Basic Motor Theory

Diagnostic Faults in Electrical Motor

Theory Application to Electric Motors

The Basic of Electric Motor

Rotate Magnetic Field Alternating current

The phase windings A, B and C are placed 120 degrees apart.

Single Phase Induction Motor

Rotate Magnetic Field Alternating current

The phase windings and number of poles

Three Phase Induction Motor

Types of Electric Motor Theory application to electric motors

AC MOTOR Asynchronous/Induction Motors • The induction motor is so named for the induced current flowing in the secondary winding (the rotor) by action of the primary winding. No direct electrical connection is made; it is a result of the magnetic field being established by the stator winding. • The principle purpose of the stator winding is to establish a rotating magnetic field in the stator core that will induce a voltage in the rotor core. • The rotor “becomes” a magnet, with a North and South pole, which in turn follows the moving magnetic fields in the stator.

Construction • The three basic parts of an AC motor are the rotor, stator, and enclosure. • The stator and the rotor are electrical circuits that perform as electromagnets.

AC MOTOR Asynchronous/Induction Motors

MAIN PARTS 

ROTOR

STATOR

OTHERS  SHAFT, BEARING, FAN, END COVER, TERMINAL BOX

Motor Construction

STATOR INSULATED ELECTRICAL WINDINGS Consist of copper wires, insulated with varnish, which are fitted into insulated slotted laminations Slots are made from high grade alloy steel to reduce the effects of eddy currents

FRAME To provide mechanical protection and support for windings

SHAFT .. Shaft is placed inside the rotor, so when rotor rotates then actually shaft rotates ..

BEARINGS -Rotator is mounted on bearings to reduce friction on both sides -Usually ball and roller bearings are used to suit heavy duty, trouble free running and enhanced service life.

FAN .. Used to for adequate circulation of cooling air ; securely keyed onto the rotor shaft ..

TERMINAL BOX .. Used for holding stator windings and rotor windings ..

END COVERS .. Provide support for the rotor assembly ..

AC MOTOR Asynchronous/Induction Motors

Stator Stator: The stationary electrical part of the motor. It contains a number of windings whose polarity is changed all the time when an alternating current (AC) is applied. This makes the combined magnetic field of the stator rotate. The stator insulation design is classified. This clas- sifi-cation is defined in IEC 62114, which have different insulation classes (temperature classes) and temperature rises (∆T). The stator can be designed to handle various voltages, frequencies and outputs and a varying number of poles.

Construction (Stator construction) The stator is the stationary electrical part of the motor. The stator core of a National Electrical Manufacturers Association (NEMA) motor is made up of several hundred thin laminations. Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core. Electromagnetism is the principle behind motor operation. Each grouping of coils, together with the steel core it surrounds, form an electromagnet. The stator windings are connected directly to the power source.

STATOR WINDING CONFIGURATION 3-PHASE

STATOR WINDING CONFIGURATION 3-PHASE

ROTOR .. rotating part of the motor ..

Two types of Rotors based on its structure : Squirrel-Cage Rotor and Slip-Ring Rotor

Rotor When the stator's moving magnetic field cuts across the rotor conductor bars, a current is produced. This current circulates through the bars and creates magnetic fields around each rotor bar. As the magnetic field in the stator keeps changing, so does the field in the rotor. This interaction is what causes the rotor to move.

Construction (Rotor construction) • The rotor is the rotating part of the electromagnetic circuit. • It can be found in two types: • Squirrel cage • Wound rotor

• However, the most common type of rotor is the “squirrel cage” rotor.

Construction (Rotor construction) Wound Rotor

Squirrel-Cage Rotor

Short circuits all rotor bars.

/rotor winding

ROTOR - Squirrel-Cage Rotor

ROTOR BAR

ROTOR - Squirrel-Cage Rotor

Enclosure The enclosure consists of a frame (or yoke) and two end brackets (or bearing housings). The stator is mounted inside the frame. The rotor fits inside the stator with a slight air gap separating it from the stator. There is NO direct physical connection between the rotor and the stator.  The enclosure also protects the electrical and operating parts of the motor from harmful effects of the environment in which the motor operates. Bearings, mounted on the shaft, support the rotor and allow it to turn. A fan, also mounted on the shaft, is used on the motor shown below for cooling.

