Electrical Basic Interview Question

7/23/2019 Electrical Basic Interview Question

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Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:

• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it

causes less losses.

• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times,

that is why current also reduces up to 3 times and hence less motor urning is caused.

• !n addition, starting tor"ue is increased and it prevents the damage of motor winding.

State the difference etween generator and alternator

#enerator and alternator are two devices, which converts mechanical energy into electrical energy.

$oth have the same principle of electromagnetic induction, the only difference is that their

construction. #enerator persists stationary magnetic field and rotating conductor which rolls on thearmature with slip rings and rushes riding against each other, hence it converts the induced emf into

dc current for e%ternal load whereas an alternator has a stationary armature and rotating magnetic

field for high voltages ut for low voltage output rotating armature and stationary magnetic field is

used.

Why &' systems are preferred over (' systems?

(ue to following reasons, &' systems are preferred over (' systems:

a. !t is easy to maintain and change the voltage of &' electricity for transmission and distriution.

. )lant cost for &' transmission *circuit rea+ers, transformers etc is much lower than the e"uivalent

(' transmission

c. rom power stations, &' is produced so it is etter to use &' then (' instead of converting it.

d. When a large fault occurs in a networ+, it is easier to interrupt in an &' system, as the sine wave

current will naturally tend to ero at some point ma+ing the current easier to interrupt.

/ow can you relate power engineering with electrical engineering?

)ower engineering is a su division of electrical engineering. !t deals with generation, transmission

and distriution of energy in electrical form. (esign of all power e"uipments also comes under power

engineering. )ower engineers may wor+ on the design and maintenance of the power grid i.e. called

on grid systems and they might wor+ on off grid systems that are not connected to the system.

What are the various +ind of cales used for transmission?

'ales, which are used for transmitting power, can e categoried in three forms:

• 0ow-tension cales, which can transmit voltage upto 1222 volts.

• /igh-tension cales can transmit voltage upto 3222 volts.

• Super tension cales can transmit voltage +5 to 13 +5.

Why ac+ emf used for a dc motor? highlight its significance.



6he induced emf developed when the rotating conductors of the armature etween the poles of

magnet, in a (' motor, cut the magnetic flu%, opposes the current flowing through the conductor,

when the armature rotates, is called ac+ emf. !ts value depends upon the speed of rotation of the

armature conductors. !n starting, the value of ac+ emf is ero.

What is slip in an induction motor?

Slip can e defined as the difference etween the flu% speed *7s and the rotor speed *7. Speed of

the rotor of an induction motor is always less than its synchronous speed. !t is usually e%pressed as a

percentage of synchronous speed *7s and represented y the symol 8S9.

%plain the application of storage atteries.

Storage atteries are used for various purposes, some of the applications are mentioned elow:

• or the operation of protective devices and for emergency lighting at generating stations and

sustations.

• or starting, ignition and lighting of automoiles, aircrafts etc.

• or lighting on steam and diesel railways trains.

• &s a supply power source in telephone e%change, laoratories and road casting stations.

• or emergency lighting at hospitals, an+s, rural areas where electricity supplies are not possile.

;. %plain advantages of storage atteries

ew advantages of storage atteries are mentioned elow:• <ost efficient form of storing energy portaly.

• Stored energy is availale immediately ecause there is no lag of time for delivering the stored

energy.

• =eliale source for supply of energy.

• 6he energy can e drawn at a fairly constant rate.

12. What are the different methods for the starting of a synchronous motor.

Starting methods: Synchronous motor can e started y the following two methods:

• $y means of an au%iliary motor: 6he rotor of a synchronous motor is rotated y au%iliary motor. 6henrotor poles are e%cited due to which the rotor field is loc+ed with the stator-revolving field and

continuous rotation is otained.

