves allow n1otors to be connected, for example, as either two- or four-pole motors. This only provides a step

change in speed and allows no other control. If other speeds are required then possibly a second winding has to be used. For example, one winding could give both two and four poles while the second winding gives six poles. It is however still only speed changes in three steps. While satisfactory for some uses such as lathe work, it is not

suitable for any project such as rolling mills where incremental changes may be required. The method of speed change in the ratio of 2: I for single windings was developed many years ago. More recently the advent of

240

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL T control of the operator, and can be done man1

a control knob on the unit. The speed range ol is from 1% to 200% of normal running speeds and efficiency are high throughout the complete The unit's operating principle is to convert ti phase alternating current supply to direct current

to generate a three-phase supply at any desired f1 within the range of the equipment. The units 1 reversing facilities, and provision can be made fo control stations. The units are available in a

sizes. The photograph of a typical unit is shown i 12.35.

12.10 Motor braking The main power equipment in most electrical sys rotating electrical machine. There are many app where it is necessary to bring the machine tc

quickly, to keep a load from moving or to enable: reversal of direction. In general there are two

braking-mechanical and electrical. 12.10.1 Mechanical braking Mechanical brakes are mostly solenoid operated tension holds the brakeshoes or band against ad it operates as a stopping device as well as a parkir This is necessary to prevent undesired movemen

..

. IJ Fig. 12.34 A two-stage secondary resistance starter. The direct-on-line contactor is at the top, while the lower contactor short-circuits the rotor resistors when operated by the timer.

pole amplitude modulation (PAM) represented an advance on this concept in that it gave closer ratios in the order of 6:4 and 8:6 and other similar ratios. Note however it is still step changes and not incremental. The PAM motor is made in all sizes and its "secret" lies in the internal connections of the pole groups. Six leads are brought out in a similar fashion to two-speed motor windings. The windings are halved and each half is connected in star across the supply lines, giving double or parallel star for one speed, while the windings are all connected together in series delta for the other.

could occur with a crane holding a suspended loa power is applied to the driving motor, the soleno activated, so releasing the brake and allowing tr full control. Light duty brakes may adopt the di configuration but these have limited application: 12.10.2 Electrical braking There are two types of electrical braking and e; also be used in conjunction with mechanical bra

Dynamic braking In this method the motor converts its energy o into electrical energy. For a three-phase inductic

Another recent development for speed control of squirrel-cage motors is the electronic control unit. Provision is usually made in the unit for adjustable rates of current increase to control starting currents, and the

voltage supplied by the unit to the motor is stabilised. The motor is accelerated from standstill at a predetermined current rate. Once running under load, the motor speed can be

varied by changing the frequency. This is usually under

Fig. 12.35 An incremental speed control unit for i motors. It uses an electronically gener ASI variable frequency.

241

THREE-PHASE INDUCTION MOTORS the most common method is to disconnect the a.c. supply and reconnect the windings to a source of d.c., as shown in Figure 12.36. An inspection of the circuit will show that the main and brake contactors are electrically interlocked with a time delay to switch the d.c. off after a short period. Because the rotor conductors are still rotating and the d.c. field is stationary, there is a generated voltage and a circulating current in the rotor bars. This creates a torque in the opposite direction to the rotation of the n1otor, resulting in a rapid slowing down. The rate of slowing down is fairly constant, unlike the d.c. n1otor with dynan1ic braking, where the braking effect din1inishes markedly as the speed reduces.

Satisfactory operation of three-phase motors on a three-phase supply depends on several factors: I. three equal voltages at the correct phase displace1nent. Under normal operating conditions the phase displacement is a function of the generating equipment and stays relatively fixed, but the line voltages can vary depending on the individual loads connected at that tin1e. For balanced loads, such as three-phase motors, unbalanced phase voltages lead to unbalanced currents flowing in the motor windings. As a consequence, circulating currents are set up, heating is increased and uneven, and torque is reduced.

2. the stator windings being correctly connected in either star or delta. Phase currents become unbalanced,

Plug braking

windings generate increased heat, and torque is greatly reduced. Refer to section 12. l I. I for more details.

Plug braking for an induction motor is simply a matter of reconnecting for the reverse direction of rotation while it is still rotating in a forward direction. While the d.c. motor needed some form of current limiting, the induction motor does not since the current drawn is substantially the same as the D.O.L. starting current. Once the motor has stopped, some form of switching is needed to prevent it accelerating up to speed in the reverse direction. This is often achieved by a friction driven contact on the shaft of the motor.

3. the three line voltages being connected to the 1notor windings. When any one supply line is not able to supply current to the winding to which it is connected, the condition known as "single-phasing" occurs. For further details refer to section 12.1 1.2. 4. the condition of.both windings in the 1noto1: The stator windings connected to the supply are pron1inent and obvious areas of concern. Noisy operation and reduced torque of a three-phase n1otor can mean that the bars of the rotor-the second winding-are in need of attention. Many cages consist of aluminium cast into shape in the laminations, and little can be done in the way of niaintenance; many of the larger motors, however, have prefabricated bars and rings of copper which are welded into place. It is possible to repair dan1age to these ite1ns, whether they are broken or sin1ply loose in the rotor.

12.11 Abnormal operating conditions The following applies specifically to three-phase motors. For further information on abnormal operating conditions applying to both single and three-phase motors see section 13.6 in the next chapter.

