Steel Plant Layout

STEEL PLANT LAYOUT AN OVERVIEW OF VSP: Visakhapatnam Steel plant, the first coastal based steel plant of India is located, 16km south west of city of destiny i.e., Visakhapatnam. Bestowed with modern technologies, VSP has a installed capacity of 3 million Tonnee per annum of liquid steel and 2.656 million tones of saleable steel. VSP exports quality pig iron & steel products to Sri Lanka, Myanmar, Nepal, Middle East, USA and south East Asia (Rig iron). Having a total manpower of about 16,613 VSP has emerged a labour productivity of 265 tone per man year of liquid steel which it the best in the country and comparable with the international levels.

MAJOR DEPARTMENTS: Raw Material Handing Plant (RMHP): VSP annually required quality saw material Viz. iron ore, fluxel (limestone, Dolomite) coking and non coking coke etc. to the tune of 12-13

1

million tones far producing 3 million tones of liquid steel. To handle such a large volume of incoming raw material received from different source and to ensure timely supply of consistent quality of feed materials to different VSP consumers, Raw material handling plant served a vital function. This unit is provided with elaborate unloading, blending, stacking and reclaiming facilities viz. wagon tippers, ground and track hoppers, stock yards crushing plants, vibrating screens, single/twin boom stockers and blender reclaimers. In VSP peripheral unloading has been adopted for the first time in the country. Coke Oven & Local chemical Plant: Blast furnaces the mother units of any steel plant require huge quantities of strong, hard and porous solid fuel in the form of hard metallurgical coke for supplying necessary heat for carrying out the reduction and refining reactions besides acting as a reducing agent. Coke is manufactured by heating of crushed coking coal (below 3mm) in absence of air at temperature of 10000c and above for 16 to 18 hours. A coke oven comprises of two hollow chambers namely coal chamber and heating chamber. In the heating chamber gaseous fuel such at Blast furnace gas, coke oven gas etc is burnt. The heat so generated is conducted through 2

the common wall to heat and carbonize the coking coal placed in the adjacent coal chamber. Number of ovens built in series one after the other form a coke oven battery. At VSP there are 3 coke oven batteries, 7 meter tall and having 67 ovens each. Each oven is having a volume of 41.6 cm and can hold upto 31.6 tonnes of dry coal charge. The carbonization takes place at 1000-10500c in absence of air for 16-18 house. The coal chemicals such as Benzole (& etc products, far landsite products), Ammonium sulphate etc. are extracted in coal chemical plant from C.O gas. SINTER PLANT: Sinter is a hard and porous ferrous material obtained by agglomeration of iron use fines, coke breeze, lime stone fined, metallurgical wastes viz. Flue dust mill scales, LD slag etc. Sinter is a better feed material to Blast Furnace in composition to iron are lumps and its usage in Blast arc and its usage in Blast furnace help in increasing productivity, decreasing the coke rate and improving the quality of hot metal produced. Sintering is done in 2nd of 312 sq.m. Sinter machines of duright lloyd type by heating the prepared feed on a continuous metallic belt made of pallets at 1200-13000C. 3

BLAST FURNACES: Hot metal is produced in Blast Furnace, which are tall vertical furnace. The furnace is named as Blast furnace as it is sum with blast at high pressure and temperature. STEEL MELTING SHOP (SMS): Steel is an alloy of iron with carbon upto 1.8% Hot metal produced in Blast furnaces contains impurities such as carbon, silicon, Manganese, sulphur and phosphorous is not suitable as a common engineering material. To improve the quality the impurities are to be eliminated or decreased by oxidation process. CONTINUOUS CASTING DEPARTMENT (CCD): Continuous capture may be defined as teaming of liquid steel in a mould with a false bottom through which practically solidified ingot/bar is continuously withdrawn at the same rate at which liquid steel is teamed in the mould. ROLLING MILL: Blooms produced in SMS-CCD do not find much applications as such and are required to be shaped into products such as Billets, rounds, squared, 4

angles, channels, I-PE beams, ME beams wire rods and reinforcements bars by rolling them in, three sophisticated high capacity, high speed, fully automated rolling mills, namely light & medium merchant mills (LMMM), wire Rod mill (WRM) and Medium Merchant and structural Mills (MMSM). LIGHT & MEDIUM MERCHANT MILL: (LMMM) LMMM comprised of two units. The unit comprised of 7 strands and 5 alternating vertical and horizontal strands. Billets are supplied from this mill to bar mill of LMMM & wire Rod Mill. The mill is designed to produce 710,000 tonnes per annum of various finished products such as sounds, refer square flats, angles, channels besides billets for sale. WIRE ROD MILL (WRM): Wire Rod mill is a 4 strand, 25 stands fully automated & sophisticated mill has a four zone combination type reheating furnace of 200TPH capacity for heating the billets received from billet mill of LMMM to rolling temp of 12000C.

