Report

ABSTRACT

In this modern world of Industrialization and automation, energy plays a major role in

the growth of any organization. The utilization of energy plays an impacting and direct role

in the growth of organizations like Visakhapatnam Steel Plant. The Telecommunication

department plays a vital role in providing & maintaining different electronic communication

systems in various departments to achieve the assigned targets and accomplishing the desired

performance in VSP.In Vizag Steel Plant, The supervisory control and data acquisition

system (SCADA) is implemented to monitoring the energy network. The objective of this

SCADA system is to monitor the energy usage and demands of various consumers of the

plant and exercise effective control over their energy consumption and optimum utilization of

in-house energy resources.

1

INDEX

S.NO CONTENTS PAGE NO

1. Introduction 6

1.1 Evolution 6

1.2 Turnaround 7

1.3 Vizag steel tomorrow 8

2. MAJOR DEPARTMENTS IN VSP 10

2.1 Raw material handling plant 10

2.2 Coke ovens 10

2.3 Sinter plant 11

2.4 Blast furnace 11

2.5 Steel smelt shop (SMS) 11

2.6 Rolling mills (RM) 12

2.7 Thermal power plant (TPP) 12

2.8 Main products of Vsp ) 12

3. COMMUNICATION SYSTEMS IN VSP 13

3.1 General purpose communication systems 14

3.2 Process communication systems 15

3.3 Motoring and signaling systems 17

2

4. INTRODUCTION TO DRIVES 20

4.1 Drive basics 20

4.2 Block diagram of a drive 20

4.3 Various types of drives used in industries 22

5. DC DRIVES 23

5.1 Analog DC drive 24

5.1.1 Components of analog DC drives(MTE) 24

5.1.1.1 POWER SUPPLY UNIT 25

5.1.1.2Control supply unit 25

5.1.1.3Synchronous supply unit 25

5.1.1.4Regulation unit 25

5.1.1.5Protection unit 25

5.1.2Need for speed control 26

5.2 Digital DC drive 37

6. COMPARISION BETWEEN ANALOG & DIGITAL DRIVES 38

7. CONCLUSION 39

8. REFERENCES 40

3

LIST OF FIGURES

FIG.NO NAME OF THE FIGURE PAGE.NO

Fig 4.1 Block Diagram of an electrical drive 20

Fig 5.1 DC drive motor connections 23

Fig 5.2 Speed torque characteristics 24

Fig 5.3 Diode Bridge circuit 27

Fig 5.4 Diode Bridge output waveform 28

Fig 5.5 Thyristor Bridge circuit 29

Fig 5.6 Output waveforms of thyristor stack 30

Fig 5.7 Controller section of the drive with all the cards 31

Fig 5.8 Speed controller in a card 32

Fig 5.9 Current controller 33

Fig 5.10 Firing module 34

Fig 5.11 Basic drive circuit 35

Fig 5.12 Forward/Reverse speed controller circuit 36

Fig 5.13 Full Forward/Reverse controller 37

Fig 5.14 Digital drive 38

4

LIST OF TABLES

TABLE.NO NAME OF THE TABLE PAGE.NO

Table 1 Products of steel plant vsp 13

5

1.INTRODUCTIONVisakhapatnam Steel Plant, an integrated steel plant under the corporate entity of

Rashtriya Ispat Nigam Limited (RINL), is the first shore based integrated Steel Plant in the

country, constructed with the then latest state of the art technology. The Plant with a rated

capacity of 3.0 Mt is a producer of steel products in the longs category like wire rods, re-

bars, angles, channels, blooms and billets. The Visakhapatnam Steel Plant strikes every one

with a tremendous sense of wonder and arrangement as it presents a wide array of excellence

in technology, in manpower, in management, in science beauty and includes all these in

product quality.The momentum decision to establish a steel plant at Visakhapatnam was

announced in the parliament in 1970 by the Prime Minister Smt. Indira Gandhi. The

announcement stone for this massive project was laid in 1971. The detailed project report

was prepared in 1977. However it was the only company in 1979 with the government of

Soviet Union offering techno economic corporation, the cabinet approved the proposal for

setting up an integrated steel plant in Visakhapatnam.The plant is located on the coast of Bay

of Bengal, 16Kms to the Southwest of Visakhapatnam port. IT lies between the northern

boundary of National Highway No.5 from Madras to Calcutta and 7Kms to the South West

of Howrah Madras railway line.