L, L, L,

I I

I t-----+---+-r-.,

r;;\ ~

l1> y ,

K2.1-2.3

-

+

Power circuit

j -

I IE I II~K2.4

I

K1/5

K2/4

K3/1

~-____. Control circuit

Fig. 12.36 Braking an a.c. motor by d.c. injection

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL 1

242

c

-8

c

A,

A A

I

B

I I Reversed phase

I

I

s,

I

I

(a)

(b)

Fig. 12.37 Reversal of one phase tor a star-connected motor

12.11.1 Phase reversal Previously in this chapter the operation of the induction motor has been based on the assumption that the motor had three identical windings and three equal currents flowing in them, all spaced at 120° E to each other. If one phase is reversed, however, as shown in Figure l 2.37(a) for a star connection, these conditions no longer hold true. Two of the three currents that flow are at 60° E to each other and the load system is unbalanced (Fig. 12.37(b)). The same condition applies to delta-connected loads, and both connections were discussed in Chapter 10. As a result of this incorrect connection, the motor loses most ofits torque and is often unable to start against even a light load. If able to start at all, it usually rotates very slowly and has unequal values of current in the phase windings. The values of current approach those drawn during normal starting, but remain high. The motor I

L,

usually emits a "growling" noise and has an a vibration due to the sustained high current valt 12.11.2 Single-phasing Single-phasing is a condition that occurs when c a three-phase supply is open-circuited and is no supply current to a three-phase load. The nar used when one of the three phase windings in open-circuited. The condition for single-phasing in a star-(

load is shown in Figure 12.38 (a), and it can be a break in either the line or the phase windin. the circuit to a single current path.

Figure 12.38(b) shows an open-circuited I delta-connected motor. There is one main cur from L 1 to L3 through phase A and another l L 1 to L3 through phases B and C in series. Bot!

~----

0 - - - - - - - , '1

-~----

I

.... L,0-----~

\ \

A

\

\ \ I

x

\

I

I

L20-X_]

,

'\

B

______

L2 o-x---~

......

_________

/

B

----

JL2 0---1--fY--VYl

\

I

I

-

-,I

JL,()----------J L, ( _ } - - - - - - - - - '

--------·--/ /

(a)

(c)

(b)

Fig. 12.38 Circuit conditions causing single-phasing with

a three-phase

motor

243

THREE-PHASE INDUCTION MOTORS are in parallel with each other, although not necessarily connected induction motor is shown with phase C open-

than normal currents in the parts of the circuit still operating, with values approaching starting current values in some circumstances. It can also emit a low-

circuited. There are two current paths-L 1 through phase A to L3 and L, through phase B to L,. In each of the cases shown in Figure 12.38, the rotating magnetic field is either destroyed or unbalanced, and causes unsatisfactory operation of the motor. The motor rotates at slower speeds, if it is able to start at all, because of a much reduced starting torque. It usually draws higher

pitched "growling" noise similar to that which occurs during a phase reversal. If single-phasing occurs while the motor is operating at normal speeds, the normal humming sound often changes to a higher-pitched whine. For any of the conditions for single-phasing outlined above, the ratios between line and phase values are no longer valid.

in phase with each other. In Figure 12.38(c), a delta-

VJ

Exercises 12.1 12.2

Briefly describe how the rotating magnetic field is produced in a three-phase motor. (a) Define the term "synchronous speed''. (b) Make a table showing the synchronous speeds of two-, four-, six- and eight-pole induction motors for frequencies of 40, 50 and 60 Hz.

12.3

Explain why an induction motor runs at less than synchronous speed.

12.4

Explain why the power factor of an induction motor increases with the load. Briefly describe the construction of the squirrel-cage and the wound-rotor.

12.5 12.6

List three methods by which the starting current of a three-phase squirrel-cage induction motor may be reduced.

12.7

What is the disadvantage in starting a squirrel-cage induction motor on a reduced voltage?

12.8

What is meant by: (a) synchronous speed of an induction motor? (b) actual speed? (c) slip speed? (d) What is the relationship between each of these three speeds? 12.9 Sketch a typical torque/speed curve for an induction motor having a normal squirrelcage rotor. At low values of slip, how does the torque vary with load? What occurs when breakdown torque is reached? 12.10 Why do squirrel-cage motors take relatively large amounts of current when connected D.0.L.? 12.11 Discuss speed control by a method of changing the number of poles in a motor. 12.12 Draw a circuit diagram for a star-delta starter using push buttons for its initiation. Discuss the operation of the circuit and list two uses for this type of starter.

Problems 12.13 Determine the percentage slip for the following three-phase, 50 Hz motors: (a) four-pole, 1420 r/min; (b) six-pole, 960 r/min; (c) eight-pole, 720 r/min. 12.14 A 15 kW, three-phase, 415 V, 50 Hz, four-pole induction motor draws 190 A when started D.O.L. in delta. Determine the starting current using: (a) the star-delta method; (b) the autotransformer method (60% tapping). 12.15 At full load the efficiency of the motor in problem 12.14 is 83%, the power factor is 0.84 and the slip is 4%. Determine: (a) the torque developed; (b) the current drawn. 12.16 Calculate the full-load torque of each of the following motors: (a) 7.5 kW, 1440 r/min;

(b) 1.5 kW, 940 r/min; (c) 12 kW, 720 r/min; (d) 5 kW, 1450 r/min. 12.17 On full load, a three-phase, 415 V, 50 Hz, six-pole motor draws 19 A at a power factor of 0.85. If the torque developed is 95 Nm and the slip is 7%, calculate the efficiency of the motor. 12.18 A cutting tool exerts a tangential force of 400 N on a 90 mm diameter steel bar which is rotating at 145 r/min in a lathe. The efficiency of the lathe gear train is 62% and the threephase, 415 V motor efficiency is 81%. Calculate the motor current if the power factor is 0.83. 12.19 The rotor speed of a 10 kW, 415 V, threephase, four-pole motor is 1455 r/min when it operates from a source of 50 Hz. Find: (a) synchronous speed; (b) slip speed; (c) frequency of rotor currents.