CONTENTS

Chapter No Description Page No

1. Over view of Viskhapatnam Steel Plant

2. About Electrical Repair Shop

3. Briefing about Sectional functioning of ERS

4. Introduction of Electrical Equipment

5. Causes for failure of Windings.

6. Testing procedures.

7. Measuring Instruments available in ERS

Conclusion

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CHAPTER-1

Over view of Viskhapatnam Steel Plant

Visakhapatnam Steel Plant, a prestigious integrated steel plant located in

Visakhapatnam on seashore of Bay of Bengal. As the name indicates it produces a

world class steel with internal coordination between core departments like RMHP,

COCCP, SP, BF, SMS & MILLS also service departments like CRMP, ES&F,

INSTN, ERS, CME, QATD&ETL etc.

VSP is certified to all the three international standards of quality. ISO- 9001

for Quality management, ISO-14001 for Environmental Management system and

OHSAS-18001 for Occupational health and safety. The certificates covers quality

systems of all operational maintenance service units besides purchase systems,

training and marketing functions spreading over four regional marketing offices &22

stock yards located all over country

Visakhapatnam steel plant is operated by so many machinery like conveyors,

rolling stands, dust extract system, casting machines, tilting devices, blowers,

compressors, circulating fans, draught fans, cranes, induction furnaces, heating ovens,

transfer trolleys etc.

Such equipment is driven by electric motors with appropriate protective systems.

Those electric motors are of different types depending on their application and

characteristics like 3phase A.C LT/HT motors (squirrel cage and slip ring type),

Synchronous motors, Turbo generators, D.C motors (shunt, series, compound type)

and AC single phase motors of different types etc.

Besides these rotary machines there are so many stationary machines like power

transformers, inductor coils, control transformers, hot and cold magnets etc.

These motors, though they are protected by suitable protective devices when

subjected to voltage surges or over loads may get damaged badly which leads to

partial or complete rewinding of that equipment.

That defected electrical equipment will send to Electrical Repair Shop (ERS), which

undertakes repair, rewinding and testing of the same.

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CHAPTER-2

ABOUT ELECTRICAL REPAIR SHOP

ERS is popularly known as Electrical Repair Shop, it is under services department,

which undertakes repair, rewinding and testing of all types of electrical equipment.

It consists of different sections to perform different activities namely:

1. Planning

2. Assembling& Dismantling

3. Pre rewinding activities

4. Rewinding (AC LT/HT, DC)

5. Testing

6. Varnishing and Impregnation

7. Maintenance

8. Machine section

9. Small motor division

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CHAPTER-3Briefing about Sectional functioning of ERS

Planning Section:The functions of planning section are

(a)Receiving: Receives the electrical equipment for repair,oveahauling and testing

along with duly filled Work Order & failure report from the customer departments.

Then allot unique ID for that equipment. The work order copy consists of 4 sets

viz.White:for planning Section Office, Yellow: SMD /Planning, Pink: Shift in

Charge room,Blue:Concerned department. The work order copy consists of Name

plate details of the machine, status of the job, repair required, reference w/o no and

details accessories and its condition. According to that spares requirement record will

be prepared.

(b)Scheduling: After giving the work order number, allot the job to the concerned

section for the necessary work to be carried out. As per the received jobs quantity,

monthly schedule will be prepared. According to the customer department urgency,

allot the job on priority. They will conduct MOU meetings to the internal customers.

(c)Failure Analysis: Along with the work order copy customer department submits

failure report. According to that motor failure analysis will be generated and

communicate to the customer department

(d)Dispatch: After completion of all respects of the job, the job will be declared

RFD(Ready for dispatch).The owner department will collect the same along with

final test report.

(e)Generating Reports: In addition to the above functions generating report for

spares,Preperation of Daily,Weekly,Monthly,and Annual reports. Performance report

also generated with highlights of the current year. Keeping all those records for

future reference purpose.

Assembling and Dismantling Section:

Dismantling: After allotment of the job to the A&D section, Inspect the job physical

condition and note the status. Whatever the repair, First dismantle the job and prepare

the FDR(First dismantle report).After check the physical condition of the job,

cleaning will take-up with suitable cleaning agent and compressed air. And shift the

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job to the testing or if any Physical winding problem observed send the job to

PRA(Pre rewinding activities).

Assembling: After completion of all respects of the repair work of stator or rotor,

assembling of the same will be taken up. And completion of assembling the job will

keep in testing for final test.

Pre Rewinding Activities:

This section receives jobs from either A&D section directly or from the

testing section after Preliminary Test. In this section, before strip out the winding

Data collection of the stator/Rotor winding will take up and prepare the Data sheet.

After collection of the data, thoroughly clean the stator/rotor or both and apply

insulation coating on the overhang side of the both ends. Along with Data sheet the

job will be shift to respective rewinding section.

Testing Section:

In the testing section, Three types of tests will be performed.

Preliminary Test:

In this test, after receiving the job first note down the name plate details of the

job. Check the I.R (Insulation Resistance) value w.r.t windings to body and measure

winding resistance also. If both are ok then pass the rated current and check the spot

heat of the winding and field rotation. In addition to that pole formation test,HV test

and surge test will be performed. If the job ok in all respects the job will be declared

ok and advise to apply protective insulation coating. All test reports will be enter in

to the testing record for further reference.

