1

A

Report on

Study of New InverteR

Drive

IN PLTCM Under the Guidance of

Mr. Nirbhay Kumar Gupta

(Maintenance Expert Group)

Submitted by

ABHISHEK DAS

AKSHAY

NEHA SAMEEN

B.Tech 4th year

Electronics and Telecommunication Engineering

ORISSA ENGINEERING COLLEGE,BHUBANESHWAR

2

CERTIFICATE

This is to certify that the project report entitled “STUDY OF NEW

INVERTER DRIVES-PLTCM”, being submitted by NehaSameen,

Akshay and Abhishek Das of Orissa Engineering College,

Bhubaneswar to TATA STEEL, as a part of summer training course

of B.Tech curriculum is a bonafide record of work carried out by

them under my supervision and guidance. The sincerity and

sense of dedication shown by them during the project is

commendable.

Mr. Nirbhay Kumar Gupta

Sr. Technologist, Jamshedpur

Maintenance Expert Group

TATA STEEL, limited

3

CONTENTS

1. Acknowledgment

2. Action plan

3. Introduction to TATA STEEL

4. Steel making process

5 . Overview of PLTCM

5.1 Entry section

5.2 Pickle section

5.3 Tandem cold reduction section

5.4 Exit inspection section

6 . Tandem cold mills

7 . Drives

7.1 Dc drives

7.2 Ac drives

7.3 Comparison between ac and dc drives

8 . Speed and frequency control for drives

9 . Drives used in PLTCM

9.1 Cyclo converter

9.2 Voltage source inverter

9.2.1. Two level inverter

9.2.2. Three level inverter

9.3 . Difference between Cyclo converter and VSI

10 . CRM PLTCM mill stand DRIVE database

11 .Conclusion

4

1.ACKNOWLEDGEMENT

With a great pleasure we would like to express our deep sense of gratitude to

Mr. Ajit Kar, Chief, MEG; Mr. Arghay Deb, Head, MEG and to our guide and

training co-ordinator Mr. Nirbhay Kumar Gupta, Sr.Technologist, MEG, TATA

STEEL Limited, for their valuable instructions, guidance and illuminating

criticism throughout our project. Without their involvement and supervision we

could not have been able to complete this project.

We would like to express our sincere thanks to Mr. GAURAV, Sr. Engineer, in

PLTCM; Mr. V.G. Rao, Consultant, MEG; Mr. O.P. Gupta, MEG and so many

other countless people of TATA STEEL Limited, Jamshedpur for helping us in our

project during our entire internship.

We would also like to thanks Mrs. MitrabindaNayak, head of Training and

Placement Department, OEC, Bhubaneswar for providing us with the

opportunity of undertaking our training at TATA STEEL Limited, Jamshedpur.

Last but not the least we would like to thank all of our friends and the

employees of Maintenance Expert Group for their sincere co-operation and

help throughout our training. Thanks to everybody and to almighty for giving us

this opportunity in our lifetime.

ABHISHEK DAS (VT201301174)

AKSHAY (VT201301661)

NEHA SAMEEN (VT20130210)

B.Tech 4th

year

ORISSA ENGINEERING COLLEGE

5

2.ACTION PLAN

WEEKS WORK PLAN

WEEK 1 Study of documents on TATA STEEL and introduction of safety.

WEEK 2 Study of motor and visit of Electrical Repair shop.

WEEK 3 Study of document on drive. Visit to CRM-PLTCM.

WEEK 4 PROJECT WORK

6

3.INTRODUCTION TO TATA STEEL

Type Public

Traded as

NSE: TATA STEEL, BSE: 500470

(BSE SENSEX Constituent)

Industry Steel

Founded 1907

Founder(s) Dorabji Tata

Headquarters Mumbai, Maharashtra, India[1]

Area served Worldwide

Key people

Cyrus Pallonji Mistry

(Chairman)

Hemant M. Nerurkar

(Managing Director)

Products Steel, flat steel products, long steel

products, wire products, plates

7

Tata Iron and Steel Company was established by Dorabji Tata on

August 26, 1907, as part of his father Jamsetji'sTata Group. By 1939 it

operated the largest steel plant in the British Empire. The company

launched a major modernization and expansion program in 1951. Later,

the program was upgraded to 2 MTPA project. In 1990, it started

expansion plan and established its subsidiary Tata Inc. in New York. The

company changed its name from TISCO to TATA STEEL in 2005.

In August 2004, TATA STEEL agreed to acquire the steelmaking

operations of the Singapore-based NatSteel for S$486.4 million in cash.

The acquisition was completed in February 2005.

In 2005, TATA STEEL acquired a 40% stake in the Thailand-based

steelmaker Millennium Steel for $130 million from Siam Cement.

On 20 October 2006, TATA STEEL signed a deal with Anglo-Dutch

company, Corus. On 19 November 2006, the Brazilian steel company

CompanhiaSiderúrgicaNacional (CSN) launched a counter offer for

Corus at 475 pence per share, valuing it at £4.5 billion. On 11 December

2006, Tata preemptively upped its offer to 500 pence per share, which was

within hours trumped by CSN's offer of 515 pence per share, valuing the

deal at £4.9 billion. The Corus board promptly recommended both the

revised offers to its shareholders. On 31 January 2007 TATA STEEL won

their bid for Corus after offering 608 pence per share, valuing Corus at

£6.7 billion.

In 2007 TATA STEEL through its wholly owned Singapore subsidiary,

NatSteel Asia Pte Ltd acquired controlling stake in two rolling mills: SSE

Steel Ltd, Vinausteel Ltd located in Vietna.