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MEDIUM MERCHANT & STRUCTURAL MILL (MMSM): This mill is a high capacity continuous mill consisting of 20 stands assigned in 3 trains. The feed material to the mill is 250X250mm size bloom, which is heated to rolling temperatures of 12000C in two walking beam furnaces. The mill is designed to produce 8, 50,000 tonnes per annum of various products. Below are the Auxiliary facilitates in Steel Plant: 1. Power Generation and Distribution: The average power demands at all

units of VSP when operating the full capacity will be 221 MW. The captive generation capacity of 270MW is sufficient to meet all the plant needs in normal operation time. 2. Water Management: The total water requirement at full capacity

utilization of 3MT/yr is 70 MGD which is met from yeluru water scheme of A.P.

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3. Traffic Department: A Steel plant of the size of VSP has to handle

around 60-65 MT traffic comprising of incoming traffic, outgoing traffic and in process traffic. 4. Engineering shop and Foundry (ESAF): Engineering shop are set up to

meet the requirements of ferrous & non ferrous spare of different departments. 5. Quality Assurance Technology Development: The department has

been set up to take case of activities pertaining to quality control of Raw material, semi finished product and finished products. 6. Calcining & Refractory Material Plant: CRMP consists of two units-

Calcining plant & Brick plant. In Calcining plant limestone & dolomite are calcined for producing lime calcined dolomite which are used for refining of steel in the converters. The brick plant has two LAE is 1600 Tonne presses & a tampering kiln (upto 3000C temperature) for making bricks. 7. Roll shop & Repair shop (RS &RS): Roll shop and Repair shop is in

the complex of Rolling mills catering to the needs of mill in respect of roll assemblies guides few maintenance spares and roll pass design.

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8. Field Machinery Department (FMD): Field Machinery is meant for

meeting the requirements of earth moving equipment, mechanical handling equipments, and vehicles of works departments of VSP. 9.

Power Engineering Maintenance: Power engineering maintenance department is doing capital repairs, breakdown maintenance preventive maintenance of Rotary equipments like turbo-generators, turbo-blowers, turbo compressor high capacity exhauster’s fans pumps and hydraulic coupling.

10. Instrumentation Department: Instrumentation helps us in motoring

and controlling the process so that product quality is improved, yield is maximized, energy consumption is optimal and safety of the plant is ensured. 11. Electrical Repair Shop (ERS): Electrical Repair shop is provided for

medium & capital repair of different types of LT & HT AC motors, DC motors, lifting magnets, transformers coils etc. 12. Electro Technical Laboratory (ETL): In view of the high degree of

sophistication and automation used in VSP a specialized group for supporting electronics in drives and PLCs necessitated. ETL precisely does the above function. 8

13. Central maintenance Mechanical: CMM department is one of the

major service departments in VSP which is carrying out major mechanical maintenance and conveyer belt replacement activities throughout the plant. 14. Central Maintenance Electrical (CME): CME department is also one

of the major service departments in VSP which is carrying out major electrical maintenance activities throughout the plant.

Brief description of Visakhapatnam Steel Plant

Department

Unh/facility 1.5 MT stage

Additional upto 3.0 MT stage

Annual capacity in thousand tones Under 1.5 MT

Under 3.0 MT

Coke ovens

Two batteries of 67 ovens each with useful coke chamber volume of 41.6 cum and height of 7m

One battery of 1130 67 ovens with useful coke chamber volume of 41.8 cum and height of 7m

2261

Sinter plant

One sinter machine of 312 sq.m grate area.

One sinter 2628 machine of 312 sq.m grate area

5256

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Blast furnace

One furnace of One furnace of 3200 cum useful 3200 cum volume useful volume

Steel Two LD melting shop converters each of 133 cum volume 3 four strand continuous casting machines Rolling mills

7 stand breakdown mill

Light & medium merchant mill

8 stand roughing mill

1700

3400

One LD converter of 133 cum volume

1500

3000

3 four strand continuous casting machines

1410

2820

1367

1857

710

710

5 stand intermediate mill (2-stand rolling) 4 stand finishing mill (single strand rolling)

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3-Φ INDUCTION MOTOR

SIGNIFICANCE OF INDUCTION MOTORS: The most common type of a.c. motor being used throughout the world today is the induction motor.