1.1EVOLUTIONVisakhapatnam Steel Plant was conceived in the year 1970 as a unit of Steel

Authority of India Limited (SAIL) to augment its long products capacity and to service the

southern markets. Announcement for Visakhapatnam Steel Plant was made in the Parliament

in the year 1970 and the foundation stone was laid in 1971 by the late Prime Minister Smt.

Indira Gandhi.

The feasibility report of the plant was made in 1973 and the Indo-Soviet

Agreement was signed in 1979. The comprehensive detailed project report was made in 1980

and the project was sanctioned by Government of India in 1982. In the same year, a separate

company called Rashtriya Ispat Nigam Limited was formed.

The plant was to be commissioned by 1986 as per original schedule. However,

because of severe cash crunch, in the year 1986, a rationalized concept was adopted to lower

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the capital costs. Some of the envisaged facilities were dropped and the nameplate capacities

of steel making and rolling mills were increased for making revenue generation more

attractive.

Finally, the plant was fully commissioned in the year 1992. Due to the long

gestation period of 22 years from concept to commissioning stage, the capital cost of the

project went up from original estimate of Rs.2256 crs in 1979 to Rs.8594 crs in 1992. As a

result of high capital cost and large borrowings the company had to bear high interest and

depreciation burden resulting in continuous losses. This has resulted in huge cost over runs

and high capital related charges and Visakhapatnam Steel Plant at the time of commissioning

itself had a net loss of over Rs.2000 crs.

While the plant was picking up production, the South East Asian financial crisis

severely affected the steel market which led to continuous drop in steel prices both in

domestic and international markets. Also from 1998, the steel industry, worldwide, was

affected by recession. Steel demand declined, forcing the steel producers to throttle

production levels. Many unviable production units in the world had to close down. India’s

exports were hit adversely and domestic consumption remained almost stagnant. Sales

realizations plummeted and profitability of the Indian steel producers was adversely affected.

All these conditions brought a tremendous pressure on the financials of the company.

1.2THE TURN-AROUND:Despite these adverse conditions, the Vizag Steel collective rose to the occasion

and over the last few years Vizag Steel’s performance has improved on all fronts including

production, techno-economics, marketing and financials. The emphasis initially had been

towards total employee involvement, then to technology up gradation and process

management and then to managing external environment.

At this juncture, the leadership played a crucial role in providing direction

focusing on critical issues and empowering employees. A number of sustenance and

performance improvement programmes were initiated to put the company on the growth

trajectory. The emphasis was on attaining rated capacity at the earliest, improving techno-

economic performance, improving health of equipment, cost reduction and process

innovations.

7

Systematic and standard operation practices and structured systems were

developed and Vizag Steel was the first integrated steel plant in the country to be accredited

with all the three International Standards for Quality (ISO 9001), for Environment

Management (ISO 14001) and for Occupational Health and Safety (OHSAS-18001).

All these initiatives gave positive results and the company started its journey to

excellence by crossing its rated capacity levels in the year 2001-02. Since then the company

has been operating consistently beyond the rated capacities. Currently, the plant is operating

at 120% of its rated capacity. The Plant turned around in the year 2002-03 by achieving for

the first time a net profit of Rs.521 crs. The improved performance saw Vizag Steel become a

Zero debt company and is a net positive company today having wiped off all its accumulated

losses.

All these efforts of Vizag Steel were well recognized by one and all. For its

excellent performance in the year 2002-03, Vizag Steel was conferred with the Prime

Minister’s trophy for the best Integrated steel plant in the country. During the year 2002,

Vizag Steel was awarded with `SCOPE’ excellence award for turnaround for its outstanding

performance. Vizag Steel has been winning the National Energy Conservation award for the

last seven years and has also won the Special Prize for National Energy Conservation for

successfully annexing the first prize for three consecutive years.

Vizag Steel has been constantly achieving excellent MOU rating from the

government for its excellent performance. It was bestowed with the World Quality

Commitment Award at Paris. In 2005, Vizag Steel bagged six Vishwakarma Rashtriya

Puraskar Awards out of the 28 awards announced by Ministry of Labour, a tribute to the

involvement of its employees and participatory approach of the management. Best Water

Management, Best Financial Management and Best Safety & Occupational health awards

from Confederation of Indian Industries are some of the prestigious awards received.

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1.3  VIZAG STEEL TOMORROW:Armed with a vision to become world class integrated steel plant, the Vizag Steel Collective

is charged with a steely resolve to face a challenging tomorrow. Some of the initiatives taken

to make Vizag Steel a world class organization are:

Adoption of business excellence through CII-Exim Business Excellence Model. Initiation for BPR & ERP. Implementation of Six Sigma methodology. Implementation of ‘5S’ for outstanding housekeeping. Bench marking with world class companies. Introduction of Knowledge Management to harness the inner potential of employees. Introduction of e-commerce.