• $y providing damper winding: /ere, ar conductors are emedded in the outer periphery of the rotor

poles and are short-circuited with the short-circuiting rings at oth sides. 6he machine is started as a

s"uirrel cage induction motor first. When it pic+s up speed, e%citation is given to the rotor and the rotor

starts rotating continuously as the rotor field is loc+ed with stator revolving field.

11. 7ame the types of motors used in vacuum cleaners, phonographicappliances, vending machines, refrigerators, rolling mills, lathes, powerfactor improvement and cranes.



ollowing motors are used: -

• 5acuum cleaners- >niversal motor.

• )honographic appliances /ysteresis motor.

• 5ending machines Shaded pole motor.

• =efrigerators 'apacitor split phase motors.• =olling mills 'umulative motors.

• 0athes (' shunt motors.

• )ower factor improvement Synchronous motors.

1. State 6hevenin9s 6heorem:

&ccording to thevenin9s theorem, the current flowing through a load resistance

'onnected across any two terminals of a linear active ilateral networ+ is the ratio open circuit voltage

*i.e. the voltage across the two terminals when =0 is removed and sum of load resistance and

internal resistance of the networ+. !t is given y 5oc @ *=i A =0.

13. State 7orton9s 6heorem

6he 7orton9s theorem e%plains the fact that there are two terminals and they are as follows:

• Bne is terminal active networ+ containing voltage sources

• &nother is the resistance that is viewed from the output terminals. 6he output terminals are

e"uivalent to the constant source of current and it allows giving the parallel resistance.

6he 7orton9s theorem also e%plains aout the constant current that is e"ual to the current of the short

circuit placed across the terminals. 6he parallel resistance of the networ+ can e viewed from the

open circuit terminals when all the voltage and current sources are removed and replaced y theinternal resistance.

14. State <a%imum power transfer theorem

6he <a%imum power transfer theorem e%plains aout the load that a resistance will e%tract from the

networ+. 6his includes the ma%imum power from the networ+ and in this case the load resistance is

eing is e"ual to the resistance of the networ+ and it also allows the resistance to e e"ual to the

resistance of the networ+. 6his resistance can e viewed y the output terminals and the energy

sources can e removed y leaving the internal resistance ehind.

1C. %plain different losses in a transformer.

6here are two types of losses occurring in transformer:

• 'onstant losses or !ron losses: 6he losses that occur in the core are +nown as core losses or iron

losses. 6wo types of iron losses are:

o eddy current loss

o /ysteresis loss.

6hese losses depend upon the supply voltage, fre"uency, core material and its construction. &s long

as supply voltage and fre"uency is constant, these losses remain the same whether the transformer is

loaded or not. 6hese are also +nown as constant losses.

• 5ariale losses or copper losses: when the transformer is loaded, current flows in primary and



secondary windings, there is loss of electrical energy due to the resistance of the primary winding,

and secondary winding and they are called variale losses. 6hese losses depend upon the loading

conditions of the transformers. 6herefore, these losses are also called as variale losses.

1. %plain different types of (.' motors? #ive their applications(ifferent type of (' motors and their applications are as follows:-

• Shunt motors: !t has a constant speed though its starting tor"ue is not very high. 6herefore, it is

suitale for constant speed drive, where high starting tor"ue is not re"uired such as pumps, lowers,

fan, lathe machines, tools, elt or chain conveyor etc.

• Service motors: !t has high starting tor"ue D its speed is inversely proportional to the loading

conditions i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. 6herefore,

motor is used in lifts, cranes, traction wor+, coal loader and coal cutter in coalmines etc.

• 'ompound motors: !t also has high starting tor"ue and variale speed. !ts advantage is, it can run at

7!0 loads without any danger. 6his motor will therefore find its application in loads having high inertiaload or re"uiring high intermittent tor"ue such as elevators, conveyor, rolling mill, planes, presses,

shears and punches, coal cutter and winding machines etc.

1E. %plain the process of commutation in a dc machine. %plain what areinter-poles and why they are re"uired in a dc machine.