The TataGroup of Companies has business operations (114 companies and

subsidiaries) in seven defined sectors – Materials, Engineering,

Information Technology and Communications, Energy, Services,

Consumer Products and Chemicals.TATA STEEL with its acquisition of

Corus has secured a place among the top ten steel manufacturers in the

world and it is the Tata Group’s flagship Company. Other Group

Companies in the different sectors are – Tata Motors, Tata Consultancy

Services(TCS),Tata Communications,Tata Power, Indian Hotels, Tata

Global Beverages and Tata Chemicals.

8

Tata Motors is India’s largest automobile company by revenue

and is among the top five commercial vehicle manufacturers

in the world. Jaguar andLandrover are now part of Tata

Motor’ s portfolio.

Tata Consultancy Services) is an integrated software

solutions provider with delivery centres in more than 18 countries.

It ranked fifth overall, and topped the list for IT services, in

Bloomberg Business week's 12th annual 'Tech 100', a ranking of

the world's best performing tech companies.

Tata Power has pioneered hydro-power generation in India and is the

largest power generator(production capacity of 2300 MW) in India in the

private sector.

Indian Hotels Company(Taj Hotels, resorts and palaces)

happens to be the leading chain of hotels in India and one of the

largest hospitality groups in Asia. It has a presence in 12 countries in 5

continents.

Tata Global Beverages (formerly Tata tea) its major

acquisitions like Tetley and Good. Earth is at present the second

largest global branded tea operation.

TATA STEEL is headquartered in Mumbai, Maharashtra, India and

has its marketing headquarters at the Tata Centre in Kolkata, West

Bengal It has a presence in around 50 countries with manufacturing ope

rations in 26 countries including: India, Malaysia, Vietnam Thailand,

Dubai, Daggaron, Ivory Coast, Mozambique, South Africa, Australia,

United Kingdom The Netherlands, France and Canada.

TATA STEEL primarily serves customers in the automotive, construction,

consumer goods engineering, packaging, lifting and excavating, energy

and power, aerospace, shipbuilding, rail and defense and security sectors

TATA STEEL has set a target of achieving an annual production capacity

of 100 million tons by 2015; ; it is planning for capacity expansion to be

balanced roughly 50:50 between greenfield developments and

acquisitions. Overseas acquisitions have already added an additional 21.4

million tons of capacity, including Corus (18.2 million tons), NatSteel (2

million tons) and

9

Millennium Steel (1.2 million tons). Tata plans to add another 29 million

tons of capacity through acquisitions.

Major greenfield steel plant expansion projects planned by TATA STEEL

include:-

A 6 million ton per annum capacity plant in Kalinganagar,

Odisha, India

An expansion of the capacity of its plant in Jharkhand, India

from 6.8 to 10 million tons per annum.

A 5 million ton per annum capacity plant in Chhattisgarh, India

(TATA STEEL signed a memorandum of understanding with

the Chhattisgarh government in 2005; the plant is facing strong

protest from tribal people).

A 3 million ton per annum capacity plant in Iran;

A 2.4 million ton per annum capacity plant in Bangladesh;

A 10.5 million ton per annum capacity plant in Vietnam

(feasibility studies are underway);

A 6 million ton per annum capacity plant in Haveri, Karnataka

10

4.STEEL MAKING PROCESS

Iron ore fines are not suitable

for use in the Blast Furnace.

Hence, the iron ore fines

areagglomerated into larger

porous lumps, which is

suitable for use in the Blast

Furnace. A greenmix of

carefully proportioned iron

ore fines, fluxes and coke

breeze is prepared in granular

formin Mixers. Heat

generated through combustion

within the mass itself produces

large lumps ofhot Sinter. This

Sinter is cooled, sized and

stored for use in the blast

furnace.Iron ore fines are

recycled to make sinter, to

help produce hot metal of

predictable andstandard

quality in the Blast Furnace.

Naturally found coal contains

Fixed Carbon (FC), Volatile

matter (VM), Ash, Moisture

and otherimpurities. Its poor

crushing strength and the volatile matter content makes it unsuitable foruse in

Blast Furnace. Hence, naturally found coal is converted into coke in the coke

oven for usein Blast Furnaces.Heating coal in the absence of air carbonizing it to

form a hard porous mass, devoid of volatilematter produces coke. Coal after

carbonization, which gives blast furnace quality coke, is called metallurgical

coal. Coal is graded as prime, medium and bendable, based on its

cokingproperties. Blending of different grades of coal is necessary in order to

conserve metallurgicalcoal, yet ensure uniform coking properties. Currently,

there are 6 batteries of coke ovensupplying coke to the blast furnaces.

The coke plant blends coal from different sources, converts coal to coke and cuts

to the correctsize for use in the blast furnace.

11

The Blast Furnace is a ceramic refractory lined tall reactor, used for the

production of liquidiron called Hot Metal. Iron oxide, present in the iron bearing

raw materials, is reduced insidethe reactor by coke and carbon monoxide.

Coke is used for combustion to attain the high temperatures required for

reduction. Coke oncombustion generates carbon monoxide, which acts as the

reducing agent and converts theiron oxides into molten iron. Fluxes are used to

make low melting slag and control the quality ofHot Metal.

Hot Metal and Slag are collected in the hearth and tapped periodically. Blast

Furnaces A, B, C, D,E and F together produce 2.8 million tons of hot metal,

annually. G-Blast Furnace produces 1.30million tons of hot metal annually. Blast

Furnace F has been rebuilt in 2002 to enlarge itscapacity to 1 million tons.Blast

Furnaces are used for producing Hot Metal.

Hot Liquid Iron (commonly called Hot Metal in TATA STEEL) is converted to

Steel in the SteelMelting Shops. Hot Metal from the Blast Furnace is stored in

Mixers in LD#1 shop. The HotMetal is converted to Steel in the LD converters

by removing its carbon, silicon, sulphur andphosphorous contents.