Intermediate Test:

In this test, after repair or re winding the above mentioned tests will be

carried out and noted the same in testing record.

Final Test:

After assembling of the motor, final test will be carried out. Final test is

namely No load running test. In this test rated voltage will be applied and run the

motor as per the duty cycle. And observe the bearing condition, noise

level,vibration,temperature of the motor body etc.If it is a DC motor observe the

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sparking level. In all respects the motor is ok,then finally prepare Test report and the

job will be declared RFD (ready for dispatch).

Varnishing and Impregnation Section:

Varnishes and impregnating compounds are important insulating materials

for electrical machines. They are employed for the dual purpose of moisture proofing

and increasing the dielectric strength of fibrous insulating materials.

After preliminary testing or intermediate test the job will be shifted to

Varnishing and impregnation testing for varnishing and applying finishing on the

winding outer surfaces.

This section performs IR improvement, Varnishing of the rewound stator or

rotor, applying finishing varnish coatings. After completion of the varnishing the job

will be shifted to A&D section for assembling.

Maintenance:

To carry out the work smoothly, ERS is having coil winding machines,EOT

(Electrically Operated over head trolley cranes),testing equipment,oh

lighting,Ovens,winding coil puller, welding machines and other allied equipment.

For maintenance of the above for trouble free performance, this section is operating.

Machine Section:

This section will carry all mechanical works like welding, shaft repairs,

turning of mechanical parts, Over hang Insulation banding ,Magnets preparation etc.

Small Motors division:

Some of the motors like below 11kw (1-Ø,3- Ø motors,fans,blower motors

etc will be given outside party for repair. For maintaining the records and allotment

of jobs to different contract agencies, this section is operating.

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CHAPTER-4

Introduction of Electrical Equipment

Types of Electrical Machines:

Electrical machines are two types.

1.Dynamic devices –Motors/Generators(Motional emf)

2.Static devices-Transformers(Statically induced emf)

The basic structure of an electro magnetic rotating electrical machine consists of the

following parts.

(a)Magnetic circuit: It provides the path for the magnetic flux and consists of air

gap, stator and rotor teeth, and stator and rotor cores(Yokes).

(b)Electric circuit: It consists stator and rotor windings. The winding of a

transformer or a rotating machine conveys electrical energy or from working region

and is concerned with production of emf and development electromagnetic force.

(c)Dielectric circuit: The dielectric circuit consists of insulation required to isolate

one conductor to another and also winding from the core.

(d)Thermal circuit: The thermal circuit is concerned with mode and media for

dissipation of heat produced inside the machine on account of losses.

(e)Mechanical parts: The important mechanical parts of a machine are its frame,

bearings and shaft.

Classification of AC Motors:

With the almost universal adoption of A.C system of distribution of electrical

energy for light and power, the field of application of A.C motors has widened

considerably.

As regards their principle of operation

Synchronous:

(i)Synchronous motors (ii) Synchronous Generators

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Asynchronous:

(a) Induction Motors- (i)Squirrel cage-Single Cage/Double Cage

(ii)Slip-Ring(External Resistance)/Wound rotor motor.

Principle of operation of Asynchronous/Induction motor:

In an induction motor, there is no electrical connection to the rotor, but

currents are induced in the rotor circuit and therefore the rotor conductors carry the

current in the stator magnetic field and thereby have a force exerted up on them

tending to move them at right angles to the field. When the stator or primary winding

of a 3 phase induction motor is connected to a 3 phaseAC supply, a rotating magnetic

field is established which rotates at synchronous speed.

STATOR: -

Stator is made up of number of stampings, which are slotted to

receive the winding. The stator carries a 3-phase winding and is fed from a 3-phase

supply. It is wound for a definite number of poles .The exact numbers of poles is

determined by the requirement of the speed. Greater the number of poles, lesser is the

speed and vice versa. The stator winding when supplied with 3 phase currents,

produce a magnetic flux, which is of constant magnitude but revolves at a

synchronous speed and induces an emf in the rotor by mutual induction

The synchronous speed is given by

NS = (120f)/p

N= synchronous speed

f=supply frequency

p=number of poles

ROTOR: -

a) Squirrel cage rotor: -

About 90% of the motors have squirrel cage type of

construction because rotor is simplest and both rugged construction imaginable and

almost indestructible The rotor consists of cylindrical laminated core with parallel

slots for carrying rotor conductors .The rotor bars are brushed or electrically welded

or bolted to two heavy and stout short circuiting end rings. Here rotor bars are

permanently short circuited on them selves. Hence, it is not possible to add any

external resistance in series with rotor for starting purpose.

The rotor slots are lightly skew as

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It makes the motor run quietly by reducing magnetic hum. It helps in reducing

locking tendency of the rotor. That is the tendency of the rotor teeth to remain

under stator teeth due to the direct magnetic attraction between the two.