Induction motors

rugged, require less maintenance, and are less expensive than D.C. motors of equal kilo-watt and speed ratings. Induction motors are manufactured both for 1-phase and 3-phase operation. Three-phase induction motors are widely used for industrial applications such as in lifts, cranes, pumps, line shafts, exhaust fans, lathes etc., where as 1-phase induction motors are mainly for domestic electrical appliances such as fans, refrigerators, grinders, washing machines etc.

CONUSTRUCTIONAL FEATURES: An induction motor is a rotating machine which converts the electrical energy into mechanical energy. It is most commonly used due to the following advantages. (a)

Simple, rugged and unbreakable construction. 11

(b)

Its cost is low and is reliable.

(c)

It has sufficiently high efficiency.

(d)

It requires less maintenance.

(e)

It is self starting.

Of course the induction motor also has few disadvantages like its speed cannot be varied without scarifying some efficiency, Speed decreases with increase in load and the starting torque is inferior to a d.c.shunt motor.

All induction motors essentially consists of the two main parts: (i)Stator and (ii)Rotor STATOR: Constructionally the stator of an induction motor is, the same as that of a synchronous Motor or generator. It is an outer, stationary, hollow cylindrical structure made of laminations of sheet steel having slots on the inner periphery. The insulated conductors are placed in the stator slots and are suitably connected to form a balanced 3-phase star or delta connected circuit or all the six terminals of the 3-phase winding are brought out to

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terminal box so that operator can connect the motor as per the requirement the 3-phase stator winding is wound for a definite number of poles as per the requirement of speed . Greater the number of poles lesser the speed and vice-versa according to the equation Ns=120f/P. when 3-phase supply is given to the stator winding, a rotating magnetic field of constant magnitude is produced this rotating magnetic field flux induces an e.m.f. in the rotor by mutual induction principle.

The sturdy construction and the ample provision for air circulation and cooling is especially important in induction motor operation because, the temperature rise in the winding is a very definite limiting factor of motor output.

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ROTOR: It is the rotating part of the motor. There are two general types of construction for rotor of an induction motor: -

Squirrel-cage rotor and

-

Phase wound or wound rotor.

SQUIRREL-CAGE ROTOR:

Almost 90 percent of induction motors are squirrel-cage type, because this type of rotor has the simplest and most rugged construction and is almost indestructible. The rotor-core is a laminated steel cylinder having slots on the outer periphery.

A common practice in constructing the

squirrel-cage is to place the assembled core in a mould and then force the molten conducting material aluminum or copper into the slots. The rotor conductors need not be insulated from the core, since the current flow through the least resistance path i.e. conductors. The rotor bars are shortcircuited at both ends by end rings. The rotor slots are not made parallel to the rotor shaft axis, they are skewed at a certain angle to reduce magnetic 14

noise during working, to produce a more uniform torque, and to prevent possible magnetic locking also called as cogging of the rotor with the stator.

In some cases the heavy conductor bars (not wire) are driven into the slots with a tight fit and project a short distance from each end of the core. Enb rings, with holes lining up with projecting conductors, are then forced over the latter, after which conductors and end rings are soldered or welded together. The fig 1.3 shows the construction of the squirrel rotor.

PHASE WOUND OR WOUND ROTOR: It consists of a laminated cylinder having slots on the outer periphery and is provided with 3-phase distributed winding insulated to the rotor slots similar to stator winding. The rotor is wound for as many poles as the number of stator poles and is always wound for 3-phase, even the stator is wound for two-phase. The three-phase are starred internally, the other three winding terminals are brought out and connected to three insulated slip-rings mounted on the same shaft with brushes resting on them. These brushes are connected to a 3 –phase star connected rheostat as shown in fig 1.4. This arrangement makes it possible to add external resistance to each phase of the rotor circuit during the starting period for increasing 15

starting torque.

Under normal running conditions, the slip-rings are

automatically short-circuited by means of a metal collar which is pushed along the shaft and connects all the slop tings together. Then the rotor is short-circuited on itself like squirrel-cage rotor. automatically to reduce friction, wear and tear. construction of wound rotor.