The liquid steel from the converter is converted to billets using Continuous

Casting machine. Asmall portion of steel is teemed into ingots through Bogie

Bottom Poring process using cast ironmoulds. The liquid steel is treated in On

Line purging, Ladle Refining Furnace or Argon Rinsingstation before continuous

casting. Special grades of steel, which are cast as ingots, areprocessed in On-line

purging, followed by the Vacuum Arc degassing & refining unit.The Steel

Melting Shop requires an Oxygen Plant to cater to the requirement of oxygen for

steelmaking. The Lime Calcining Plant and the Tar Dolo Plant are auxiliary units

required for themanufacture of Steel.

Hot metal is converted to Steel and cast into Billets.

The LD#2 Shop has three Converters of 140 tons capacity each, producing 2.6

million tons ofcrude steel per annum. Hot Metal is brought from A, D, E, F and G

Blast Furnaces in Torpedoladles. The metal from the Torpedo ladle is taken into

the Hot Metal for Desulphurization. It isthen charged into the vessel.Primary

refining of steel is done in the Ladle Furnace (LF) and RH Degasser (RH) to make

cleanersteel of different value added grades.LD 2 makes superior & cleaner

grades of steel required to process Flat products of world-classstandards.

12

5.Overview of PLTCM

Cold rolling mills are used for deforming various metals by passing

them through rollers at a temperature below its re-crystallization

temperature. This process of rolling increases the yield strength and

hardness of a metal by introducing defects into the metal's crystal

structure. These defects prevent further slip and also reduces the

grain size of the metal.Cold Rolling mills are used to obtain desired

level of thickness reduction.

Basically Pickle Line Tandem Cold Mill consists of four sections:

1.Entry Section

2.Pickle(cleaning) Section

3.Tandem Cold Reduction Section

4.Exit Section

5.1Entry Section:

Its basic operation is divided into two parts:

1.CALL TRANSFER:-This system

optimizes logistics and transport

from the hot strip mill,reducing the

potential for damage to the bands that

can occur prior totheir arrival at the

PLTCM. Additionally, a horizontal

storage positionallows for better heat

transfer for faster processing time.

2.COIL TRACKING SYSTEM:-A sophisticated tracking system uses ―smart‖ cranes to

track themovement of the steel between the hot strip mill and the PLTCM.Automatic band

identification is made based on weight, width and barcodeto guarantee that the correct

coils are delivered to the customer.

There are four parts in this section:-

Automated Coil Preparation Station:- The automated strap

removal and crop shearprocess not only assures operator

safety, butoptimizes product yield by cutting off only

theportions determined by the band quality data.

SMS Siemag Coil Handling:-Built by

SMS Siemag, this system uses

automatedmotion to move and lift the product,reducing the risk

of product mixing and damage.

13

Miebach HSL 19 Laser Welder:- In order to roll AHSS

grades, this laser weldercan join high-alloy coils with absolutely

minimalstrip breaks. The result is superior weld qualityand

increased reliability and on-time delivery.

Tension Leveler in Pickle Section:-The

tension leveler removes scale to provideuniform surface quality

regardless of producttype and flattens the strip.

5.2Pickle(Cleaning) Section:-The PLTCM uses a four-tank,

cascading type hydrochloric acid pickling section with surface agitation to

remove iron fines. This, coupled with afour-stage rinse section utilizing a

demineralized water final rinse spray before drying, ensures a perfectly clean and

dry strip.

It basically has two parts:-

1. Mitsubishi-Hitachi Immersion BoxTurbulent

Pickling:-This system uses strip motion to achieveoptimum

fluid turbulence and minimized pickling time. No moving

parts assure effective scale removal and guarantee superior

reliability.

2. Automated Turret Side Trimmer:-This fully

automated system provides precisewidths and ideal trim

quality. Optional

automatic set-up for gap and overlap leadsto improved edge

trim quality while allowingfor optimized product yield.

14

5.3 Tandem Cold Reduction Section:-

It basically consists of two parts:-

1. Independent Main Mill Motots:-Five independent

motors, inconjunction with two tension reelmotors, provide

the capability to processa full range of current and future

productrequirements, and allow maximum

coldreductioncapability.

2. Automated Roll change equipment:- All work rolls

and intermediate rolls can bechanged without stopping the

strip in the pickleline, optimizing surface quality. Rolls

aredelivered directly into the roll shop to bereconditioned

for further use.

Apart from that it has Five stand, six high universal crown mill.

By incorporating smaller diameter work rolls, the millcan realize heavy reduction

rolling and provide a widerrange of product thickness, width and hardness.

Thisallows us to produce products from ultra low carbonto advanced high

strength steel. Since the mill utilizesa straight roll at the number five stand, we are

able toprovide significant improvement in shape control.The five-stand, six-high

universal crown mill is theperfect partner to the pickle line. Together, they

areredefining the shape of today’s steel solutions.The Universal Crown Mill’s

cylindricalrolls create a more consistent relationship between strip speed and roll

speed, improving texture consistency across the width.

5.4 Exit Inspection Section:-

This section has three parts:-

15

1. Carousel Tension Reel:-The twin-exit reel system

insures a constantthread position and single mode of

operationfor greater consistency and

uninterruptedproduction from coil to coil.

2. Strip-Surface Inspection Station:-This station

provides an optimal view of thestrip-surface quality

and enables comprehensivesurface inspection under

tension withoutdisrupting processing.

3. SMS Siemag Exit Coil Handling System:-Utilizing

a system very similar to the entrycoil handling system, this

automated exit coilhandling system allows for the

automaticdelivery of finished product. After weighingand

strapping to safely restrain the coil, thesystem

automatically marks, and applies barcode tags to the

finished coils.