Another construction of rotor consists of a solid cylinder of steel

without any conductor or slot at all .The motor operation depends up on the

production of eddy currents in the steel rotor

b) Phase wound rotor: -

This type of winding is provided with 3 phase double

layer distributed winding consisting of coils similar to stator winding .The three

windings are brought out and connected to the three insulated slip ring mounted on

the shaft with brushes resting on them. These brushes are further internally connected

to a 3 phase star connected rheostat. This makes possible the introduction of

additional resistance in rotor circuit during starting conditions. When running the slip

rings are automatically short circuited by means of metallic collar which is perished

on the shaft and connected to the rings.

Starting torque TST = 3 E22 R2

2NS (R22+X2

2)

NS = Synchronous Speed

R2 = Rotor Resistance

E2 = Rotor Voltage

X2 = Rotor Reactance

The supply voltage is constant.

The starting torque is proportional to external resistance and inversely

proportional to impedance but resistance effect dominates. Here after speed is

gathered, the external resistance is cut out.

R=X id the value of resistance improved to attain starting torque.

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Introduction of Additional Resistance in the rotor circuit

The three phase winding is displaced in space by 120

degrees and are fed by three phase current, displaced in time by 120 degrees. They

produce resulting magnetic flux, which rotate in space as if actual magnetic poles are

being rotated mechanically. 1,2and 3 are the fluxes due to three phases and m is

the maximum value of flux. The resultant flux is sum of 1,2and 3.

R =1.5m

The resultant flux is a constant value (1.5m) that is 1.5 times

maximum value of flux due to any phase.

The resultant flux rotates around the stator at synchronous speed

given by N=120f/p.

Graph of Rotating flux

As seen the positions of the resultant phases have been

shown at an interval of 60 degrees only. The resultant flux produces a field rotating

in clockwise direction.

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Principal of rotation of induction motor:

When a three phase stator winding is fed by a three phases

supply then maximum flux of constant value but rotating at synchronous speed set up

the flux passes through the air gap, sweeps past the rotor surface and so cut the rotor

conductors which as yet are stationary .Due to the relative speed between the rotor

flux and the stator conductors, an EMF is induced in the latter part without any

change in frequency. This is according to faradays laws of electromagnetic induction.

Since rotor bars form closed path, rotor current is produced, whose direction is given

by Lenz law is such as to oppose the very cause of producing it .In this case the cause

which produces the rotor current is the relative velocity between the rotating flux of

the stator and the stationary conductors. Hence to reduce relative speed, rotor starts in

the same direction as that of the flux and tries to catch up with the rotating flux.

In practical the rotor never succeed up catching with the stator field if

it really did so then there will be no relative speed between the two. Hence no rotor

current and rotor torque is induced .The difference between synchronous speed and

speed of the rotor is known as slip.

%Slip = ( Ns-N ) *100

Ns

.Introduction of synchronous motor:

A synchronous motor has the same relationship to an alternator as a dc motor

has to a dc generator i.e. if an alternator is supplied ac power it is capable of rotating

as a motor and doing mechanical work. If the mechanical power supplied to a

rotating alternator is removed while dc field remains energized, and an ac supply is

then connected across the armature terminals, torque will be developed and the

alternator will continue to rotate at a speed determined by the ac supply frequency

and the number of poles on the synchronous machine. Changes in mechanical load

with in the machine’s rating will not cause change in speed.

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D.C Motor principle:

If a current carrying conductor is placed in a magnetic field,

mechanical force is experienced on the conductor, the direction of which is given by

Fleming’s left hand rule and hence the conductor moves in the direction of force.

When the motor armature rotates, the conductors also rotate and hence

cut the flux. In accordance with the law of electromagnetic induction, e.m.f is

induced in them whose direction, as found by Fleming’s right hand rule, is in

opposition to the applied voltage, because of its opposing direction; it is referred to as

counter emf/back emf Eb.

Transformer Principle:

A transformer is a static piece of apparatus by means of which electric power

in one circuit is transformed in to electric power of the same frequency in another

circuit. It can raise or lower the voltage in a circuit but with a corresponding decrease

or increase in current. The physical basis of a transformer is a mutual induction

between two circuits linked by a common magnetic flux. A transformer is a device

that transfers electric power from one circuit to another. It does so without change of

frequency. It accomplishes this by electromagnetic induction and where two electric

circuits are in mutual inductive influence of each other.

Electro Magnets:

Electro magnets are used for the purpose of pulling, lifting and holding. The

general principle of operation and design are the same for all types of

electromagnets irrespective of their application.

(a) Core material: Soft materials are used for construction of core of the

electromagnets. Most of these materials contains the Ferro magnetic

materials like iron, nickel and cobalt in various combinations.

(b) Electromagnet coils: Coils are used in electromagnets as an exciting source

for production of magnetic field. A coil usually, consists of wire wound like

a helical thread to form a layer. The usual material for the conductor is

copper. In some cases aluminum is used. The cross section of coil is

generally rectangular and the cross section of the conductor is round except

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in coils made of heavy wire where a square, or a rectangular section with

round corners is used.

CHAPTER-5

CAUSES FOR WINDING FAILURES

A single phased winding failure is the result of an open in one of the

phases that supply power to the motor. The open is usually caused by a bad fuse, bad

connection, a open contactor or a broken power line.

These six photos show insulation failure typically due to vibration,

voltage surge, abrasives, contamination by hostile environments

. Thermal deterioration of insulation in one phase of the winding can

result from unequal voltage between phases. Unequal voltages usually are caused by

unbalanced loads on the power source, a poor connection at the motor terminal, or a

high resistance contact. NOTE: A one-percent voltage unbalance can result in a six to

ten percent current unbalance.

