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

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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

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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.

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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.

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.

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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,

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

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for heating the billets received from billet mill of LMMM to rolling temp of

12000C.

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.

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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.

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.

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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.

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.

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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.

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 Annual capacity in thousand tones

1.5 MT stage Additional upto 3.0 MT stage

Under 1.5 MT

Under 3.0 MT

Coke ovens Two batteries of 67 ovens each with useful coke

One battery of 67 ovens with useful coke

1130 2261

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chamber volume of 41.6 cum and height of 7m

chamber volume of 41.8 cum and height of 7m

Sinter plant One sinter machine of 312 sq.m grate area.

One sinter machine of 312 sq.m grate area

2628 5256

Blast furnace

One furnace of 3200 cum useful volume

One furnace of 3200 cum useful volume

1700 3400

Steel melting shop

Two LD converters each of 133 cum volume

One LD converter of 133 cum volume

1500 3000

3 four strand continuous casting machines

3 four strand continuous casting machines

1410 2820

Rolling mills

Light & medium merchant mill

7 stand break-down mill

8 stand roughing mill

5 stand intermediate mill

(2-stand rolling)

4 stand finishing mill

(single strand rolling)

1367

710

1857

710

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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.

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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.

(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:

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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

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

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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 short-

circuited 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

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

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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

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. The brushes are lifted

automatically to reduce friction, wear and tear. Fig. 1.2 shows the

construction of wound rotor.

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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

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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

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

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stationary; the relative motion of the rotor with respect to the stator is anti-

clockwise. 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)

From the above discussion it is clear that an induction motor is a self-

starting 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.

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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 of

adjustable speed drives. In variable torque applications that are

frequently required in HVAC industry, a tremendous cost saving can

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be developed by using adjustable speed drives. If a fan could be

slowed by as little as 20% of its

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 the

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

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AC does not require reversing contactor for reversing

AC motors offer more flexible motor enclosure

Individual isolation transformer not required

VARIOUS DRIVE CONCEPTS:

Variable voltage and constant frequency

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 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 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 )

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A small variable

frequency

drive(VFD) is

shown in figure

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

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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.

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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:

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

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.

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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 fixed-

speed 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

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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

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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

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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

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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

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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

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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 (one-

off) 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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