'ommutation: !t is phenomenon when an armature coil moves under the influence of one pole- pairF it

carries constant current in one direction. &s the coil moves into the influence of the ne%t pole- pair, the

current in it must reverse. 6his reversal of current in a coil is called commutation. Several coils

undergo commutation simultaneously. 6he reversal of current is opposed y the static coil emf andtherefore must e aided in some fashion for smooth current reversal, which otherwise would result in

spar+ing at the rushes. 6he aiding emf is dynamically induced into the coils undergoing commutation

y means of compoles or interpoles, which are series e%cited y the armature current. 6hese are

located in the interpolar region of the main poles and therefore influence the armature coils only when

these undergo commutation.

1G. 'omment on the wor+ing principle of operation of a single-phasetransformer.

Wor+ing principle of operation of a single-phase transformer can e e%plained as

&n &' supply passes through the primary winding, a current will start flowing in the primary winding.

&s a result, the flu% is set. 6his flu% is lin+ed with primary and secondary windings. /ence, voltage is

induced in oth the windings. 7ow, when the load is connected to the secondary side, the current will

start flowing in the load in the secondary winding, resulting in the flow of additional current in the

secondary winding. /ence, according to araday9s laws of electromagnetic induction, emf will e

induced in oth the windings. 6he voltage induced in the primary winding is due to its self inductance

and +nown as self induced emf and according to 0ene9s law it will oppose the cause i.e. supply

voltage hence called as ac+ emf. 6he voltage induced in secondary coil is +nown as mutually

induced voltage. /ence, transformer wor+s on the principle of electromagnetic induction.



1;. (efine the following terms:-

• =eliaility,• <a%imum demand,• =eserve-generating capacity,

• &vailaility *operational.

=eliaility: !t is the capacity of the power system to serve all power demands without failure over long

periods.

<a%imum (emand: !t is ma%imum load demand re"uired in a power station during a given period.

=eserve generating capacity: %tra generation capacity installed to meet the need of scheduled

downtimes for preventive maintenance is called reserve-generating capacity.

&vailaility: &s the percentage of the time a unit is availale to produce power whether needed y the

system or not.

2. <ention the disadvantages of low power factor? /ow can it eimproved?

(isadvantages of low power factor:

• 0ine losses are 1.CE times unity power factor.

• 0arger generators and transformers are re"uired.

• 0ow lagging power factor causes a large voltage drop, hence e%tra regulation e"uipment is re"uired

to +eep voltage drop within prescried limits.

• #reater conductor sie: 6o transmit or distriute a fi%ed amount of power at fi%ed voltage, the

conductors will have to carry more current at low power factor. 6his re"uires a large conductor sie.

1. State the methods of improving power factor?

<ethods of improving power factor:

• $y connecting static capacitors in parallel with the load operating at lagging power factor.

• & synchronous motor ta+es a leading current when over e%cited and therefore ehaves li+e a

capacitor.

• $y using phase advancers to improve the power factor of induction motors. !t provides e%citing

ampere turns to the rotor circuit of the motor. $y providing more ampere-turns than re"uired, the

induction motor can e made to operate on leading power factor li+e an overe%cited synchronousmotor.

. State the factors, for the choice of electrical system for an aero turine.

6he choice of electrical system for an aero turine is guided y three factors:

• 6ype of electrical output: dc, variale- fre"uency ac, and constant- fre"uency ac.

• &ero turine rotational speed: constant speed with variale lade pitch, nearly constant speed with

simpler pitch- changing mechanism or variale speed with fi%ed pitch lades.

• >tiliation of electrical energy output: in conHunction with attery or other form of storage, or

interconnection with power grid.



3. What are the advantages of 5S' wind electrical system?

&dvantages of 5S' wind electrical system are:

• 7o comple% pitch changing mechanism is needed.

• &ero turine always operates at ma%imum efficiency point.

• %tra energy in the high wind speed region of the speed duration curve can e e%tracted

• Significant reduction in aerodynamic stresses, which are associated with constant speed

operation.