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Thermal deterioration of the insulation in all phases of the stator

winding typically is caused by load demands exceeding the rating of the motor.

.

Severe thermal deterioration of the insulation in all phases of the

motor normally is caused by very high currents in the stator winding due to a locked

rotor condition. It may also occur as a result of excessive starts and reversals.

Insulation failures like this usually are caused by voltage surges.

Voltage surges are often the result of switching power circuits, lightning strikes,

capacitor discharges and solid-state power devices.

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6.1 Good Stator Winding

Unfavorable operating conditions electrical, mechanical or

environmental–can dramatically shorten the life of a three-phase stator winding. The

winding failures illustrated below typify what can happen in such circumstances.

They are shown here to help you identify the causes of failure, so that, where

possible, you may take preventive measures.

Compare the new stator winding (above) with the failed windings pictured below.

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2.2 Winding Single-Phased

(Wye(Y)-Connected)

A single-phased winding failure is the result of an open in one phase of

the power supply to the motor. The open is usually caused by a blown fuse, an open

contactor, a broken power line or bad connections

.

6.3 Winding Phase-to-Phase Shorted

This type of insulation failure is typically caused by contaminants, abrasion,

vibration or voltage surge.

6.2 Winding Single-Phased

(Delta-Connected)

A single-phased winding failure is the result of an open in

one phase of the power supply to the motor. The open is usually caused by

a blown fuse, an open contactor, a broken power line or bad connections.

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2.5 Winding Shorted Turn-to-Turn

This type of insulation failure is typically caused by contaminants,

abrasion, vibration or voltage surge.

2.6 Winding With Shorted Coil

This type of insulation failure is typically caused by contaminants,

abrasion, vibration or voltage surge.

2.7 Winding Grounded at Edge of Slot (a)

This type of insulation failure is typically caused by contaminants,

abrasion, vibration or voltage surge.

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2.8 Winding Grounded at edge of Slot (b)

This type of insulation failure is typically caused by contaminants,

abrasion, vibration or voltage surge.

2.9 Winding Grounded in the Slot

This type of insulation failure is typically caused by contaminants,

abrasion, vibration or voltage surge.

2.10 Shorted Connection

This type of insulation failure is typically caused by contaminants,

abrasion, vibration or voltage surge.

2.11Phase Damage Due to Unbalanced Voltage

Thermal deterioration of insulation in one phase of the stator winding

can result from unequal voltage between phases. Unequal voltages usually are caused

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by unbalanced loads on the power source, a loose connection at the motor terminal,

or a high resistance contact.

Note: A one-percent voltage unbalance may result in a six- to ten-percent current

unbalance.

2.12 Winding Damaged Due to Overload

Thermal deterioration of the insulation in all phases of the stator

winding typically is caused by load demands exceeding the rating of the motor.

Note: Under-voltage and over-voltage will result in the same type of insulation

deterioration.

2.13 Damage Caused by Locked Rotor

Severe thermal deterioration of the insulation in all phases of the

motor normally is caused by very high currents in the stator winding due to a locked

rotor condition. It may also occur as a result of excessive starts or reversals.

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2.14 Winding Damaged by Voltage Surge

Insulation failures like this usually are caused by voltage surges.

Voltage surges are often the result of switching power circuits, lightning strikes,

capacitor discharges and solid-state power devices.

2.15 Motor Rewinding

The life of a three-phase stator winding can be shortened dramatically

when the motor is exposed to unfavorable operating

conditions - electrical, mechanical or environmental. The

winding failures illustrated below are typical of what can

happen in such circumstances. They are shown here to help

in identifying the causes of failure so that, whenever possible,

preventive measures may be taken. A new stator winding is

pictured at right for purposes of comparison. Descriptions of the causes of failure. are

provided below.

A single-phased winding failure is the result of an open in one phase

of the power supply to the motor. A blown fuse, an open contractor, a broken power

line or bad connections usually cause the open circuit.

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These photos illustrate insulation failures that typically are caused by contaminants, abrasion, vibration or voltage surge.

Insulation failures like this usually are caused by voltage surges.

Voltage surges are often the result of switching power circuits, lighting, strikes,

capacitor discharges and solid-state power devices.

Faults Occurring in DC Machines: The probable failure reasons in the dc motors

are identified separately in the field and armature circuit.

Faults in Field winding:

The faults in the field winding may be

(i) an open circuit

(ii) an earth fault or shorting of a coil either completely or some of its

turns.

Location of open circuit fault in field winding:

An open circuit fault in the field winding of dc shunt motor will either cause

in an interruption of supply to the motor owing to operation of over current

protection or tremendous increase in speed if it is running light. Such a fault in dc

series motor will cause the motor to stop. In a dc compound motor the effect will be

depend on whether the break is an series or shunt field winding.

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Location of earth fault in field winding :

An earth fault in the field winding of a motor with effective earth leakage

protection will cause the disconnection of supply to the motor. The presence of earth

fault can be verified by test between end terminal to the motor frame with an

insulation tester. Under these circumstances an earth fault will be indicated by zero

reading.