6.Tandem Cold Mills

The main advantages of Tandem Cold Mills are:-

16

Reduced material loss can be attributed to thefollowing:

Thick ends eliminated by continuous operation

Coil handling damage reduced due to less handling

Minimized off gauge lengths as a result of constantprocess conditions.

Yield increased by 1.5–2.0%

Improved product quality is attained due to thefollowing criteria:

Constant process conditions result in higher quality and

more consistent products

Reduced strip-edge damage due to less coil handling

Higher proportion of coil with stable operatingconditions

Surface finish flexibility

Flatness tolerance: below 15 I-Units (thickness- andwidth-dependent)

Gauge tolerance: standard tolerance (DIN standard)

Surface quality: roughness, cleanness improvements

Reduced operating cost is reached due to:

Continuous, compact plant requires fewer operators

40% longer roll service life because of less damage dueto reduced threading

and tailing outReduction in energy consumption of 10% by anoptimized

process due to fewer acceleration anddeceleration operations

Reduced roll oil consumption by 32–40%

Reduced maintenance costs due to constant operating

conditions and less threading and tailing-out operations

Mill Power:-Since mill speed had to be increased to meet the capacitytargets, a

power upgrade of the existing mill drives from

10,800 kW to 22,100 kW was necessary. The optimizationof the speed cone

required an adaptation of the gearratios and hence new gears.

Mill Stand:-The 5th stand uses smaller work-rolls than stand no. 1–4which is

required to obtain the aimed maximum reduction

of approximately 6% in particular for rolling of stronghardening, high strength

materials down to the minimumthickness of 0.3 mm. Since the maximum total

reductionfrom hot gauge to cold gauge can reach 84%, the materialhardening is

significantly high. The required reductionson the last stand could therefore not be

performed withconventional work-roll diameters. To compensate forthe decrease

in flexural stiffness of the work-/back-uproll assembly due to the smaller

work-roll diameters, thediameter of the back-up rolls was increased accordingly.

17

Working Principle of Tandem Cold Mill:-

Application of the UCM-MILL makes possible

to roll all products with straight rolls, which

straight rolls, which allow to get rapid start up.

The operator derives most stable rolling and

shape controllability. Thanks to horizontal

rigidity created by shifting of intermediate roll,

the user can control strip thickness deviation

without jeopardizing the shape of the strip.

The mill is equipped with high response

Hydraulic Roll Position Device

(HYROP-F).Laser Doppler type speed

measuring device and X-ray type thickness

gauge meters. By using these equipment, high

performance up-to-date Automatic Gauge

Control (AGC) system, which includes the

mass flow control, can obtain the higher standard of the finished thickness. At the

exit of the mill , a modern designed shape measuring sensor and Automatic Shape

Control (ASC) system is installed. They can guarantee the required flatness of the

finished coil and the qualified products can be produced.

Technical specification of Motors Used in Tandem Mills:-

Stand

#No

Type Power Current Voltage Speed Stator

Insulation

Rotor

Insulation

1. Synchronous 2300KW 1565A 900V 132/600RPM CLASS F CLASS H

2. Induction 5000KW 3000V 440/1200RPM

3. Synchronous 3500KW 1551A 1350V 201/600RPM CLASS F CLASS H

4. Synchronous 3500KW 1551A 1350V 201/600RPM CLASS F CLASS H

5. Synchronous 3500KW 1551A 1350V 201/600RPM CLASS F CLASS H

18

7.DRIVES Whenever the term electric motor or generator is used, we tend to think

that the speed of rotation of these machines are totally controlled only by

the applied voltage and frequency of the source current. But the speed of

rotation of an electrical machine can be controlled precisely also by

implementing the concept of drive. The main advantage of this concept

is, the motion control is easily optimized with the help of drive. In very

simple words, the systems which controls the motion of the electrical

machines, are known as electrical drives. A typical drive system is

assembled with a electric motor (may be several) and a sophisticated

control system that controls the rotation of the motor shaft. Now a days,

this control can be be done easily with the help of software. So, the

controlling becomes more and more accurate and this concept of drive

also provides the ease of use. This drive system is widely used in large

number of industrial and domestic applications like factories,

transportation systems, textile mills, fans, pumps, motors, robots etc.

Drives are employed as prime movers for diesel or petrol engines, gas or

steam turbines, hydraulic motors and electric motors.

Types of Drives:

DC Drives

AC Drives 7.1 DC DRIVE CONTROL SYSTEM:

A basic DC drive

control system

generally contains a

drive controller and

DC motor as shown

inthe figure given

below:

The controls allow

the operator to start,

stop, and change

direction and speed of the motor byturning potentiometers or other operator

devices. These controls may be an integral part of thecontroller or may be

remotely mounted.The drive controller converts a 3-phase AC voltage to an

adjustable DC voltage, which is thenapplied to a DC motor armature.

19

The DC motor converts power from the adjustable DC voltage source to rotating

mechanicalforce. Motor shaft rotation and direction are proportional to the

magnitude and polarity of theDC voltage applied to the motorThe tachometer

(feedback device) shown in Figure converts actual speed to an electrical

signalthat is summed with the desired reference signal. The output of the

summing junction providesan error signal to the controller and a speed correction

is made.

FEATURES: • Field orientation via mechanical commutator.

• Controlling variables are Armature Current and Field Current, measured

DIRECTLY from themotor.

• Torque control is direct.

OPERATION: The magnetic field is created by the current through the field winding in the

stator. This field is always at right angles to the field created by the armature

winding. This condition, known asfield orientation, is needed to generate

maximum torque. The commutator-brush assemblyensures this condition is

maintained regardless of the rotor position.Once field orientation is achieved, the

DC motor’s torque is easily controlled by varying thearmature current and by

keeping the magnetizing current constant. Torque is the inner control

loop and speed is the outer control loop

TYPES OF DC DRIVES: Armature and Field Controlled DC Drives The motor is armature voltage controlled for constant torque-variable HP

operation up to basespeed. Above base speed the motor is transferred to field

current control for constant HPreduced torque operation up to maximum speed.