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The brushes are lifted Fig. 1.2 shows the

WORKING PRINCIPLE: The two essential parts of a 3-phase induction motor, as mentioned earlier are stationary part knows as stator and rotating part as known rotor. When 3-phase stator winding is fed from 3-phase supply, a rotating magnetic field of constant magnitude and rotating at synchronous Ns=129f/p speed is produced. This rotating flux passes through the air gap and cuts the stationary rotor conductors. Due to the relative speed between the rotating flux and stationary rotor conductors, an e.m.f. is induced according to Faraday’s Laws of Electromagnetic Induction. The frequency of the induced e.m.f. is same as the supply and proportional to the relative speed between the flux and the rotor conductors and its direction is given by Fleming’s Right-hand rule. Since the rotor conductors form a closed circuit and has no external path to the induced current, whose direction as given by Lenz’s Law, such as to oppose the very cause producing it. In this case the cause which producing it is the relative speed. Hence, to reduce the relative speed the rotor starts rotating in the same direction as that of stator flux and tries to catch it, but it never do so. (Suppose if the rotor catches the stator field or rotates at synchronous speed, the relative speed increases and again 17

the rotor picks up the speed. Likewise the rotor tries to catch the synchronous speed always but it never does so).

The working principle or the torque developed in the rotor can also be explained as below: Let us assume that the stator field is rotating in clockwise direction as shown in Fig. 1.5(a). Consider the instant when the rotor is stationary; the relative motion of the rotor with respect to the stator is anticlockwise. By applying Fleming’s Right-hand rule, the direction of the induced e.m.f. in the rotor is found to be towards the observer or outwards. Hence the direction of the rotor flux is anti-clockwise as shown in Fig. 1.5(b). Now, by combining the two fields, the flux strengthens on left and weakens on right of the rotor conductors. The property of magnetic lines is to travel in straight line. Due to this property the flux try to travel in straight line pushing the rotor conductors towards right i.e. clockwise. OR by applying Fleming’s left-hand rule, the rotor conductors experience a force . Its magnitude is clock wise direction. Hence the rotor is set into rotation in the same direction as that of the stator rotating flux as shown in fig. 1.5(c)

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From the above discussion it is clear that an induction motor is a selfstarting motor. The rotor of an induction motor never rotate at synchronous speed, hence, it is also referred to as Asynchronous motor.

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CHARACTERISTICS: TORQUE/SPEED CHARACTERISTICS:

The torque developed by a conventional 3-phase motor depends on its speed but the relation between the two cannot be represented by a simple equation. It is easier to show the relationship in the form of a curve (fig. 1.9(a)). In this diagram, T represents the nominal full-load torque of the motor. As seen, the starting torque (at N=0) is 1.5T and the maximum torque (also called breakdown torque) is 2.5T.

At full-load, the motor runs at a speed of N. when mechanical load increases, motor speed decreases till the motor torque again becomes equal to the load torque. As long as the two torques are in balance, the motor will run at constant (but lower) speed. However, if the load torque exceeds 2.5 t, the motor will suddenly stop. TORQUE/SLIP CHARACTERISTICS:

1. Motoring mode: 0≤S≤1 For this range of slip, the load resistance in the circuit model of fig 1.9(b) is positive, i.e. mechanical power is outputted or torque developed is in the direction in which the rotor rotates. Also: (a) Torque is zero at s=0.

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(b) The torque has a maximum value, called the breakdown

torque, (TBD) at slip max, t. The motor would decelerate to a halt is loaded with more than the breakdown torque. (c) At s1, i.e. when the rotor is stationary, the torque corresponds to the starting torque, Ts. In normally designed motor Ts is much less than TBD. (d) The normal operating point is located well below TBD. The full-load slip is usually 2.8%. (e) The torque-slip characteristic from no-load to somewhat beyond full-load is almost linear. 2. Generating model: s<0 Negative slip implies rotor running at super-synchronous speed (n>ns). The load resistance is negative in the circuit model of fig 1.9(b) which means that mechanical power must be put in while electrical power is put out at the machine terminals.

3. Breaking mode: s>1 The motor runs in opposite direction to the rotating field (i.e. n is negative), absorbing mechanical power (breaking action) which is dissipated as heat in the rotor copper. 22

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INTRODUCTION TO AC DRIVES The use of adjustable speed in industrial Equipment is increasing due to the need for better equipment Control and for energy saving where partial power is required drive systems are widely used in industries for all application such as pumps, fans, paper and textile mills, steel and cement mills etc. The electrical machine, that converts electrical energy into mechanical energy, and vice-versa, is the workhorse in a drive system.