Acquisition of captive mines in India and abroad through joint venture, so as to have

a level playing field with its competitors.

In line with its vision to become a continuously growing company, Vizag Steel

recast its expansion plan to double its capacity from its present 3 mT to 6.3 mT by 2008-09.

The approval for its recast expansion plan was obtained on 28th Oct ’05 in a record time of 10

months. The expansion has been planned to further strengthen its long product leadership in

the country through production of special bars, wire rods and structurals, in view of its high

brand image and also envisaged demand in line with the infrastructure growth of the country.

A seamless tube plant has been planned for the first time in an integrated steel plant to cater

to the growing oil and gas industry and reduce dependence on imports of seamless tubes. The

next phase of expansion to 10 mT is planned for completion by 2012-13 which will include

flat products also to provide a rich product mix.

With excellent performance levels continuing during the current year also and the

ensuing expansion wherein Vizag Steel has set up an ambitious target of completion of its

expansion in a record 36 months at international standards, Vizag Steel looks to new

horizons. Vizag Steel collective has proved its mettle time and again and is all set to take up

new challenges and soar to new heights to emerge as a World Class steel plant.

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2.MAJOR DEPARTMENTS IN VSP

2.1 RAW MATERIAL HANDLING PLANT (RMHP) :The RMHP receives the basic materials iron ore, fluxes (lime stone,

dolomite),coking and non coking coals etc. required for the steel making process from

various sources through wagons which are stacked and reclaimed by stackers-cum reclaimers

and distributed to various departments of Visakhapatnam steel plant through conveyor

systems.

2.2 COKE OVENS (CO):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 of the reduction and refining reactions besides acting as a

reducing agent.

Coke is manufactured by heating of crushed coking coal (below 3mm) is in the

absence of air at a temperature of 1000deg centigrade and above for period of 16 hrs to 18

hrs. A coke oven comprises of two hollow chambers namely coal chamber and heating

chamber in the heating chamber a gaseous fuel such as blast furnace gas, coke oven gas etc.

is burnt. The heat so generated is conducted through the common wall to heat and carbonize

the coking coal placed into adjacent coal chamber. Number of ovens built in series one after

the other form a coke oven battery.

At VSP there are three coke oven batteries, 7 meter tall and having 67 ovens

each. Each oven is having volume of 41.6 cu meters and can hold up to 31.6 tonnes of dry

coal charge. The carbonization takes place at 1000 to 1050 deg centigrade in the absence of

air for 16 to 18 hours.

Red hot coke is pushed out of the oven and sent to coke dry cooling plants for

cooling to avoid its combustion. There are three coke dry cooling plants (CDCP) each having

four cooling chambers. The capacity of each cooling chamber is 50 to 52 TPH. Nitrogen gas

is used as the cooling medium. The heat recovery from nitrogen is done by generating steam

and expanding in two back pressure turbines to produce 7.5 power each.

10

The coal chemicals such as benzol, tar, ammonium sulphate etc. are extracted in

coal chemical plant from co gas. After recovering the coal chemicals the gas is used as a

byprioduct fuel by mixing it with gases such as BF gas, LD gas etc. A mechanical, biological

and chemical treatment plant takes care of the effluents.

2.3 SINTER PLANT : Sintering is one of the most widely used and economic agglomeration techniques.

Sinter is a hard and porous lump obtained by agglomeration of fines of iron ore, coke,

limestone , and metallurgical waste. Sinter increases the productivity of blast furnace,

improves the quality of pig iron and decreases the consumption of coke rate .Two 312 square

meter sinter machines with 420 square meter straight stand type coolers for annual

production of 5.26 MT sinter.

2.4 BLAST FURNACE (BF) :Pig iron or hot metal is produced in the Blast Furnace .The furnace is named as

BF as it is run with blast at high temperature and pressure of 1500 deg. C. Raw materials

required for pig iron and iron are iron making ore, sinter, coke, and lime stone. There are

two 3200 cubic meter blast furnace to meet 3.0 MT annual metal requirement. with bell-

Each furnace is provided with a set of four hot blast furnace stoves designed for supplying air

blast up to 1300 deg .C. Three turbo blowers, one for each furnace and one stand by

common to both furnaces are provided with 12 MW top pressure recovery turbo generating

power. BF gas is produced from each furnace is being cleaned in gas cleaning plant

comprising dust catcher, high pressure scrubber and is distributed through out the plant as a

fuel.