20

APPLICATIONS: DC drives were used for VARIABLE SPEED CONTROL DRIVES because they could easily achieve a good torque and speed response with high accuracy. Variable Speed Control Drive: Basic function of a variable speed drive (VSD) is to control the flow of energy

from the mains tothe process. Energy is supplied to the process through the motor

shaft. Two physical quantitiesdescribe the state of the shaft: torque and speed. To

control the flow of energy we musttherefore, ultimately, control these quantities.

In practice, either one of them is controlled andwe speak of ―torque control‖ or

―speed control‖. When the VSD operates in torque controlmode, the speed is

determined by the load. Likewise, when operated in speed control, thetorque is

determined by the load.

ADVANTAGES: • Accurate and fast torque control

• High dynamic speed response

• Simple to control

DRAWBACKS: • Reduced motor reliability: the fact that brushes and commutators wear downand

needs regular servicing.

• Regular maintenance

• Motor costly to purchase

•Needs encoder for feedback.

- 21 -

7.2 AC DRIVES AC drives provide a very efficient and direct method of controlling the speed of

the most rugged and reliable of prime movers, the squirrel cage motor. AC drives

provide many economic and performance advantages in a wide variety of

adjustable speed drive applications.

The following are some of the benefits provided:

1. High efficiency and low operating cost.

2. Minimal motor maintenance.

3. Controlled linear acceleration and deceleration provide soft.

4. Starting and stopping and smooth speed changes.

5. Multiple motor operations are easily accomplished.

6. Current limit provides for quick and accurate torque control.

7. Adjustable speed operation can be accomplished with existing AC motors.

8. Improved speed regulation can be accomplished by slip compensation.

9. AC motors are available in a wide variety of mechanical configurations.

10. Flexibility of machine design due to the light weight and compact size of AC

motors.

11. IR compensation provides high starting torque easily and economically.

12. AC motors are available in enclosures suitable for hazardous or corrosive

environments.

13. Fewer spare motors are required since the same motor can be used for both

adjustablespeed and constant speed operations.

14. Cutler-Hammer rugged and reliable designs ensure minimum downtime

expense.

15. High speed operation can be economically accomplished using extended

frequencyoperation.

16. Reverse operation is accomplished electronically without the need for a

reversingstarter.

ADJUSTABLE FREQUENCY AC DRIVE SYSTEM INTRODUCTION An adjustable frequency AC drive system consists of an ordinary three-phase

induction motor,an adjustable frequency drive to control the speed of the motor and

an operator's controlstation.The most common motor used with an AF drive system is

a standard NEMA design B squirrelcage induction motor, rated for 230 or 460 volt,

3-phase, 60 Hz operation. The adjustablefrequency controller is a solid-state power

conversion unit. It receives 240 or 480 volt, 3-phase,60 Hz power and converts it to a

variable frequency supply which can be steplessly adjustedbetween 0 and 60 Hz. The

controller also adjusts the output voltage in proportion to thefrequency to provide a

nominally constant ratio of voltage to frequency as required by thecharacteristics of

the motor. The operator's station provides the operator with the necessary controls for

starting and stopping the motor and varying the motor speed. These functions canalso

be performed by a wide variety of automatic control systems. There are

severalclassifications of adjustable frequency AC drives. Some common types of

drives are VariableInput (VVI) sometimes called Six Step drives, current source input

(CSI), pulse widthmodulated (PWM) drives, Sensorless Vector drives, Field Oriented

drives and Closed LoopVector drives.

- 22 -

PRINCIPLES OF ADJUSTABLE FREQUENCY MOTOR OPERATION The operating speed of an AC induction motor can be determined by the

frequency of theapplied power and the number of poles created by the stator

windings. Synchronous speed isthe speed of the magnetic field created in the

stator windings. It is given by:

N = 120f /P

Where:

n = speed in RPM

f = operating frequency

P = number of poles

When the frequency is changed, the voltage must also be changed, based on the

formula forreactance and Ohm’s Law.

XL = 2πfL

Where L = inductance

XL = reactance

V = voltage

Im = magnetizing current

Im = V/XL

Combining the above equations yields:

Im = (V/f).(1/2πfL)

For steady-state operation, a constant volts per hertz ratio must be maintained.

This is equal tothe motor rated voltage divided by the rated frequency.

For the magnetizing current to remain constant, the V/f ratio, or the volts per hertz

ratio, mustremain constant. Therefore, the voltage must increase and decrease as

the frequency increasesand decreases.

INDUCTION MOTOR SPEED CONTROL Standard induction motors (NEMA design B) have approximately 3% slip at full

load.If the drive only controls the output frequency, the motor speed will deviate from

the set speeddue to slip. For many fan and pump applications, precise speed control is

not needed.

Vector controlled drives need speed feedback of the rotor. For Sensorless Vector, the

rotorspeed is calculated based on a model of the motor stored in the drive. For Closed

Loop Vector,a digital encoder is added to the motor to provide actual rotor speed.

- 23 -

VOLTS PER HERTZ REGULATION In order to operate the motor with the desired speed/torque curve, we must apply

the propervoltage to the motor at each frequency. As we have already seen, it is

necessary to regulatemotor voltage in proportion to the frequency at a constant

ratio. In reality, this requirement forconstant volts/hertz does not apply to the

motor terminals, but to a hypothetical point insidethe motor. The voltage at this

point is called the air gap voltage. The difference between air gapvoltage and

motor terminal voltage is the IR voltage drop.