Following points put forth the need for an ac drive

 Machine or process requirements - Occasionally a machine or

a process

will require other than base speed operation.

Energy savings - This is by far the greatest single application adjustable

speed

drives.

In variable

of

torque applications that are

frequently required in HVAC industry, a tremendous cost saving can be

developed

by using adjustable speed drives. If a fan could be

slowed by as little as 20% of its 24

base speed, an energy savings of 50% can be developed.

 Automated Factory Concept - Adjustable speed drives allow

industries to communicate

information

from one point to

another and to react to the information communicated.  Productivity increase - The adjustable speed drives utilize

resources more efficiently increasing the productivity. The evolution of ac variable speed drive technology has been partly driven by the desire to emulate the performance of a dc drive such as fast

torque

response and

speed

accuracy,

while utilizing

advantages offered by the standard ac motor,

AC DRIVES FEATURES:

 No commutator / brushes

 Ac motors are more available than DC  Power factor is constant across speed range  Low rotating inertia per frame size  AC does not require reversing contactor for reversing

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the

 AC motors offer more flexible motor enclosure  Individual isolation transformer not required

VARIOUS DRIVE CONCEPTS: Variable voltage and constant frequency

Variable voltage and constant frequency drives (Stator voltage control): This control is used for motor starting and helps in limiting the inrush of current during starting. These are also called Soft starters.

Variable frequency and constant voltage drives:

As the frequency is increases the air gap flux and rotor current decreases and correspondingly, the developed torque also decreases. Similarly the frequency is decreases the air gap flux tends to saturate and causes excessive stator current, the machine behaves like a dc series motor.

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Variable Voltage and Variable Frequency drives:

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VARIABLE FREQUENCY DRIVE CONCEPT OPERATING PRINCIPLE : The principle of speed control for adjustable frequency drives is based on the following fundamental formula for a standard AC motor: Ns = 120 F / P Where

Ns = synchronous speed ( rpm ) F = frequency ( cps )

A small variable frequency drive(VFD) is shown in figure

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P = no . of poles. The number of poles of a particular motor is set in its design and manufacture. The adjustable frequency system controls the frequency( F ) applied to the motor. The speed ( Ns ) of the motor is then proportional to this applied frequency. Control frequency is adjusted by means of a potentiometer or external signal depending on the application. The control can automatically maintain the required volts/cycle( V / Hz ) ratio to the motor at any speed. This provides maximum motor capability throughout the speed range. The frequency output of the control is infinitely adjustable over the speed range and therefore the speed of the motor is infinitely adjustable. BLOCK DIAGRAM: The main parts of a variable frequency drive are 1) A Rectifier 2) Filter 3) Inverter All the three i.e. the rectifier, filter and the inverter are connected in cascade. Three phase a.c. supply is given to the rectifier this rectifier converts the applied three phase a.c. voltage to d.c. voltage .This d.c.output voltage is given as input to the filter, this filter filters the waveform and gives the pure d.c. voltage .this d.c.voltage is given as input to the inverter .this inverter 29

converts the applied d.c.voltage into a.c. voltage with variable frequency output, which is given as input to the induction motor.

While operating v/f drive the ratio of voltage to the frequency is maintained constant. In an A C motor, torque is given by

Where: E/F proportional to motor flux I is current drawn by the motor V/F ratio should be kept constant to maintain air gap flux constant. 30

The air gap flux of the machine is kept constant to get higher starting torque. Here machine behaves like a D.C shunt motor.

Characteristics of D.C shunt motor torque for constant flux producing current:

Τ=ΚΦΙa*sin (d)

In order that the magnetic flux is kept constant for any frequency the applied voltage to the induction motor must be adjusted in proportion with

DC

frequency i.e. the ratio of applied voltage over frequency should be constant. The inverter frees the I.M. from its inherent limitation of single speed.

If

IaIa 31

Z

Z

Voltage has to be boosted before it is given to the v/f drive. In low frequency region the air gap flux is reduced by the stator impedance drop. In this region the stator impedance drop must be compensated by an additional boost so as to restore the torque

VFD SYSTEM DESCRIPTION:

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VFD system A variable frequency drive system generally consists of an AC motor, a controller and an operator interface. VFD motor : The motor used in a VFD system is usually a three-phase induction motor. Some types of single-phase motors can be used, but three-phase motors are usually preferred. Various types of synchronous motors offer advantages in some situations, but induction motors are suitable for most purposes and are generally the most economical choice. Motors that are designed for fixedspeed mains voltage operation are often used, but certain enhancements to the standard motor designs offer higher reliability and better VFD performance. VFD controller: Variable frequency drive controllers are solid state electronic power conversion devices. The usual design first converts AC input power to DC 33

intermediate power using a rectifier bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit. The rectifier is usually a three-phase diode bridge, but controlled rectifier circuits are also used. Since incoming power is converted to DC, many units will accept single-phase as well as three-phase input power (acting as a phase converter as well as a speed controller); however the unit must be rerated when using single phase input as only part of the rectifier bridge is carrying the connected load.