4. %plain the terms real power, apparent power and reactive power for accircuits and also the units used.

• =eal )ower: !t is the product of voltage, current and power factor i.e. ) I 5 ! cos H and asic unit of

real power is watt. i.e. %pressed as W or +W.

• &pparent power: !t is the product of voltage and current. &pparent power I 5 ! and asic unit of

apparent power is volt- ampere. %pressed as 5& or J5&.

• =eactive )ower: !t is the product of voltage, current and sine of angle etween the voltage and

current i.e. =eactive power I voltage K current K sinH or =eactive power I 5 ! sin H and has no other

unit ut e%pressed in 5&= or J5&=.

C. (efine the following: &verage demand, <a%imum demand, (emandfactor, 0oad factor.

• &verage (emand: the average power re"uirement during some specified period of time of

considerale duration is called the average demand of installation.

• <a%imum (emand: 6he ma%imum demand of an installation is defined as the greatest of all the

demand, which have occurred during a given period. !t is measured accordingly to specifications, over

a prescried time interval during a certain period.

• (emand actor: !t is defined as the ratio of actual ma%imum demand made y the load to the rating

of the connected load.

• 0oad actor: !t is defined as the ratio of the average power to the ma%imum demand.

. %plain forward resistance, static resistance and dynamic resistance ofa pn Hunction diode.

• orward =esistance: =esistance offered in a diode circuit, when it is forward iased, is called

forward-resistance.

• (' or Static =esistance: (' resistance can e e%plained as the ratio of the dc-voltage across the

diode to the direct current flowing through it.

• &' or (ynamic =esistance: !t can e defined as the reciprocal of the slope of the forward

characteristic of the diode. !t is the resistance offered y a diode to the changing forward current.

E. /ow does Lener phenomenon differ from &valanche rea+down?

6he phenomenon when the depletion region e%pands and the potential arrier increases leading to avery high electric field across the Hunction, due to which suddenly the reverse current increases under



a very high reverse voltage is called Lener effect. Lener-rea+down or &valanche rea+down may

occur independently or oth of these may occur simultaneously. (iode Hunctions that rea+down

elow Cv are caused y Lener ffect. Munctions that e%perience rea+down aove Cv are caused y

avalanche-effect. 6he Lener-rea+down occurs in heavily doped Hunctions, which produce narrow

depletion layers. 6he avalanche rea+down occurs in lightly doped Hunctions, which produce widedepletion layers.

G. 'ompare M69s and <BS69s.

'omparison of M69s and <BS69s:

• M69s can only e operated in the depletion mode whereas <BS69s can e operated in either

depletion or in enhancement mode. !n a M6, if the gate is forward-iased, e%cess-carrier inHunction

occurs and the gate-current is sustantial.

• <BS69s have input impedance much higher than that of M69s. 6hus is due to negligile small

lea+age current.• M69s have characteristic curves more flat than that of <BS6 is indicating a higher drain

resistance.

• When M6 is operated with a reverse-ias on the Hunction, the gate-current !# is larger than it

would e in a comparale <BS6.

32. %plain thin film resistors and wire-wound resistors

a. 6hin film resistors- !t is constructed as a thin film of resistive material is deposited on an insulating

sustrate. (esired results are otained y either trimming the layer thic+ness or y cutting helical

grooves of suitale pitch along its length. (uring this process, the value of the resistance is monitoredclosely and cutting of grooves is stopped as soon as the desired value of resistance is otained.

. Wire wound resistors length of wire wound around an insulating cylindrical core are +nown as

wire wound resistors. 6hese wires are made of materials such as 'onstantan and <anganin ecause

of their high resistivity, and low temperature coefficients. 6he complete wire wound resistor is coated

with an insulating material such as a+ed enamel

31. What is a differential amplifier? &lso, e%plain '<==.

(ifferential &mplifier: 6he amplifier, which is used to amplify the voltage difference etween two input-

lines neither of which is grounded, is called differential amplifier. 6his reduces the amount of noise

inHected into the amplifier, ecause any noise appearing simultaneously on oth the input-terminals as

the amplifying circuitry reHects it eing a common mode signal.