AC DRIVE APPLICATIONS MATCHING THE AC DRIVE TO THE MOTOR PWM and Vector AC Drives are designed for use with any standard squirrel cage

motor. Sizingthe drive is a simple matter of matching the drive output voltage,

frequency and current ratingsto the motor ratings.

OUTPUT VOLTAGE AND FREQUENCY Most modern AC Drives are designed for use with various voltages and

frequencies. Byadjusting the V/Hz properly, almost any 3-phase motor can be

used.

OUTPUT CURRENT AC drive full load currents are matched to typical full load motor current ratings.

Usually an ACdrive can be matched to an AC motor by their hp ratings; however,

actual motor currentrequired under operating conditions is the determining factor.

If the motor will be run at fullload, the drive current rating must be at least as high

as the motor current rating. If the drive isto be used with multiple motors, the sum

of all the full load current ratings must be used, andadding up the hp ratings of the

motors will usually not provide an accurate estimate of thedrive needed.

MOTOR PROTECTION Motor overload protection must be provided as required by the applicable codes.

Motorprotection is not automatically provided as part of all AC drives. It may be

provided as astandard feature on one model or it may be an optional feature on

another. The best means ofmotor protection is a direct winding over temperature

protection such as an over temperatureswitch imbedded in the motor windings.

Direct over temperature protection is preferredbecause overheating can occur at

normal operating currents at low speeds. Most AC drives areequipped with

electronic overcurrent protection, such as I2t protection, similar to a

conventionoverload.

In multiple motor applications, individual motor overload protection must be

provided evenwhere electronic protection is provided by the drive. In some cases,

short circuit protectionmay be required.

MOTOR WINDING DAMAGE The voltage output of AC drives contains voltage steps. In modern PWM drives,

the dV/dt of amotor causes can cause very large voltage spikes. Voltage spikes of

- 24 -

1500 volts or more aretypical for a 460 volt motor. This can cause the end

windings of a Non-Inverter Duty or standardinduction motor to fail. This problem

gets worse as the cable length from the drive to the motorgets longer. Corrective

action is normally required for cables longer than 150 feet. Load sidereactors,

installed at the drive output terminals, will reduce the voltage spikes at the

motorterminals. Most drive manufacturers have load side reactors available as an

option.

7.3 COMPARISON BETWEEN AC DRIVES AND DC DRIVES

1. The Dc motor is complicated and requires a lot of maintenance, which makes it

expensiveto run; it also has a lower degree of protection. The AC motor, on the

other hand, is simpleand sturdy, does not need much maintenance, is therefore

less expensive, and possesses ahigher degree of protection into the bargain.

2. In contrast to the AC standard motor with fixed basic speeds (synchronous

speeds of 3000/1500/1000/... rpm at 50 Hz), the DC motor's basic speed can be

designed from approx. 300 rpmto about 4000 rpm for each working point.

3. Power limitation is caused by the breakdown torque of AC motor decreasing as

the square ofspeed (1/n2).

Power limitation is caused by the commutation of DC motor.

8. Speed and Frequency control for Drives:-

Both voltage and frequency

reference are fed into a

modulatorwhich simulates an

AC sine wave and feeds this to

the motor’sstator windings. This

technique is called Pulse

WidthModulation (PWM) and

utilises the fact that there is a

- 25 -

dioderectifier towards the mains and the intermediate DC voltageis kept constant.

The inverter controls the motor in the formof a PWM pulse train dictating both

the voltage and frequency.

Significantly, this method does not use a feedback devicewhich takes speed or

position measurements from the motor’sshaft and feeds these back into the

control loop.Such an arrangement, without a feedback device, is calledan

―open-loop drive‖.

To emulate the magnetic

operating conditions of a DC

motor,i.e. to perform the field

orientation process, the

flux-vectordrive needs to

know the spatial angular

position of the rotorflux

inside the AC induction

motor.With flux vector PWM drives, field orientation is achieved byelectronic

means rather than the mechanical commutator/brush assembly of the DC

motor.Firstly, information about the rotor status is obtained by feedingback rotor

speed and angular position relative to the statorfield by means of a pulse encoder.

A drive that uses speedencoders is referred to as a ―closed-loop drive‖.

Also the motor’s electrical characteristics are mathematicallymodelled with

microprocessors used to process the data.The electronic controller of a

flux-vector drive creates electricalquantities such as voltage, current and

frequency, which arethe controlling variables, and feeds these through a

modulatorto the AC induction motor. Torque, therefore, is controlled indirectly.

9. Drives used in PLTCM

In PLTCM basically two types of drives are used,they are cycloconverter and

Hitachi VSI drives.

9.1 CYCLO CONVERTER:-Cycloconverter isnothing but basically a

Variable Speed drive.This type of VSD makes a direct conversion from

constant frequency, constant voltage to variable frequency,

variablevoltage in one stage, without resorting to an intermediate DC

linkwith energy storage. By supplying each phase of the motor winding

from a reversibleconverter, a low frequency AC drive system is formed,

as shown in Figure 1.16. Although thisVSD is complex, cycloconveters

use a large number of thyristor switches but they do notrequire forced

commutation circuits and thus can use relatively inexpensive,

converter-gradethyristors. The generated structure of cycloconverters

presented in Figure 1.16a, shows a largenumber of power switches and

the need for a special three-phase secondary transformer.

- 26 -

A cycloconverter is a device that converts alternating current, or AC, power

at one frequency into AC power of an adjustable but lower frequency without

any direct current, or DC, stage in between. It can also be considered as a

static frequency changer and typically contains silicon-controlled rectifiers.

The device consists of an array containing back-to-back, parallel, connected

switches, which are used to fabricate the desired output AC waveforms. It's

possible to control the frequency of these output AC waveforms by opening

and closing the switches in a controlled fashion.