PWM VFD Diagram

AC motor characteristics require the applied voltage to be proportionally adjusted whenever the frequency is changed in order to deliver the rated torque. For example, if a motor is designed to operate at 460 volts at 60 Hz, the applied voltage must be reduced to 230 volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 V/Hz in this case). For optimum 34

performance, some further voltage adjustment may be necessary, but nominally constant volts per hertz are the general rule. This ratio can be changed in order to change the torque delivered by the motor. The usual method used for adjusting the motor voltage is pulse width modulation PWM. With PWM voltage control, the inverter switches are used to divide the quasi-sinusoidal output waveform into a series of narrow voltage pulses and modulate the width of the pulses. Operation at above synchronous speed is possible, but is limited to conditions that do not require more power than nameplate rating of the motor. This is sometimes called "field weakening" and, for AC motors, is operating at less than rated volts/hertz and above synchronous speed. Example, a 100 Hp, 460V, 60Hz, 1775 rpm (4 pole) motor supplied with 460V, 75Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 rpm) = 100% power. An embedded microprocessor governs the overall operation of the VFD controller. The main microprocessor programming is in firmware that is inaccessible to the VFD user. However, some degree of configuration 35

programming and parameter adjustment is usually provided so that the user can customize the VFD controller to suit specific motor and driven equipment requirements. At 460 Volts, the maximum recommended cable distances between VFDs and motors can vary by a factor of 2.5:1. The longer cables distances are allowed at the lower Carrier Switching Frequencies of 2.5 kHz. The lower Carrier Switching Frequencies can produce audible noise at the motors. The 2.5 kHz and 5 kHz Carrier Switching Frequencies cause less motor bearing problems than caused by Carrier Switching Frequencies at 20Hz. shorter cables are recommended at the higher Carrier Switching Frequencies of 20 kHz. The minimum Carrier Switching Frequencies for synchronize tracking of multiple conveyors is 8 kHz. VFD operator interface: The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed. Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of 36

the drive. An operator interface keypad and display unit is often provided on the front of the VFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most are also provided with input and output (I/O) terminals for connecting pushbuttons, switches and other operator interface devices or control signals. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored and controlled using a computer. VFD Operation : When a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. When a VFD starts, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while drawing only 50% of its rated current. When a motor is simply switched on at full voltage, it initially draws at least 300% of its rated current while producing less than 50% of its 37

rated torque. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed while drawing only 150% current. With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit to dissipate the braking energy or return it to the power source. Available VFD power ratings; Variable frequency drives are available with voltage and current ratings to match the majority of 3-phase motors that are manufactured for operation from utility (mains) power. VFD controllers designed to operate at 110 volts to 690 volts are often classified as low voltage units. Low voltage units are typically designed for use with motors rated to deliver 0.2kW or 38

1/4 horsepower (Hp) up to at least 750kW or 1000Hp. Medium voltage VFD controllers are designed to operate at 2400/4160 volts(60Hz), 3000 volts(50Hz) or up to 10kV. In some applications a step up Transformer is placed between a low voltage drive and a medium voltage load. Medium voltage units are typically designed for use with motors rated to deliver 375kW or 500Hp and above. Medium voltage drives rated above 7kV and 5000 or 10,000Hp should probably be considered to be one-of-a-kind (oneoff) designs.

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CONCLUSION We learnt applications of variable frequency speed control drive 3- Φ induction motor in VISHAKA STEEL PLANT at Visakhapatnam. The operation of slip ring induction motor using variable frequency drive control in the plant is clearly studied. Variable frequency speed control drives are mostly preferred for operation of slip ring induction motor in industries. These drives have special features than the other drives. Those are  Power factor is constant across the speed range.  Low rating inertial per frame size.  These drives doesn’t require reversing contactor for reversing.  Variable frequency control gives large torque with reduced current for the complete range of speeds.  These drives are high efficient compare to other.  Individual isolation transformer not required.

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