Location of shorted coil in field winding:

The normal supply is connected to the motor through potentiometer in order

to keep test current through the coils there will be a voltage drop across each coil and

it is measured with the voltmeter.

Faults in armature windings: The possible faults that can be developed in the

armature windings are illustrated below.

6.3.1 Short circuited coil

6.3.2 Open circuited coil

6.3.3 Earthed coil

Location of faults in armature windings: Armature faults can be located by using

special test device known as GROWLER or by applying DROP VOLTAGE test.

Troubles in DC motors: Several troubles may arise in a dc motor and are described

below.

Sparking at brushes: It may be due to troubles in brushes,commutator or armature

or excessive load.

Over heating: It may be due to excessive loading, sparking at brushes, short

circuited field or armature coils, poor ventilation, incorrect voltages or too frequent

starts and reversals.

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CHAPTER-6

TESTING PROCEDURES

Testing of Induction Motor:

Various tests, which are performed on a poly phase induction motor, for

determination of healthiness and its operating characteristics.

Insulation resistance test:

For LT motors(<415Volts) the winding insulation resistance to be

measured with 500 Volts Megger.The winding insulation resistance should be More

than 5MΩ with respect to body. And the same is measured between the each phase.

If the megger reads below the mentioned value, the winding should be thoroughly

cleaned and dried. The stator and slip ring rotor of the induction motor has a three-

phase winding wound on core. Each phase has a starting and ending. Each phase of

the winding is insulated from the other and form the core .To check the perfect ness

of the insulation the insulation resistance is measured .The megger is a piece of

instrument used to measure the insulation resistance .It works on the principle of

dynamo. A liver connected rotor is rotated rotor is rotated in the magnetic field and

current is passed to the testing terminals.

Insulation resistance between core and winding

insulation

winding

A2

B1

A1

B2

core

1 2

The testing terminals of the megger is connected to A1and B1

Initially the megger is under the open circuit condition indicating infinite

resistance between open terminals. After connecting liver is rotated and observed

weather the needle moves to zero then we can assume that there is insulation failure.

Similarly it is tested for windings of different phases.

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Insulation Resistance between phases

1 2

B2

Y

C1

B

A2 C2B1

R1 2

A1

1 2

Testing terminals of megger are connected.

For HT motors(>415Volts i.e. 3.3kv,6.6kvand 11kv) winding insulation

resistance to be measured with >500Volts megger i.e. 1000kv,2.5kv and 5kv.The

winding insulation resistance should be >100MΩ.In addition to that, P. I

Value(Polarization Index) for HT motors to be measured. This test indicates

healthiness of the Insulation.

P.I Value (K)= IR 60Seconds/IR 15 Seconds= >1.3

DC cold resistance( Winding Resistance) @ at ambient temperature:

The stator and slip ring motor has three phase winding wound on it.

Each phase has starting and ending terminals. This test is meant for measuring the

resistance of the three phases .The standard values of the resistances differs with the

rating of the motor. If the three values of the resistances are equal then test is

conformed.

R=V/I

R=resistance measured

I=amount of current passed through the testing terminals

V=amount of voltage applied

Phase to phase, Winding resistance to be measured with Ohm meter or micro

ohm meter. According to the capacity of the motor, type of connection (either star or

delta) the resistance may vary. For LT motors the winding resistance in ohms, where

as in HT motors the resistance will be in milli ohms.

Inductance test:

Rated AC voltage is applied to each and every phase of winding and

amount of current flowing is measured with the help of clamp meter

Z=V/I

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V=sinusoidal voltage applied between phase and neutral

I= current flowing through the winding

XL = √ (Z22-R2

2)

XL=2 f L

L = XL / (2 f)

High Voltage Test:

This test will be performed, to know the dielectric strength of the insulation.

The insulation will be withstand sudden voltage surges and at the time of starting

(high voltages will be induced at the time of starting).

Amount of HV applied is V=2U+1 (Where “U” is rated voltage of the

machine)

Surge Test:

This test will be performed to know the earth fault,inter turn short, reverse

coil connection and phase to phase short.

This test is conducted on surge kit. This kit can supply voltage to the phase

windings individually. It consists of a C.R.O display in it, in order to observe the

waveforms of the current passing through the winding. A surge voltage is applied to

the winding terminals .The surge consists of a train of impulses.

V

t

Impulse diagram

Current is passed through the R, Y, and B phases of the winding and corresponding

wave forms are observed on the CRO screen.

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Surge kit

Good winding

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Inter turn short fault

Short to ground fault

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Open winding

Reverse coil connection

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Phase to Phase Short

This test is sensitive even in detecting a minute fault and the winding in

which fault has occurred. This test is conducted to the single turn and can detect the

fault turn.

1 2

R

ia/2

B

Y1 2

ia/21 2

ia

Fig 4.5:- KCL Application in Winding

Current is passed to phase the R phase of the winding. Then current

returns through Y&B phases of the winding.

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The waveforms of the currents flowing through Y&B phases are observed in a duel

mode simultaneously. These two waveforms are superimposed on one other. So if the

two waveforms are exactly same, then it appears to be a single waveform.