This converter converts single-phase or three-phase AC power to

single-phase or three-phase power having a variable frequency and

magnitude. Typically, the output frequency of the AC power is lower than

the input frequency. A cycloconverter has the capacity to operate with loads

of variable power factors and also allows bidirectional power flow. They can

be broadly classified into two types — phase-controlled cycloconverters and

envelope cycloconverters. In the former, control of the firing angle is

accomplished through adjustable gate impulses, while in the latter, the

switches remain in an on state and conduct in consecutive half cycles.

They are mostly used to control the speed of drives and for converting

variable input frequency power into constant frequency output, such as in

very high-power applications, including driving synchronous motors and

induction motors. Some of the places where cycloconverters are employed

include cement mill drives, mine winders, and ore grinding mills. They are

also utilized in ship propulsion drives, scherbius drives, and rolling mill

drives.

Offering many advantages, a cycloconverter can be used in quite a few

low-speed applications and is also a compact system. Its ability to directly

affect the frequency conversion of power without any intermediate stage

involving DC power is another huge advantage. If the cycloconverter

experiences a commutation failure, the results are minimal, such as the

blowing off of individual fuses.

It also has the capacity of regeneration, covering the total range of speeds.

Another huge advantage of the cycloconverter is its ability to deliver a

sinusoidal waveform at a lower output frequency. This advantage comes

from its ability synthesize the output waveform using a large number of

segments of the input waveform.

This technology does have some disadvantages, though. Firstly, the

frequency of the output power is around one third or less of the input

frequency. It's possible to improve the quality of

the output waveform if a larger number

of switching devices are employed. A

cycloconverter requires quite a complex control

mechanism and also uses a large quantity of

thyristors. Its use is also limited by severe

harmonics and the low-output frequency range.

- 27 -

Cycloconverters are used for high power machines (above 1MW) with low

frequency operation(e.g. rolling mills, cement kilns). The output frequency is

typically below 25Hz since thequality of the voltage waveforms degrades as the

output frequency increases. They can operatedown to zero speed and they can be

used both with induction and synchronous motors. Themain disadvantages are

the complex circuit design and the low power factor at low speed.

9.2 VSI DRIVES:-With fast switching semiconductor devices like the insulated

gate bipolar transistors (IGBT) it is possible to build inverter based high voltage

power supplies for electrostatic precipitators. Comparing conventional SCR

(Silicon controlled rectifier) based technology the average corona power can be

increased significantly to improve the precipitator efficiency. Additionally,

during flashovers the fast current control of IGBT power inverters improves the

precipitator performance due to fast voltage recovery resulting in further

increasing of the peak and average precipitator voltage. In a new approach, the

advantages of higher distances up to 400 mm between the discharge and

collecting electrodes could be addressed by a voltage up to 150 kV applied to the

precipitator. Due to the exact voltage control of the IGBT inverter a smooth DC

voltage can be generated and therefore, the overvoltage capability of the system is

much lower than it would have to be with a conventional thyristor based high

voltage generation system. Thus, the IGBT inverter solution becomes more

economical or less expensive to operate than the conventional supply. With the

availability of the latest generation of integrated IGBT modules a very compact

IGBT inverter has been developed to meet the design requirements by operating

at a frequency up to 10 kHz. The new IGBT types have lower saturation voltages

than the previous modules resulting in lower power losses. The HV-transformer

has been designed with the required rating and stray inductance.

The three-phase voltage source inverter (VSI) is used to control AC-motors in the

lower andmedium power ranges, from small high dynamic performance servo

drives with speed andposition control capability (<10kW) to most auxiliary

drives in industry, ranging up to several hundred kW. The VSI is suitable for

supplying induction, as well as synchronous motors. Figure 1.10 shows a

simplified diagram of the basic three-phase voltage source inverter. Theinput

rectifier serves to produce a DC supply, and the relatively large electrolytic

capacitor isinserted to filter ("stiffen") the DC voltage which feeds the inverter.

Typically, the capacitor of2 to 20 milifarads, is a mayor cost item in the system.

Additionally, it is usual to insert areactance between the rectifier and the AC

supply to limit the fault current and to reduce theharmonic distortion produced by

the rectifier. The inverter module converts the DC voltage to avariable frequency,

variable voltage output.

The variable voltage inverter (VVI) uses an SCR converterbridge to convert the

incoming AC voltage into DC. The SCRsprovide a means of controlling the value

of the rectified DC voltage from 0 to approximately 600 VDC. The L1 choke

And C1 capacitor(s) make up the DC link section and smooththe converted DC

voltage. The inverter section consists ofsix switching devices. Various devices

- 28 -

can be used such as thyristors, bipolar transistors, MOSFETS, and IGBTs.

Thefollowing schematic shows an inverter that utilizes bipolartransistors. Control

logic (not shown) uses a microprocessorto switch the transistors on and off

providing a variable voltageand frequency to the motor.

This type of switching is often referred

to as six-step becauseit takes six 60°

steps to complete one 360° cycle.

Although themotor prefers a smooth

sine wave, a six-step output can

besatisfactorily used. The main

disadvantage is torque pulsationwhich

occurs each time a switching device,

such as a bipolartransistor, is switched.

The pulsations can be noticeable at

lowspeeds as speed variations in the motor. These speed variationsare sometimes

referred to as cogging. The non-sinusoidalcurrent waveform causes extra heating

in the motor requiring amotor derating.

Classification of Voltage Source Inverter:-

Voltage source inverters can be

classified according to different

criterions. They can be classified

according to number of phases

they output. Accordingly there

are single-phase or three-phase

inverters depending on whether

they output single or three-phase voltages. It is also possible to have inverters

with two or five or any other number of output phases. Inverters can also be

classified according to their ability in controlling the magnitude of output

parameters like, frequency, voltage, harmonic content etc. Some inverters can

output only fixed magnitude (though variable frequency) voltages whereas some

others are capable of both variable voltage, variable frequency (VVVF) output.