Stator Rotor Air gap

Construction (Enclosure)

A Stator

B

Current of Each Winding

Flux

C EMF A+B+C

Rotor

Inducing Rotor

Current of Rotor

EMF Stator

Force of Rotor

Nr >> Slip Fr = S*Fs

Rotating Magnetic Field When a 3 phase stator winding is connected to a 3 phase voltage supply, 3 phase current will flow in the windings, which also will induced 3 phase flux in the stator. These flux will rotate at a speed called a Synchronous Speed, ns. The flux is called as Rotating magnetic Field Synchronous speed: speed of rotating flux

ns 

120 f p

Where; p = is the number of poles, and f = the frequency of supply

RMF(Rotating Magnetic Field) a

Fc

c’

b

b’ Fa c

1.5

F

1 0.5

Fb

a’

Fa

0

-1 -1.5 -93

F Fb c’

a

a’ t = t1

10

113

216

Space angle () in degrees Fc

Fb

b’ c

b

Fc

t = t0= t4

Fb

-0.5

t = t0= t4

F

c’

a

b’

c’

Fa

F

b Fc ’ a t = t2

c

a

b’ c

b t = t3

Fc a’

F

Fb

Induction Motor Speed At what speed will the IM run?  



Can the IM run at the synchronous speed, why? If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced

Synchronous Speed ROTOR

ns 

120 f p

If, for example, the frequency of the applied power is 50 Hz, the synchronous speed is 3000 min-1 for a 2-pole motor.

Induction Motor Speed  

IM will always run at a speed lower than the synchronous speed The difference between the motor speed and the synchronous speed is called the Slip

Where, nslip = slip speed n s lip  n s y n c  n m nsync = speed of the magnetic field nm = mechanical shaft speed of the motor

Slip So far, so good. But of course we already know that AC motors are known as asynchronous motors. This is because the rotor field does not follow the stator field in perfectly synchronous motion.

This difference in speed between rotor and stator fields, is called slip and is measured in %. Slip is a key factor and is necessary to produce torque. The greater the load - torque - the greater slip.

Slip and Rotor Speed Slip s The rotor speed of an Induction machine is different from the speed of Rotating magnetic field. The % difference of the speed is called slip. s 

Where;

ns  nr ns

OR

n r  n s (1  s )

ns = synchronous speed (rpm) nr = mechanical speed of rotor (rpm) under normal operating conditions, s= 0.01 ~ 0.05, which is very small and the actual speed is very close to synchronous speed. Note that : s is not negligible

Slip and Rotor Speed Rotor Speed  When the rotor move at rotor speed, nr (rps), the stator flux will circulate the rotor conductor at a speed of (ns-nr) per second. Hence, the frequency of the rotor is written as: fr  (ns  nr ) p

Where;

 sf

Note : At stator

:

s = slip f = supply frequency

ns 

120 f

 f 

p

ns p

.....( i )

120 At Rotor :

ns  nr   fr 

( ii )  ( i ) :

120 f p

(ns  nr ) p

.....( ii )

120

f r  s. f

When the rotor is blocked (s=1) , the frequency of the induced voltage is equal to the supply frequency On the other hand, if the rotor runs at synchronous speed (s = 0), the frequency will be zero

Where does torque and speed come from?

Nameplate

EXERCISE TIME

HOW TO READ MOTOR NAME PLATE

EXERCISE TIME

EXERCISE TIME

EXERCISE TIME

EXERCISE TIME

EXERCISE TIME

EXERCISE TIME

EXERCISE TIME

EXERCISE TIME

EXERCISE TIME

Frequency of motor excitation Magnetic field speed, RPM (Ns) = 120 x F / # pole Slip frequency (SF) = Magnetic Field Speed - Actual Speed Pole pass frequency (Fp) = Slip frequency X # poles Rotor bar pass frequency (RBPF) = # bars x RPM Stator Slot Pass Frequency

= # stator slot x RPM

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Example • Sebuah motor listrik di name plate tertera 1480 RPM. Jumlah rotor bar = 40.

Berarti motor listrik ini mempunyai 4 pole, sehingga : Magnetic Field speed (Ns) = 120 x 50 / 4 = 1500 RPM Slip frekuensi (SF) = 1500 - 1480 RPM = 20 RPM = 0.33 Hz Pole pass frekuensi (Fp) = 4 x 20 RPM = 80 RPM RBPF = 40 x 1480 RPM = 59200 RPM = 986.67 Hz.

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