Current balance test:

Sinusoidal voltage corresponding to the rated current is applied across

the phases of the winding that is RY, YB, BR and currents flowing in phases R, Y, B

are noted .If the values represents approximately the same, then the test is

conformed. If not re winding to be done.

Field rotation:

Three-phase supply is connected to the three phases of the winding and the

formation of the field is tested. A needle made of magnetic material indicating north

and south is mounted on a non magnetic bar .The arrangement is such that it the

needle is pivoted on the non magnetic bar and is free to rotate .when ever this bar

with needle is placed in the synchronous rotating magnetic needle. It rotates as the

rotor rotates in the motor.

Pole formation:

Only two phases are connected to the terminals of the three-phase supply. Then

one winding will be kept unexcited, hence continuous magnetic field will not be

developed, and then the number of poles formed can be measured.

Spot Heat: The three phase winding of rotor (or) stator is connected to power

supply. The winding will draw rated current at nominal voltage i.e. around 30-40%

of the rated voltage and keep on the supply for 1 minute for identifying the spot heat.

The internal connections of windings are not tightly joined then that spot will

be heated. The temperature on the core is sensed with hand .If any part is having a

high temperature than the winding at that spot is not good. The fault is detected by

surge test kit also.

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In this test the winding healthiness is checked. Full load current is

passed through the windings and the temperature at different parts of the stator or

rotor is sensed. If the joints of the winding or not correctly binded, then heat loss

takes place. The difference in temperature indicates the faulty spot. By this test the

exact position of faulty winding is detected.

Lead marking or Phase sequence:

It determines the starting and ending terminals of the winding .The

voltage v is applied to the two windings.

Fig 4.6: Lead Marking

If voltmeter reads V then A2 is ending and A3 is starting. Otherwise if voltmeter

reads zero then both terminals are either starting or ending.

No load running test:

This test is conducted on over hauling motor or completely

assembled motor. In this test the three phase supply is connected to the terminal and

currents in the phases are noted with the help of a clamp meter.

1. Speed is determined with the help of tachometer

2. Temperature of the motor is checked

3. Bearing conditions checked

4. Vibrations are checked

The no load test is performed with different values of applied voltage below and

above rated voltage, while the motor is running light(without load)

Voltage ratio test:

This test can only performed on a wound rotor motor by exciting the stator

winding at rated voltage and frequency (the rotor circuit being kept open –circuited

and stand still).The ratio of rotor to stator voltage can be measured by means of volt

meter. It is to be noted that emf appearing at the slip rings (induced emf in rotor )is of

the supply frequency because the rotor is at the standstill.

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V

Blocked rotor test:

This test is performed to determine the short circuit current Isc with normal

applied voltage to stator. In this test rotor is held firmly (rotor windings are short

circuited at slip rings in case of wound rotor motor )and stator is connected across

supply of variable voltage. This test is just equivalent to SC test on transformer.

Heat Run Test(Temperature rise Test):

The life of the insulation of the electrical equipment depends up on the

temperature attained during operation. The objective of this test is to find out the

actual maximum temperature attained while the machine is operating under certain

load conditions. The temperature is measured both while the motor is operating and

after its shutdown.

The above mentioned test will be performed for slip ring and squirrel cage motors

according to their nature of construction.

Testing of Transformers:

The performance characteristics of a transformer can be determined by

conducting simple tests are called the open circuit test or no load test and short

circuit test or impedance test involving very little power consumption. Other tests to

be conducted on transformers are polarity test and voltage ratio test.

Testing of polarity:

Polarity test is performed to determine the terminals having the same

instantaneous polarity (assuming the terminals are not marked).Polarity test in the

field can be conveniently carried out by using a dc battery, a switch and a dc volt

meter. The switch on the primary side is closed, the primary current increases, and so

do the flux linkages of both the windings ,inducing emfs in them. The positive

polarity of this induced emf in the primary is at the end to which the battery is

connected (according to Lenz’s law).The end of secondary which simultaneously

acquires positive polarity, as determined by the dc voltmeter is the similar polarity

end. The reverse happens on opening of the switch i.e. the similar polarity end is that

end which acquires negative potential.

Voltage Ratio Test:

The true ratio is based on turn-ratio. If the secondary and primary voltages are

measured on no load ,their ratio is very nearly to the true value. Measurement of

primary and secondary currents in short circuit test also gives fairly accurate results

voltage ratio (V2/V1=I2/I1).

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Open circuit Test:

There are four main parameters in a transformer. They are

1. Equivalent resistance

2. Equivalent leakage reactance

3. Core loss conductance

4. Magnetizing susceptance

These parameters can be determined by two tests.

1. Open-Circuit test

2. Short circuit test

Open circuit test:

In this test we can find core loss and no load Io One winding of the

transformer (High voltage winding) is left open and other is connected to supply

voltage. A wattmeter W and voltmeter V and an ammeter A are connected low

voltage winding. When the voltage applied normal flux will be setup in the core.

The wattmeter reading shows the iron losses.

V1

V 2

W=V1Io cos oI= Io sino, Iw= Io cos oXo = V1/ I and Ro=V1/Iw

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A

V

Short circuit test:

In this test we can find

1. Equivalent impedance (Zo1 or Zo2) leakage reactance (Xo1 or Xo2) and total

resistance.