2.5 STEEL MELT SHOP (SMS) :Steel is an alloy iron and carbon, where carbon should be less than 2%. Hot metal produced

in B.F contains impurities like carbon, sulphur, phosphorous, silicon etc., these impurities

will be removed in steel making by oxidation process. These are the three L.D converters to

convert hot metal in to steel. The steel melt shop complex comprising two 1300-ton hot

metal mixers, three 130-ton LD converters (two operating) and six 4-stand bloom casters.

Each converter is being provided with gas cleaning plant for cleaning and recovery of LD

gas, which will be used as fuel in plant.

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2.6 ROLLING MILLS (RM):Blooms cannot be used as they are in daily like. These blooms have to be reduced

in size and properly shaped to fit for various jobs. Rolling is one of the mechanical processes

to reduce large size sections in to smaller ones. The cast blooms from CCM are heated and

rolled in to long products of different specifications like high capacity, sophisticated high-

speed rolling mills. The rolling mill complex comprises:

Light and medium merchant mill (LMMM)

Wire rod mill (WRM)

Medium merchant and structured mill (MMSM).

Each mill is well equipped with required number of walking beam furnaces for

heating of walking beam furnaces for heating of blooms or billets and except for wire rod

mills, each furnace is provided with evaporative cooling system for generation of steel for

plant consumption.

2.7 THERMAL POWER PLANT (TPP):The estimated power requirement for V.S.P in 280 at 3.0 MT stages, the peak

load being 292 MW essential loads being 49 MW. The generating capacities 286.5 MW. A

captive power having 3x60 MW turbo-generator sets and 5x330 ton/hr steam generators. In

this plant, 6000 Nm^3/min turbo blowers are being provided for supplying cold air blast

furnaces.

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2.8 MAIN PRODUCTS OF VSP( PRODUCT MIX)

Unlike many other steel plants VSP is an integrated steel plant , producing many

products like Blooms,Channels,Angles etc., other than these VSP also produces many by-

products that are derived during the production of coke and iron .The main products and the

by-products that are produced are shown in the table below :

TABLE –1:

Steel productsBy-Products

Angles Nut coke Granulated slag

Billets Coke dust Lime fines

Channels Coal tar Ammonium sulphate

Beams Anthracene oil

Squares HP napthalene

Flats Benzene

Rounds Toluene

Re-bars Zylene

Wire rods Wash oil

Table1: Products of steel plant vsp

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3.COMMUNICATION SYSTEMS IN VSP

In this modern age of industrialization telecommunications plays a very important role in

coordinating the activities of various departments / sections and in achieving the set targets

and also in improving the performance of the organization. In Visakhapatnam Steel Plant,

different types of communication systems are being used to meet the internal and external

communication needs. These are broadly classified as follow:

a) General purpose communication systems.

b) Process communication systems.

c) Monitoring & Signaling Systems.

Apart from the above facilities Telecom department maintains the following cable networks

also

a) DATACOM cable network

b) .Telephone cable network in plant and township

3.1GENERAL PURPOSE COMMUNICATION SYSTEMS:The following facilities are provided under category of general purpose communication

systems:

4000 lines IP based Telephone in Plant.

3000 lines Electronic Exchange in Township.

100 lines Electronic Exchange in Visakha Steel General Hospital.

44 lines Electronic exchange in Hill Top Guest House

14

2500 Lines Electronic Exchange of Bharat Sanchar Nigam ltd (BSNL) in Project

Office is catering to the needs of Plant area and Sectors-I to VII in township. Another 2000

Lines Electronic Exchange of Bharat Sanchar Nigam ltd (BSNL) in Township is catering to

the needs of Sectors-VIII to XI in Township.

The 3000 Lines electronic exchange in plant and 3000 lines exchange in township

are having the following facilities:Extension (subscriber) to extension call, Auto call back,

Hot lines, Music on hold, Reminder Alarm, Automatic line testing facility, Faults man ring

back, Call consult facility, Malicious call tracing facility, 3 party conference Facility and

Howler Tone alert etc.

All exchanges working in the steel plant are interconnected by means of junction

lines and have closed numbering scheme.The 3000 lines exchange in township is

interconnected to the BSNL network. Due to this interconnection all the subscribers of this

exchange can receive incoming calls from any part of the world. A few subscribers are

provided with facility to contact subscribers connected to the BSNL network and cellular and

mobile phones in and around Visakhapatnam.