'<==: !t can e defined as the ratio of differential voltage-gain to common made voltage gain. !f a

differential amplifier is perfect, '<== would e infinite ecause in that case common mode voltage

gain would e ero.

What is the Scope of Electrical Engineering?

[ This question is mostly asked to check your communication. Speak

casually. Tell the importance of electrical and electronics engineering



course, what are the topics to be covered in the course, what are the job

prospects etc.]

nswer! "lectrical "ngineering has a very broad scope with the science

and technology which involves electricity, electronics, and

electromagnetism in order to design, construct, and maintain products,

services, and information systems.

"lectrical "ngineering is a branch that deals with the #eld associated with

power transmission and motor control.

"lectrical engineering is one of the core #elds of engineering, so it is

always evergreen in terms of job prospects.

"lectrical "ngineering is the heart of whole engineering discipline.

$owadays, we see "lectronics playing an important role in the

implementation and control of "lectrical %achines. &eeping this fact #le in

mind the course have transformed from core electrical "ngineering into a

'usion of "lectronics and "lectrical and has been renamed as "lectronics

and "lectrical engineering.

"lectricity has become the basic need for survival, globally. 'rom

household to industrial plants, communication and satellite navigation

system, electronic equipment, computers etc., all require electricity."lectrical engineering thus deals with study and application of electrical

systems for use in these di(erent environments. )t equips you with the

knowledge of transmission and generation of electrical power, electrical

circuit design, electronics, instrumentation, control system, understanding

electrical and electronic networks etc.

The course also covers the study of electronic devices and circuits

involved in measurement, instrumentation, control and protection of

electrical equipments and conversion systems. *oncept of computer and

recent applications of computer based systems in design, analysis and

e+cient operation of power system, maintaining quality and security, also

included in the course.

What does '230V AC Supply' really mean? Is it the RMS or peak

olta!e?

)f the peak value is meant it should be clearly stated, otherwise assume it

is the RMS alue. The supply is -/ * means -/ 0%S, so the peak

voltage of the mains is about -/.



What does the multimeter in olt meter mode sho"? Is it the RMS

or peak olta!e?

)f the multimeter is in voltmeter mode and when it connects to * circuits

it shows the RMS alue of the voltage or current. 1hen it connects to

varying 2* signals it will show 0%S value only.

#o" does Indu$tion Motor "ork?

)nduction motors are also known as rotating transformer.3 4rimary winding

is stator and secondary winding is rotor5.

1hen the - phase stator windings are fed by - phase supply a magnetic

6u7 of constant magnitude, rotating at synchronous speed is generated.

These 6u7 passes through the air gap, sweeps past the rotor surface, cuts

stationary rotor conductors.ccording to 'araday8s laws of electro9magnetic induction, an emf is

induced in the rotor. This emf frequency is same as supply frequency.

s rotor conductors form a closed circuit, the induced emf produces a

current. )t8s direction will oppose the cause producing it 3 according to

len:8s law5.

)n this case, the cause is relative velocity between the rotating 6u7 of the

stator and stationary rotor conductors. So to reduce the relative speed,

the rotor starts to run in the same direction of the stator 6u7.

Special motors (Permanent Magnet Synchronous Motor) Interview

Questions

Special Machines (Permanent Magnet Synchronous Motor) Interview

Questions

N1O What is the )<S<?