2. Copper loss at full load

In this test one winding usually low voltage winding is short-circuited. A low

voltage (5 to 10 % of primary voltage) at rated frequency is applied to the

primary and is increased till full load current flows in the primary and

secondary.

2Copper loss W=I1 Ro1

2

Ro1 = W/I 1

Xo1 =(Zo1 – Ro1)

V1

DC Motor testing: After repair, testing section will perform healthiness of the field

winding and armature winding separately. After assembling No load test and Load

test for some of the motors also to be performed.

Testing of field winding:

For the field winding, first check the IR values with the 500 volts

megger.Field winding consists of Main poles, inter poles, series winding and

compensatory winding. With the help of megger,IR value to be checked all the

windings wrt frame, and between windings also. If the IR value is <0.75MΩ@75°C

the windings should be cleaned and dried properly.

Apply the rated field voltage and check the field current how much the

windings drawn. And also check the polarity. Keep the power for 2-3 minutes and

measure the temperature of the field windings.

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A

V

Testing Armature winding:

Armature is having commutator and its winding. Before going to rewinding

of the armature, Commutator healthiness to be checked by Lamp test method. In this

test commutator segment short if any will be noticed. In addition to that wrt body IR

value to be checked. And HV test also conducted @1kv dc for 1 minute time.

After completion of the armature rewinding, Passing 25-30% of rated dc

current will be given for drop voltage test to check the healthiness of the armature.

In all respects the winding is ok then declared for varnishing.

After assembling the field and armature, no load test will be performed.

MNA position test:

The position of magnetic neutral axis is found by using brush rocker setter or

MNA meter. If the position of the magnetic neutral axis is deviated, then by adjusting

the position of the slip rings, MNA is brought to the correct position.

a) Field supply because flux is inversely proportional to speed.

b) The armature voltage is gradually raised to its rated value and checked for

any damage in resistance.

c) Sparking level

d) The speed in RPM

e) Bearing noise.

f) Vibration

HOPKINSON’S TEST (BACK –TO- BACK TEST)

By this method, full-load test can be carried out on two identical

shunt machines. There are two machines are mechanically coupled and are so

adjusted electrically that one of them runes as motor and other as generator the motor

drives the generator and the electrical out put of the generator is feed to the input of

the motor

If there were no losses in the ‘machines they would have run

without any electrical power supply but due to the losses, generator output is not

sufficient to drive the motor. The loss are supplied either by an extra motor which is

belt connected to the M G Set or electrically from the supply mains.

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A2 I2 I1 S I1

(I1+I2)A4 V1 A1 A3

I4

R2 R1

M G

Machine ‘M’is started up from the supply mains with the help of a starter. Switch

‘S’ is kept open . its speed ;is adjusted to normal value by means of field regulator

the motor drives the machine ‘G’ as generator and its ;voltage is read on ‘V1’. The

voltage of ‘G’ is adjusted by its field regulator until ‘V1’ reads zero their by showing

that its voltage is same, in polarity and magnitude with respective to the; main supply

Then ‘S’ is closed to parallel to the machines . By adjusting the respective field

regulators any load can now be thrown on to the machines . Generator current ‘I1’

can be adjusted to any desired value by increasing the excitation of ‘G’ or by

reducing the excitation of ‘M’ and the corresponding values of different ammeters

read.The electrical out put of a generator plus the small power taken from the supply,

is taken by the motor and is given out as mechanical power after supplying the motor

losses.

Motor in put =V (I1+I2)

Generator output =VI1

Assuming that both machine have the same efficiency

Output of motor = x input = V (I1+I2) = Generator input

Out put of the generator = x input = x V (I1+I2)

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TESTING OF LT &HT MOTORS

Procedure: - planning section receives the electric motors in ERS, for medium

repairs and delivers in to testing section for preliminary test

1) Testing section carries out the external inspection checking of presence of

all components and tight ness of connections etc

2) Testing section also carry out checking for reliability and tightening of all

threaded connections lead terminal conditions, presence of protective

covers, casing and packing etc.

3) In case of slip ring motors slip ring condition is checked for fitting,

varnishing, paints etc.if the condition is not ok then it is recommends for

slip ring turning.

4) Condition of brushes and brush holder is checked, width of the brush

should not exceed the width of slip ring

5) Before starting of machine, small voltage for short time is given for

checking balance current in lines and proper direction of rotation.

Subsequently supply is raised to nominal voltage

6) Bearing condition and temperature raise of bearings are checked. By

rotating the shaft of the motor physically free rotation of machine is

checked. Noise of bearings, brushes etc if any are observed

7) Line currents, voltages, speed and other measurements are recorded on

test record

8) Any defects raised during or indicated in daily report testing

9) Measurement of insulation resistance value between windings and with

reference to body are measured .For stator winding insulation resistance

values between phase windings, phase to earth are measured with

megger .In case of slip ring rotors winding IR value with respect to stator

windings, earth, slipring insulation with respect to body and between

rings are measured. Continuity of winding is also measured

10) For LT motors measuring IR value with 500v megger .IR value

should be more than 1 mega ohm .If IR value is less then 1 mega ohm, it is

indicated in daily report testing and recommended for IR improvement .Job

is shifted to concerned section for IR improvement.