3.2 PROCESS COMMUNICATION SYSTEMS:To facilitate coordination, operation & management activities of production,

maintenance & service departments, the following process communication systems are

provided:

a) Dispatcher communication system

b) Loudspeaker intercom systems

c) Loudspeaker broadcasting systems

d) Loudspeaker conference communication system

e) Industrial public address system

f) Hotline communication systems

g) VHF communication systems

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A) DISPATCHER COMMUNICATION SYSTEMS:

Initially dispatcher communication is provided with a cordless switch board type

manual exchanges of electromechanical. Since manufacture of such systems and their spares

is discontinued in the country, these dispatcher systems are being replaced by Electronic

exchanges progressively. Except the ones in CCCP other systems have been replaced. These

would also be replaced very soon.

Production coordination at plant level being conducted by ED (Works) with all HOD s in the

morning every day is facilitated with the help of the digital EXCOM system provided in the

plant control room.

B) LOUD SPEAKER INTERCOM SYSTEMS:

Loudspeaker intercom systems are working LMMM & WRM. These systems are

used for communication between various sections of the same production shop.

Communication is made possible using microphones and loudspeakers provided in the

subscriber stations. This system is very useful in noisy environment where conventional

telephones are ineffective. This system helps to establish communication between any two

stations having interconnectivity on selection basis. By using group call facility it is possible

to communicate to all the subscribers in the group at a time.

C) LOUDSPEAKER BROADCASTING SYSTEMS:

This system consists of centralized amplifier rack with amplifiers, desktop micro

phone with press to talk switch and a network of loud speakers connected to the

amplifiers .this is useful to make general announcements to the entire working area as to pass

on important instruction from the control room.

Loudspeaker broad casting systems are provided in C&CCD, BF, SP and SMS departments.

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D) LOUDSPEAKER CONFERENCE COMMUNICATION SYSTEMS:

Loudspeaker conference communication systems are working in CCCP. These

systems are provided with both paging and private channel communication facilities. In case

of paging a general announcement can be made which is heard on all the stations. In the

private mode communication is possible between two selected stations only. Here also

communication is carried out by means of microphones and loudspeakers provided in the

subscriber stations.

E) INDUSTRIAL PUBLIC ADDRESS SYSTEM:

Industrial Public Address System is working in TPP. It is a combination of

loudspeaker broadcasting system and conference communication system. From the main

control room it is possible to make announcements which are heard on the shop floor. From

certain locations the communication can also be established through handsets in private mode

with the main control room.

F) HOTLINE COMMUNICATION SYSTEMS:

To ensure direct telephone communication between closely related critical

locations hot lines are provided. By using the hot lines specified locations are connected

permanently. Communication is possible only between these two locations. When one

subscriber lifts his telephone the other will immediately get a ring and communication can be

had without any loss of time.

This is useful to pass-on urgent messages. These hot lines are initially provided

with direct line communication systems which are electro mechanical systems. Due to

obsolescence electronic systems are now being used for most of the locations. Hot lines are

working in CCCP, BF, SMS, LMMM, WRM, MMSM, TPP, PPM, DNW and WMD

departments.

17

G) VHF COMMUNICATION SYSTEMS:

VHF communication systems are used in VSP to establish two way

communications between two or more when either or one of them is moving. There are three

models working in our plant. They are hand-held units (Walkie-talkie), vehicle mounted –

mobile units and base station units.

Walkie-talkies are used by operation and service personnel in almost all of the

production shops. Vehicle mounted units are being used by DNW, CISF (Fire) and CISF

(security). Base station units are used by CISF (fire), CISF (Security), Administration, DNW

and largely by CCCP departments. In CCCP these can be seen in pusher cars, charging cars,

door extractors, electric locos, lifters and CDCP areas.

These sets are very essential and useful in answering proper communication and

coordination during alignment of oven machines while charging and pushing the ovens and

carrying out these operations safely.

Handheld VHF sets are extensively used for establishing instantaneous communication and

ordination of operation or maintenance activities on different departments throughout the

steel plant.

3.3 MONITORING AND SIGNALING SYSTEMS:To facilitate monitoring production, maintenance & service activities, the

following monitoring and signaling systems are provided:

a) Closed Circuit Television Systems (CCTV)

b) Central fire alarm signaling system

c) SCADA system

d) Shift change Announcement Siren System

A) CLOSED CIRCUIT TELEVISION SYSTEMS (CCTV):

For monitoring critical operations in different production units continuously from

the concerned control rooms / pulpits CCTV systems are used in SP, BF, SMS, LMMM,

WRM and MMSM departments. CCTV system comprises of CCTV camera with a lens and a

CCTV monitor.

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In some cases a video switcher, a central control console, pan and tilt head and

zoom lens are also used. These are interconnected by means of control cables and / or coaxial

cables. Necessary protection is provided for the CCTV equipment depending on the locations

where they are used.