!n medium and small sie synchronous motors, (' field can e produced y

permanent magnets. Such +ind of motors are called as )<S<

NO What are the advantages and disadvantages of )<S<

Avantages

limination of field copper loss

/igher power density

0ower rotor inertia

=oust construction

!isavantages

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)ower rating low

<otor power factor can not e controlled ecause of field e%citation cannot e

controlled

N3O What are the classification of )<S<

$ased on construction

Surface mounted )<S<

!nterior mounted )<S<

$ased on the nature of voltage induced

Sinusoidally e%cited

6rapeoidally e%cited

N4O #ive some applications of )<S<

=oots

<achine tools

NCO (efine tor"ue angle

6he angle etween e%citation emf and supply voltage. 6his angle is called tor"ue

angle or load or power angle .

NO What are the different classifications of synchronous motor?

*i Wound field synchronous motor

'ylindrical rotor construction Salient pole rotor construction

*ii )ermanent-<agnet Synchronous <otors

Surface mounted *)roHecting 6ype and !nset 6ype

!nterior $uried 6ype

*iii Synchronous =eluctance <otors

*iv /ysteresis motors

NEO What are the different modes employed to achieve variale fre"uency control in

synchronous motors?

Separate or true synchronous mode

Self controlled mode

NGO Why a Synchronous <otor without damper winding is used, in the drive system

employing a 'ycloconverter?

& Synchronous <otor without the damper winding is used, ecause the damper

winding reduces the inductance of the machine, and therefore is aility to filter out

harmonics in the output voltage of 'ycloconverter. Since the drive operates in self-

controlled mode, the damper winding is not needed for its converter. Alternators related Interview Questions with Answers

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"re#uently As$e Interview Questions on Alternators

!n this post we had given few &lternators related !nterview "uestions with solutions.

We elieve that it will e helpful for your interview preparation. )lease leave your

comments in comments section.

NPO What is meant y synchronising the alternators?

6he process of connecting two or more alternators in parallel for supplying a

common load is called synchronising.

NPO What are the conditions to e satisfied for proper synchronising of alternators?

*i 6he terminal voltage of the incoming alternator must e the same as us-ar

voltage.

*ii 6he speed of the incoming alternator must e such that its fre"uency e"uals to

the us-ar fre"uency

*iii 6he phase se"uence of the incoming alternator must e the same as that of the

other alternators or usars.

NPO What are the advantages of )arallel operation of alternators?

*i !t ensures continuity of power supply to consumers in case of rea+down of an

alternator in an generation station.

*ii 6his is economical and improves the efficiency of the generating station.

*iii When the demand of power increases, new alternators can e installed to

operate in parallel.

*iv !t is not possile to uilt single large sie alternator to meet the re"uirement.

NPO What are types of rotors used in alternators?

6here are two types of rotors used in alternators namely

*i Salient pole rotor

*ii Smooth cylindrical type rotor

NPO What are the advantages of providing damper winding?

6he damper winding is useful in preventing the hunting * momentary speed

fluctuations in generators.

6he damper winding also used to maintain alanced 3 phase voltage under

unalanced load conditions.

NPO What are the various methods to determine the voltage regulation of the large

alternators?

!n case of small machines, the regulation can e found y direct loading.

or large alternators, to find the voltage regulation indirect methods are used. 6hey

are

*iSynchronous !mpedance <ethod.

*ii 6he &mpere-turn method.*iii Lero power factor or )ointer <ethod.



NPO What is the asic principle of alternators?

&lternators is noting ut a &' generators. 6hey operate on the fundamental principle

of electromagnetic induction as dc generators.

ie, when the rotor rotates, the emf is induced in the stator.

NPO Which type of rotor is suitale for low speed alternators? salient pole type or

cylindrical type rotor?

Salient pole type alternators are suitale for low and medium speed alternators. !t

has large numer of proHecting poles. !t has large diameters and short a%ial lengths.

NPO What is meant y turo alternators?

/igh speed alternators are called as 6uro alternators.

&s it runs at very high speed, salient pole rotors are not used. Smooth cylindrical

type rotor is suitale for turo alternators.

Induction Motor Interview Questions: Part 1

$asic !nterview Puestions on !nduction <otors:

NPO /ow does the !nduction motor wor+? *B= Why does the =otor rotate?