Output of some voltage source inverters is corrupted by significant amount of

many low order harmonics like 3rd

, 5th

, 7th

, 11th

, 13th

order of the desired

(fundamental) frequency voltage. Some other inverters may be free from low

order harmonics but may still be corrupted by

some high order harmonics. Inverters used for ac motor drive applications are

expected to have less of low order harmonics in the output voltage waveform,

even if it is at the cost of increased high order harmonics. Higher order harmonic

- 29 -

voltage distortions are, in most ac motor loads, filtered away by the inductive

nature of the load itself.

Inverters may also be classified according to their topologies. Some inverter

topologies are suitable for low and medium voltage ratings whereas some others

are more suitable for higher voltage applications. The inverters shown in Figs.

33.3(c), 33.4(a) and 33.4(b) are two level inverters as the pole voltages may

acquire either positive dc bus or negative dc bus potential. For higher voltage

applications it may not be uncommon to have three level or five level inverters.

In VSI drives Very High Voltage control and very Fast Switching Device is

required ,henceIGBT is used in VSI Drives.

Insulated Gate Bipolar Transistor (IGBT):-

It is a Power transisitor employed in the power conversion master circuit.This

system uses 33.3kV,1200 A IGBTdevices.IGBT has the characteristics of both

bipolar transisitor and MOSFET.In other words, it is characterized with

comparatively high withstand voltage and high current capacity by means of

bipolar action, high-speed switching by MOS gate, and easy gate drive at low ON

voltage as ideal requirements. Since this device provides a self arc extinguishing

function, it does not require any forced commutation circuit to ensure compact

design of device.

Block Diagram of Hitachi VSI :-

There are two types of inverter main circuit used.

They are:-

3 level type circuit

2 level type circuit

9.2.1--2 Level type Circuit:-The IGBT drive

employs a 2-level system conversion circuit.This

conversion circuit connects 2 serial switching

devices between P bus and N bus.

- 30 -

9.2.2--3 level type Circuit:-The IGBT converter employs a 3-level system

conversion circuit. This 3-level system conversion circuit divides the DC power

voltage into 2 parts via the neutral point,and connects serial switching devices to

the positive bus-neutral point and the neutral point-N bus as a feature.

From the figure when switch QP and QPC is ON the the output voltage is

+Edc,when the switch QPC is ON but QP is OFF then the output voltage is zero.

Similarly when the switch QNC and QN is ON then

the output voltage is –Edc, when the switch QN is

OFF but QNC is ON then the output voltage is zero.

In this circuit when the switch QP is ON then the

output voltage is +E/2 and when the switch QN is ON

then the outut voltage is –E/2.

9.3 Difference between CYCLOCONVERTER and VSI

CYCLOCONVERTER VSI

1. It uses tyristor switch. 2. It converts constant ac to variable ac directly. 3. It is complex device used large number of components. 4. Power factor is less then unity. 5. It operates in frequency of less than 25HZ. 6. It is used to operate motors of higher power range.

1. It uses IGBT switches. 2. It converter contact ac to dc then to variable ac. 3. It is less complex. 4. Power factor is nearly unity. 5. It can be operate up to frequency of 200HZ. 6. It is used to operate motors of lower and medium power ranges.

- 31 -

10. Crm-PLTCM mill stand drive database

MAKE

MODEL NO.

QTY.

AC/DC

ANALOGUE/

DIGITAL

KVA

VOLTAGE

APPLICATION

YEAR OF

INSTALLING

HITACHI

HIVECTOL

-VSI-M

1

AC DIGITAL

800 DC

1200V

MILL

ENTRY #4

BRIDLE

ROLL

1999

HITACHI

HIVECTOL

-VSI-M

1 AC DIGITAL

200 DC 600V

MILL

ENTRY

BRIDLE

ROLL

1999

HITACHI

HIVECTOL

-VSI-M

2 AC DIGITAL

400 DC 600V

MILL

ENTRY

BRIDLE

ROLL

1999

HITACHI

HIVECTOL

-VSI-M

1 AC DIGITAL

75 DC 600V

MILL EXIT

PINCH

ROLL

1999

HITACHI

HIVECTOL

-VSI-S

1 AC DIGITAL

AC1074

#1 MILL

STAND

1999

HITACHI

HIVECTOL

-CYC

4 AC DIGITAL

AC 1645

#2~5 MILL

STAND

1999

TOSHIB

A

TOSVERT 5

AC DIGITAL AC 415

X-RAY

1999

- 32 -

11. CONCLUSION

Drive is a device which gives us the power to control the motor speed

from distance. It gives the technology to control the torque-speed of

motor. In today’s world dc drive is totally replaced with ac drive. In

―TATA STEEL‖ there are total 2365 no of drive from which 2060 is ac

and only 305 is dc. Because ac drive is more efficient and gives more

features then dc.PLTCM has a total of 123 drives of which 117 are AC

while the rest 6 are DC . Of the total 9 are analogue while 114 are digital.

In PLTCM, we mainly went throughcycloconverter drive and VSI based

IGBT drive system. By examining and working with this drive we found

that the new one IGBT drive is more efficient and has many more features

compared to cycloconverter. IGBT drive has power factor nearly equal to

unity, it is less complex then cycloconverter. Now a days in ―TATA

STEEL‖ induction motor are driven by IGBT drive and synchronous

motor are driven by cycloconverter. Also the operating frequencies of

IGBT drives are much more higher than that of cycloconverters.

Therefore, in TATA STEELIGBT drive system has been adopted to go

with new technology.

At last, we would like to thank all officer of meg dept and all employee of

―TATA STEEL‖ who helped us directly or indirectly to complete this

project.