B) CENTRAL FIRE ALARM SIGNALING SYSTEM:

The central fire alarm system is provided for communicating the exact location of

outbreak of fire in any part of the steel plant complex to the central fire station and

simultaneous actuation of sirens to alert personnel of the affected plant zone. The system

employs manual call points located all over the plant.

C) SCADA SYSTEM:

To monitor the generation and consumption of various energies by various users

in the plant the SCADA system is provided. Scada system is located at Energy and Telecom

centre. The system comprises of a master servers, 23 Remote Terminal Units (RTUs)

installed in different plant units and display units. The RTUs will collect the signals from the

transducers & electrical systems and transmitting to the master servers by means of co

communication channel. The RTUs are connected to the master servers by underground laid

telephone cables.

D) SHIFT CHANGE ANNOUNCEMENT SIREN SYSTEM:

The shift change announcement siren system is provided for ensuring uniform and

accurate shift timings throughout the plant. This system consists of two quartz crystal

controlled master clocks in the Energy & Telecom Centre. The shift timings are

programmed in the Master Clock. At the specified time, the signal will be transmitted for

energizing the sirens located at strategic points in plant area simultaneously at the preset

timings.

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4. INTRODUCTION TO DRIVE

4.1DRIVE BASICS

Motion control is required in large number of industrial and domestic

applications. Systems employed for motion control are called “DRIVES”. It may employ any

of the prime movers such as diesel or petrol engines, gas or steam turbines, steam engines,

hydraulic motors and electrical motors, for supplying mechanical energy for motion control.

Drives employing for electrical motors are known as “ELECTRICAL DRIVES”

4.2 BLOCK DIAGRAM OF AN ELECTRICAL DRIVE:

FIGURE 4.1 Block diagram of a drive

Electrical drive has the following major parts:

LOAD:

There are large no of loads and each load has its own specific requirements.

Each load has its own current and voltage rating.These ratings are to be observed carefully

before operating the motor or machine.

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MOTORS: Motors commonly used in electrical drives are :

DC MOTORS:

Shunt motors

Series motors

Compound motors

1. Cumulative compound .

2. Differential compound.

AC MOTORS:

Induction motors

1. Squirrel cage

2. Slip ring or wound rotor

Synchronous motors

POWER MODULATOR:

Modulates flow of power from the source to the motor in such manner that motor

is imparted speed-torque characteristic required by the load. It consists of converters,

inverters etc.In general a pulse modulator is used in drives to supply the required power to

the DC motor.

CONTROL UNIT:

Controls for a power modulator are built in control unit which usually operates at

much lower voltage and power levels. It consists of firing circuits, which employ linear and

digital integrated circuits and transistors and a microprocessor when sophisticated control is

required. Input command of the signal, which adjusts the operating point of the drive, forms

an input to the control unit.

SENSING UNIT:

Sensing of certain drive parameters, such as motor current and speed, may be

required either for protection or for closed loop operation.

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4.3 VARIOUS TYPES OF DRIVES USED IN INDUSTRIES:

DC DRIVES: These drives are used to regulate the speed of DC motors within the

rated current and voltages.

Analog AC drives

Digital DC drives

AC DRIVES: These drives are used to regulate the speed of DC motor within the

rated current and voltages.

Analog AC drives

Digital DC drives

The Drives are to be protected from over current and over voltage. This is to be done

using Chokes and snubber circuits.

CHOKE: A choke is a coil of insulated wire often wound on a magnetic core used

as a passive inductor to block high frequency AC in an high frequency circuit.

SNUBBER: Snubbers are frequently used in electrical systems with an inductive

load where sudden interruption of current flow often leads to sharp rise in voltage

across a device creating interruption.

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LINE INPUT

MOTOR OUTPUT

Armature

Field

A1

A2

F1

F2

5.DC DRIVES

DC DRIVE: Drives employed for speed control of dc motors are called dc drives. DC

Drives convert AC line voltage into variable DC voltage with an SCR phase-controlled

bridge rectifier, to power the DC motor ARMATURE. A separate field supply provides the

motor with DC FIELD excitation.

FIGURE 5.1:DC drive motor connections

speed of dc motor can be written as

N = k Eb / @ ; Eb=(@ZN / 60) (p/a)

Eb – Back emf

@ - flux

Z - no. of turns

P- no. of poles

A – no. of parallel paths

Speed can be controlled either by varying armature voltage or field flux.

Armature controlled method is used to vary the motor speed below base speeds

Field control method is used to vary the motor speed above base speeds.