• When the 3 phase stator windings are fed y 3 phase supply, a magnetic flu%

of constant magnitude which is rotating at synchronous speed is set up.

• 6he flu% passes through the air-gap and sweeps past the rotor surface thus it

cuts the rotor conductors.

• (ue to the relative speed etween the rotating flu% and the stationary rotor

conductors, an emf is induced in the stationary rotor conductors as per the aradayQslaws of electromagnetic induction.

• 6he fre"uency of the induced emf is the same as the supply fre"uency.

• !ts magnitude is proportional to the relative velocity etween the flu% and the

conductors.

• !ts direction will e as per lemingQs right hand rule.

• Since the rotor conductors form a closed circuit the rotor current is produced.

• 6his currentQs direction will oppose the very cause producing it * as per 0enQs

law

• /ere the cause is the relative velocity etween the rotating flu% of the stator

and the stationary rotor conductors.

• /ence, in order to reduce the relative speed, the rotor egins to rotate in the

same direction as that of the rotating magnetic flu% and tries to catch up with therotating flu%. 6hus the rotor of induction motor starts to rotate.

NPO What is the general wor+ing principle of !nduction motor?

• 6he conversion of electrical power into mechanical power ta+es place in therotating part of an electric motor.

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• !n (' motors the electrical power is conducted directly to the armature

through rushes and commutator.

• 6hus the (' motor can e called as conduction motor. $ut in case of &'

motors, the rotor receives electric power, not y conduction ut y induction.

•

6his is e%actly in the same way as the secondary of two winding transformerreceives its power from the primary.

• 6hat is why such motors are +nown as induction motors.

• 6hus an induction motor is also +nown as rotating transformer * ie, one in

which primary winding is stationary and the secondary is free to rotate

NPO What is the advantage of s+ewed stator slots in the rotor of !nduction motors?

!n the induction motor design, the rotor slots are purposely made with a slight s+ew

arrangement. !t will not e parallel to the shaft.

6his is for the purpose of reducing magnetic loc+ing or reducing magnetic attraction

etween stator and rotor teeth.

!n addition to that this arrangement will help to reduce the magnetic hum and noise.

NPO What is meant y cogging in the !nduction motor? /ow to prevent the cogging?

When the numer of teeth in stator and rotor are e"ual, the stator and rotor teeth

have a tendency to align themselves e%actly opposite to each other, since this

corresponds to minimum reluctance position. !n such case the rotor may refuse to

accelerate. 6his phenomenon is called as magnetic loc+ing or cogging.

6his prolem can e prevented y proper choice of stator and rotor slots and also y

s+ewing the rotor slots y one slot pitch.

NPO What are the various methods of measuring slip?

1. $y actual measurement of rotor speed

. $y measurement of rotor fre"uency

3. Strooscopic method

NPO What are the various methods of speed control in three phase induction motors?

*i 'ontrol from stator side

1. $y changing the supply fre"uency

. $y changing the numer of stator poles

3. $y changing the supply voltage



*ii 'ontrol from rotor side

1. $y inserting resistance in rotor circuit

. $y various ways of cascade connection

3. $y inHecting <s in the rotor circuit.

NPO What is meant y crawing in the induction motor?

!n induction motors, particularly s"uirrel cage type induction motors, sometimes

e%hiit a tendency to run staly at speeds as low as one-seventh of their

synchronous speed 7s. 6his phenomenon is +nown as crawling of an induction

motor and the speed is called as crawling speed











Brushless vs. brushed motorsNeditO

$rushed (' motors have een in commercial use since 1GE;.N1ONO $rushless motors, on the other

hand, did not ecome commercially viale until 1;.N3ON4O

$rushed (' motors develop a ma%imum tor"ue when stationary, linearly decreasing as velocity

increases.NCO Some limitations of rushed motors can e overcome y rushless motorsF they

include higher efficiency and a lower susceptiility to mechanical wear. 6hese enefits come at

the cost of potentially less rugged, more comple%, and more e%pensive control electronics.

