TO STUDY THE TESTING, TROUBLE SHOOTING OF DC MOTORS

A Mini Project submitted in partial fulfillment of the

Requirement for the award of the degree of

Bachelor of technology

In

Electrical & Electronics engineering

A . Divya Bharathi (08L31A0201)

M. Gouthami (08L31A0246)

B. Rakesh (08L31A0206)

K. Siva Sekhar Kumar (08L31A0232)

Under The Guidance Of

Sri M.PARAMESWARA RAO,

ASST. MANAGER (E),

ERS, RINL-VSP

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

VIGNAN’S INSTITUTE OF INFORMATION TECHNOLOGY

DUVVADA, VISAKHAPATNAM

2011-2012

RASHTRIYA ISPAT NIGAM LIMITED

ELECTRICAL REPAIR SHOP

CERTIFICATE

This is to certify that the Mini Project titled

“TO STUDY THE TESTING, TROUBLE SHOOTING OF DC MOTORS”

Has been done by the following students of 3 / 4 B.Tech (EEE) of “VIGNAN’S INSTITUE OF INFORMATION TECHNOLOGY” in Visakhapatnam ,during the period 30rd May,2011 to 11th June,

2011 under my guidance.

1. A . Divya Bharathi (08L31A0201)

2 M. Gouthami (08L31A0246)

3. B. Rakesh (08L31A0206)

4. K. Siva Sekhar Kumar (08L31A0232)

GUIDE

M.PARAMESWARA RAO,

Asst. Manager ,

ERS/RINL,

VISAKHAPATNAM STEEL PLANT.

ACKNOWLEDGEMENT

We express my heartfelt gratitude to Sri N.SUDHAKAR,DGM, Head of

the Department of ERS,Training & Development Center Authorities and

Management of VISAKHAPATNAM STEEL PLANT for accepting the training

programme.

We have immense pleasure in expressing my sincere thanks and keep

sense of gratitude to my Guide Sri M.PARAMESWARA RAO,

Asst.Manager (ERS), for his everlasting encouragement and cooperation

offered in amiable and pleasant manner through out my project.

Finally, We extend gratitude to our Head of the department and the

Staff Members of E&E department for obtaining the permission to do the

Industrial Training programme in VISAKHAPATNAM STEEL PLANT.

Also we are in deed very thankful to Dr.K.ALICE MARY Prof.

& Head of the department of Electrical &Electronics Engineering,

Vignan’s institute of information, Visakhapatnam for her willingness to

share her valuable knowledge and constantly inspirintg us through her

suggestions.

1. A . Divya Bharathi 2 M. Gouthami 3. B. Rakesh 4. K. Siva Shekar

CONTENTS

Chapter No Description Page No

1. Over view of Visakhapatnam 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

8. Conclusion

CHAPTER-1

Over view of Visakhapatnam 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.

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

CHAPTER-3

Briefing about Sectional functioning of ERS

Planning Section:The functions of planning section are

(a)Receiving: Receives the electrical equipment for repair, overhauling 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 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 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.

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

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

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.

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.

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.

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 in coils made of heavy wire where

a square, or a rectangular section with round corners is used.

CHAPTER-6

TESTING PROCEDURES

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.

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 (BLOCK –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.

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)

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.

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.

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.

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

CHAPTER-8

CONCLUSION

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.

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.