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Stator and rotor winding resistance measurement is done with bridge or

micrometer during the above testing process, any defects or non conformity of any

device is observed, then the job is declared as not ok or ok and it is indicated in daily

report testing. Concern section will rectify the defect and keep the job for testing.

In case of HT motors partially repair, after removal of faulty coil, rest of

the coils are subjected to high voltage. The group of pitch coil lifted to remove faulty

coils will be tested after placing separately. In case of full winding repair to

placement of coils on the stator or rotor few coils at random are to be subjected to

high voltage. During complete repair process, HV is conducted in batches of 4 to 6

coils after placing all other coils, which are not under test, are earthed. After

placement of all coils and rewinding, HV test is conducted.

For conducting high voltage test IR value of the coil should not be less

then 50-mega ohm. This is measured with 2.5kv megger. If IR values less then 50-

mega ohm the coil or winding is heated. High voltage should be as per following

norms adopted for the 6.6volts.After the completion of the rewinding of job the

following tests are conducted.

a) IR value between phases and phase to earth are measured .It should not

be less then 50 mega ohm

b) Winding resistance of each phase is measured with micro oh m meter

c) Uniform magnetic field is obtained three phases AC supply is given to

the winding. This is checked with a magnetic compass or needle. This

needle rotates uniformly, close to around the core

d) Surge test

e) Current balancing, high voltage test etc.

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CHAPTER-7

MEASURING INSTRUMENTS

The various measuring instruments are using in ERS are:

1. Tong tester (clamp meter): This meter works on the principle of induction.

This meter can measure AC and DC voltage, AC and DC current, resistance

of order ohms, continuity, and temperature.

2. Milli ohmmeter: This is use to measure the resistance of conducting

materials.

3. Megger: It has an in-built dynamo, which produces high voltages of 500 V,

1KV, 2.5KV; 5KV.This is used to measure high resistances (insulators).

4. LCR meter: This is use to measure inductance, capacitance as well as

resistance.

5. Digital tachometer: This is use to measure the speed of the rotor in RPM.

6. Temperature measuring instruments: Various temperature-measuring

instruments like thermocouple, laser thermometer are used to measure the

temperature.

7. SPA meter: Vibration level or baring analysis is carried out using SPA

(shock pulse analyzer) meter. Data regarding speed, bearing type is fed into

this meter and the condition of bearing is checked.

8. Magnetic needle/compass: This is used to check pole formation, rotating

field.

9. Polarity tester: This is used for terminal identification.

10. Surge kit: this kit is used to carry out surge test.

11. HV kit: This kit is used to carry out Hi-potential test.

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MEASURING INSTRUMENTS

The various measuring instruments used for repairing DC machine are:

1. Tong tester (clamp meter): this meter works on the principle of induction. This

meter can measure AC and DC voltages, AC and DC current, resistance of order

ohms, continuity, and temperature.

2. Milli ohmmeter: this is use to measure the resistance of conducting materials.

3. Megger: It has an in built dynamo, which produces high voltages of 500V, 1KV,

2.5KV, and 5KV. This is used to measure high resistances (insulators).

4.LCR meter: This is used to measure inductance, capacitance as well as

resistance.

5.Capacitance meters: Capacitance is measured using these capacitor meters.

6.Multi meter (AVO meter): this is use to measure resistance, voltage,

current and also other parameters like diode current.

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7.Digital Tachometer: this is used to measure the speed of the rotor in RPM.

8.Temperature measuring instruments: Various temperature-measuring

instruments like thermocouple, laser, and thermometer are used to measure

the temperature.

LASERGUN

9.SPA meter: vibration level of baring analysis is carried out using SPA

(shock pulse analyzer) meter. Data regarding speed, bearing type is fed into

this meter and the condition of bearing is checked.

10.Brush rocket setter: the position of magnetic neutral axis is found using

brush rocket setter.

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11.Magnetic needle/ Compass: this is used to check the pole formation, rotating

field.

12.Growler tester: this kit is used to carry out rotor open circuit test and also for

polarity test.

13.Polarity tester: this is used for terminal identification.

14.Surge kit: this kit is used to carry out surge test.

15.HV kit: this kit is used to carry out Hi-potential test.

16.Gauss meter: this instrument is used to measure the magnetic strength.

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CONCLUSION

This report deals about the “To study the testing procedures of

various electrical equipment” We are in the modern world. The utility of

electricity and electrical equipment is much more increased. To know the

usage and performance of the electrical machines, studying the theory of

operation and procedures of electrical machines are not enough.

In addition to the theoretical knowledge, practical knowledge is

also needed to know the operating procedure, characteristics of the different

machines to deal and use the equipment in optimum level in any industry or

organization.

In this regard, I physically observed the different parts of the

machine (in dismantled condition of the various AC/DC motors) repair

procedures, trouble shooting, maintenance, testing of various electrical

equipments in ERS department of VSP.

By putting utmost observation of testing of electrical machines to

know the different types of tests performed in the every event of certain repair

like preliminary test for fault identification, intermediate test conducted after

re winding of stator/rotor, field or armature. Final test for total completion of

the repair of the motor and generating the test reports of various tests.

I conclude that, rather then class room study, in this project we

learn and gain the practical knowledge about the different electrical machines.

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