https://en.wikipedia.org/w/index.php?title=Brushless_DC_electric_motor&action=edit&section=1

https://en.wikipedia.org/wiki/Brushed_DC_motor


https://en.wikipedia.org/wiki/Brushless_DC_electric_motor#cite_note-1






https://en.wikipedia.org/wiki/Torque



https://en.wikipedia.org/w/index.php?title=Brushless_DC_electric_motor&action=edit&section=1






https://en.wikipedia.org/wiki/Torque




& typical rushless motor has permanent magnets which rotate around a fi%ed armature,

eliminating prolems associated with connecting current to the moving armature. &n electronic

controller replaces the rush@commutator assemly of the rushed (' motor, which continually

switches the phase to the windings to +eep the motor turning. 6he controller performs similar

timed power distriution y using a solid-state circuit rather than the rush@commutator system.

$rushless motors offer several advantages over rushed (' motors, including high tor"ue to

weight ratio, more tor"ue per watt *increased efficiency, increased reliaility, reduced noise,

longer lifetime *no rush and commutator erosion, elimination of ioniing spar+s from the

commutator, and overall reduction of electromagnetic interference*<!. With no windings on the

rotor, they are not suHected to centrifugal forces, and ecause the windings are supported y

the housing, they can e cooled y conduction, re"uiring no airflow inside the motor for cooling.

6his in turn means that the motorQs internals can e entirely enclosed and protected from dirt or

other foreign matter.

$rushless motor commutation can e implemented in software using

a microcontroller or microprocessor computer, or may alternatively e implemented in analogue

hardware, or in digital firmware using an )#&. 'ommutation with electronics instead of rushes

allows for greater fle%iility and capailities not availale with rushed (' motors, including

speed limiting, Rmicro steppedR operation for slow and@or fine motion control, and a holding

tor"ue when stationary.

6he ma%imum power that can e applied to a rushless motor is limited almost e%clusively yheatFNcitation needed O too much heat wea+ens the magnets NO and may damage the windingQs insulation.

When converting electricity into mechanical power, rushless motors are more efficient than

rushed motors. 6his improvement is largely due to the rushless motorQs velocity eing

determined y the fre"uency at which the electricity is switched, not the voltage. &dditional gains

are due to the asence of rushes, which reduces mechanical energy loss due to friction. 6he

enhanced efficiency is greatest in the no-load and low-load region of the motorQs performance

curve.Ncitation needed O >nder high mechanical loads, rushless motors and high-"uality rushed motors

are comparale in efficiency.Ncitation needed ONdisputed – discussO

nvironments and re"uirements in which manufacturers use rushless-type (' motors include

maintenance-free operation, high speeds, and operation where spar+ing is haardous *i.e.

e%plosive environments or could affect electronically sensitive e"uipment.

https://en.wikipedia.org/wiki/Armature_(electrical_engineering)

https://en.wikipedia.org/wiki/Watt

https://en.wikipedia.org/wiki/Brush_(electric)


https://en.wikipedia.org/wiki/Electromagnetic_interference

https://en.wikipedia.org/wiki/Microcontroller

https://en.wikipedia.org/wiki/Microprocessor

https://en.wikipedia.org/wiki/FPGA

https://en.wikipedia.org/wiki/Wikipedia:Citation_needed











https://en.wikipedia.org/wiki/Wikipedia:Disputed_statement



https://en.wikipedia.org/wiki/Talk:Brushless_DC_electric_motor



https://en.wikipedia.org/wiki/Armature_(electrical_engineering)

https://en.wikipedia.org/wiki/Watt


https://en.wikipedia.org/wiki/Electromagnetic_interference

https://en.wikipedia.org/wiki/Microcontroller

https://en.wikipedia.org/wiki/Microprocessor

https://en.wikipedia.org/wiki/FPGA







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Electrical Basic Interview Question