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100

50

25

75

750 500 750 1000

Base Speed Max.Speed

2 : 1 FIELD WEAKENING

3 : 1 FIELD WEAKENING

4 : 1 FIELD WEAKENING

FIELD WEAKENED RANGE 4 : 1

CONSTANT HORSEPOWER

HORS

EPOW

ER

CONSTANT TORQUE

TORQUE @ 100% ARMATURE AMPS

FULL FIELD

TOR

QU

E &

HO

RSE

POW

ER

SPEED (RPM)

%

The typical speed torque characteristics of dc motor is as shown in below figure.

FIGURE 5.2: Speed torque characterstics

Main drive:

The drive having both armature and field control is called main drive. In this the

motor can be operated in below & above base speeds.

Auxiliary drive:

The drive having only armature control is called auxiliary drive.

The dc drives are of either analog or digital. The power circuit is same for both. In

analog drives all the control signals are in analog form but in digital drives this controlling is

through microprocessor controller.

5.1 ANALOG DC DRIVE:

5.1.1 Components Of Motor Testing Equipment( Analog Dc Drive):1. POWER SUPPLY UNIT

2. CONTROL SUPPLY UNIT

3. SYNCHRONOUS SUPPLY UNIT

4. REGULATION UNIT

5. PROTECTION UNIT

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5.1.1.1.POWER SUPPLY UNIT:

Armature main incoming transformer the primary of which is 11KV. The

secondary of the transformer is connected to thyristor converter through HT breaker. The

converter output DC is connected to armature of the motor through HSCB and DC contactor.

Field main incoming transformer the primary of which is 415 V. The secondary

of the transformer is connected to thyristor converter through AC contactor. The output DC

is connected to field of the motor directly.

5.1.1.2. CONTROL SUPPLY UNIT: Armature control supply transformer. Primary 415-3 phase. Generation of

unregulated +/-24v DC and regulated +/-15v DC for control electronics.

Field control supply transformer Primary 415-3 phase. Generation of unregulated

+/-24v DC and regulated +/-15v DC for control electronics. Primary of the transformer is fed

with a circuit breaker.

Generation of auxiliary supply (60V DC) in armature and in field separately for

pulse generation.

5.1.1.3.SYNCHRONOUS SUPPLY UNIT:

This consists of armature synchronous transformer, primary of which is 415V-3

phase, secondary of which is connected to pulse generator card. Generation of 60V DC pulse

power supply for armature.

5.1.1.4.REGULATION UNIT:

This consist of

Speed controller, current controller for armature

EMF controller, flux controller for field

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5.1.1.5.PROTECTION UNIT:

This consists of fault monitoring ,fault registering and tripping in armature for following

units:

Main incoming transformer (over temperature, Bucholtz protection). Thyristor converter (fuse, over temperature, over current protection) Motor armature (over temperature, instantaneous over current, over speed, over

voltage, earth fault protection)

5.1.2 Need for speed control: In various applications we need to vary speed. i.e There is a need to run the motor

at more than one speed . At such applications we use drives.

The speed of a dc motor can be controlled by various methods.

Speed is directly proportional to the supply voltage & hence we can increase the speed of

motor only up to the rated voltage. Hence speeds below rated speeds are only possible.

Speed is inversely proportional to Field/flux. Field can only be weakened. Hence we can

only achieve speed above rated in this process.

The speed of a dc drive is varied by varying the input power(since the speed is

proportional to applied voltage to the motor terminals).

In general industries use three phase ac supply. But we need to convert the ac power to

corresponding dc power.

i.e... We need to convert the Input three phase supply to dc voltage.

This can be done by using bridge rectifiers.

We have many types of bridge rectifiers like

Basic diode bridges

Thyristor bridges.

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Our main aim is to regulate the speed of motor. But in a diode bridge we cannot control the

output of the bridge. Where as we can control the output of the bridge by varying the firing

angle at the gate of thyrisor.

Diode Bridge:

Figure 5.3: Diode bridge circuit

Figure 5.4: Diode bridge output waveform

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Thyristor (scr) Bridge:

Figure 5.5: Thyristor bridge circuit

In a thyristor circuit unlike in a diode bridge rectifier we can control the output

voltage to be applied to the DC motor for obtaining various rated speeds. This principle is

largely employed nowadays

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Thyristor firing and part of input conducted:

Figure 5.6: Output waveforms of thyristor stack

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From the above wave forms it is evident that the output of the Thyristor Bridge is dependent

on the position at which pulse fired. This is also known as firing angle. The complete voltage

output can be regulated by varying the firing pulse between an angle from 30-150.

30 degrees corresponds to maximum voltage firing angle.

150 degrees corresponds to minimum

Reference generation: A reference 0-10V is generated which is equated to gate pulse angle max to min

(Eg: 1500 to 300) to generate the dc voltage zero to maximum (Eg: 0V to 400V DC) which is directly proportional to speed (Eg: 0 to 1000 rpm) of the motor. 

Ramp function generation: To overcome starting inrush current, the ramp function generator is introduced. It

is also called soft start circuit. (fig) The input of the ramp function generator is 10V, the output of this varies as 0-10V with set times called ramp up and ramp down.

Figure 5.7: Controller section of the drive with all the cards

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Speed controller:

Open loop: In modern speed-control drives, open loop control proves inadequate because of sharply drooping speed-torque characteristics.

Closed loop: The closed loop control systems are widely used for maintaining the speed constant at

a desired value with better accuracy and dynamic response. A tachogenerator is often used, sometimes voltage feedback for closed loop system.

The output of the tachogenerator is compared with a preset reference (output of ramp function generator). The difference between these two voltages is fed as an actuating signal to control elements of the system.

Figure 5.8: Speed controller in a card

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Current controller:

CT feed back or Shunt feedback is used for the actual current of the motor. It is compared with the output of the speed controller, which actuates and controls the triggering of the thyristors.

There are two feedback paths in the dc control system; one is called outer speed or voltage feedback path and the other is called inner current feedback path. With the combination of two feedbacks paths the overall performance becomes more precise.

Figure 5.9: Current controller

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Firing module: The function of the firing module is to generate the firing pulses and to control the thyristor-firing angle. The sample voltage with same phase sequence of 3 Ph AC voltage is given to the firing module is called synchronous supply. The firing pulses are generated and controlled synchronously with the 3 Ph AC voltage .

Figure 5.10: Firing module

GK cards: The component in the GK (gate and cathode) card is pulse transformer, mainly to isolate the control signal with Power circuit

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Figure 5.11: Basic drive circuit from a standard manufacturer

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Fig 5.12 forward/reverse controller

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Fig 5.13 full forward/reverse controller

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5.2 Digital Drives:An advancement of technology is the present day’s digital drive. All the above blocks are

combined logically on a single board using program logic controls which provide output to

motor based on the parameters given as input. The program is fed into and operated via a

micro processor/controller. The code used in the micro controller is given to the staff so that

it can be reprogrammed at any time.Along with all the above blocks logically the firing pulse

generator is provided with necessary hardware. The block diagram of the digital drive is as

shown in figure below.

Figure 5.14 Digitaldrive

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Basic building block :op-amp supported by other analog components

Functional blocks are inter connected through wire wrapping or mother board

Set points and gains are adjusted using potentiometers provided.

When the temperature increases the values are changed.

Modification is difficult. All analog components ratings has to be changed.

Chances for failures are more.Repairing of analog cards can be possible.

Functional blocks are soft ware based program processed in a microprocessor

Blocks are interconnected through software connectors

Adjusted using parameter values directly using their parameter number

They are independent of temperature.

We can configure the drive to any application easily. Only by changing the parameter values.

Chances for failures are less.

Repairing is not possible. Only replacement is possible.

FUNCTIONALLY NO DIFFERENCE

ANALOG DIGITAL

6. COMPARISION BETWEEN ANALOG & DIGITAL DRIVES

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7.CONCLUSION:

In the time being the analog drives are being replaced by digital drives. Analog drives are difficult to maintain and are costly unlike digital drives which are very easy to maintain and cheaper cost. The repair of digital drive involves of simple steps like re programming unlike the analog drives which require component wise attention and replacement. The digital drives also present a large range of accuracy. But in component wise the replacement of components in an analog drive is easy compared to the Digital drive which involves finding out the specific IC to be replaced in case of physical damage. Due to the functional efficiency and less probability of hardware failure digital drives are replacing the analog drives in almost all the industrial applications. Even modifying parameters of a digital drive is easy compared to the analog drive which needs just changing the parameter values in the code whereas an analog drive requires the resistors and other circuitry to be changed. Hence it is always a good choice to run the machines based on availability of the drive.

8.REFERENCES

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1. B.L.Thereza,”A Text Book Of ELECTRICAL TECHNOLOGY in SI units(Vol-ii)”,S.Chand publishers,2009.

2. http://en.wikipedia.org/wiki/Dc_motor 3. http://en.wikipedia.org/wiki/Motor_controller 4. http://en.wikipedia.org/wiki/Rashtriya_Ispat_Nigam

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