common job interview questions and answers

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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 9

Electrical interview question:how to calculate capacitor bank value to maintain unity

power factor with some suitable example?

Answer:KVAR= KW(TAN(COS(-1)#e)- TAN(COS(-1)#d) )

#e= EXISTING P.F.

#d= DESIRED P.F.

Electrical interview question:Tell me in detail about c.t. and p.t. ?(Company:reliance)

Answer:The term C.T means current transformer,and the term P.T means potential

transformer.In circuit where measurements of high voltage and high current is involved

they are used there.Particularly when a measuring device like voltmeter or ammeter is

not able to measure such high value of quantity because of large value of torque due to

such high value it can damage the measuring device.so, CT and PT are introduced in

the circuits. They work on the same principle of transformer, which is based on linkage

of electromagneticflux produced by primary with secondary.They work on the ratio to

they are designed.E.g if CTis of ratio 5000\5A and it has to measure secondary current

of

8000A.then ANS=8000*5\5000=8Aand this result will be given to ammeter .and after

measuring 8A we can calculate the primary current.same is

the operation of PT but measuring voltage.

Electrical interview question:There are a Transformer and an induction machine.

Those two have the same supply. For which device the load current will be maximum?

And why?

Answer:The motor has max load current compare to that of transformer because the

motor consumes real power.. and the transformer is only producing the working flux and

its not consuming.. hence the load current in the transformer is because of core loss so

it is minimum.

Electrical interview question:what is power factor? whether it should be high or low?

why?

Answer:Power factor should be high in order to get smooth operation of the

system.Low power factor means losses will be more.it is the ratio of true power to

apperent power. it has to be ideally 1. if it is too low then cable over heating &

equipment overloading will occur. if it is greater than 1 then load will act as capacitor

and starts feeding the source and will cause tripping.(if pf is poor ex: 0.17 to meet actual

power load has to draw more current(V constant),result in more lossesif pf is good ex:

0.95 to meet actual power load has to draw less current(V constant),result in less

losses).

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ATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 1

COMMON JOB INTERVIEW QUESTIONS AND ANSWERS Our website contains common job interview questions and answers related to Electrical and electronic engineering,Computer science engineering,IT engineering and HR related questions.

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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 1Electrical interview question:What is electric traction?

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Answer:Traction means using the electric power for traction system i.e. for railways,trams,

trolleys etc. electric traction means use of the electricity for all these . now a days, magnetic

traction is also used for bullet trains. basically dc motors are used for electric traction systems.

Electrical interview question:How can you start-up the 40w tube lite with 230v AC/DC without

using any choke/Coil?

Answer:It's possible by means of Electronic choke.otherwise it's not possible to ionize the

particles in tube. light, with normal voltage.

Electrical interview question:What is "pu" in electrical engineering?

Answer:Pu stands for per unit and this will be used in power system single line diagram there it

is like a huge electrical circuit with no of componenes (generators, transformers, loads) with

different ratings (in MVA and KV). To bring all the ratings into common platform we use pu

concept in which, in general largest MVA and KV ratings of the component is considered as

base values, then all other component ratings will get back into this basis.Those values are

called as pu values. (p.u=actual value/base value).

Electrical interview question:Operation carried out in Thermal power station?

Answer:The water is obtained in the boiler and the coal is burnt so that steam is obtained this

steam is allowed to hit the turbine , the turbine which is coupled with the generator generates

the electricity

Electrical interview question:Why link is provided in neutral of an ac circuit and fuse in phase

of ac circuit?

Answer:Link is provided at a Neutral common point in the circuit from which various connection

are taken for the individual control circuit and so it is given in a link form to withstand high Amps.

But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is

calculated for the particular circuit (i.e load) only.So if any malfunction happen the fuse

connected in the particular control circuit alone will blow off.

Electrical interview question: what is the diff. btwn. electronic regulator and ordinary rheostat

regulator for fans?

Answer:The difference between the electronic and ordinary regulator is that in electronic reg.

power losses are less because as we decrease the speed the electronic reg. give the power

needed for that particular speed but in case of ordinary rheostat type reg. the power wastage is

same for every speed and no power is saved.In electronic regulator triac is employed for speed

control.by varying the firing angle speed is controlled but in rheostatic control resistance is

decreased by steps to achieve speed control.

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Electrical interview question:What is electric traction?

Answer:Traction means using the electric power for traction system i.e. for

railways,trams, trolleys etc. electric traction means use of the electricity for all these .

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now a days, magnetic traction is also used for bullet trains. basically dc motors are used

for electric traction systems.

Electrical interview question:How can you start-up the 40w tube lite with 230v AC/DC

without using any choke/Coil?

Answer:It's possible by means of Electronic choke.otherwise it's not possible to ionize

the particles in tube. light, with normal voltage.


Answer:Pu stands for per unit and this will be used in power system single line diagram

there it is like a huge electrical circuit with no of componenes (generators, transformers,

loads) with different ratings (in MVA and KV). To bring all the ratings into common

platform we use pu concept in which, in general largest MVA and KV ratings of the

component is considered as base values, then all other component ratings will get back

into this basis.Those values are called as pu values. (p.u=actual value/base value).


Answer:The water is obtained in the boiler and the coal is burnt so that steam is

obtained this steam is allowed to hit the turbine , the turbine which is coupled with the

generator generates the electricity

Electrical interview question:Why link is provided in neutral of an ac circuit and fuse in

phase of ac circuit?

Answer:Link is provided at a Neutral common point in the circuit from which various

connection are taken for the individual control circuit and so it is given in a link form to

withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed

such that the fuse rating is calculated for the particular circuit (i.e load) only.So if any

malfunction happen the fuse connected in the particular control circuit alone will blow

off.

Electrical interview question: what is the diff. btwn. electronic regulator and ordinary

rheostat regulator for fans?

Answer:The difference between the electronic and ordinary regulator is that in

electronic reg. power losses are less because as we decrease the speed the electronic

reg. give the power needed for that particular speed but in case of ordinary rheostat

type reg. the power wastage is same for every speed and no power is saved.In

electronic regulator triac is employed for speed control.by varying the firing angle speed

is controlled but in rheostatic control resistance is decreased by steps to achieve speed

control.



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Q & A Part 1

Q & A Part 2

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Q & A Part 7

Q & A Part 8

Q & A Part 9


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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 1Electrical interview question:What is electric traction?

Answer:Traction means using the electric power for traction system i.e. for railways,trams,

trolleys etc. electric traction means use of the electricity for all these . now a days, magnetic

traction is also used for bullet trains. basically dc motors are used for electric traction systems.

Electrical interview question:How can you start-up the 40w tube lite with 230v AC/DC without

using any choke/Coil?

Answer:It's possible by means of Electronic choke.otherwise it's not possible to ionize the

particles in tube. light, with normal voltage.


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Answer:Pu stands for per unit and this will be used in power system single line diagram there it

is like a huge electrical circuit with no of componenes (generators, transformers, loads) with

different ratings (in MVA and KV). To bring all the ratings into common platform we use pu

concept in which, in general largest MVA and KV ratings of the component is considered as

base values, then all other component ratings will get back into this basis.Those values are

called as pu values. (p.u=actual value/base value).


Answer:The water is obtained in the boiler and the coal is burnt so that steam is obtained this

steam is allowed to hit the turbine , the turbine which is coupled with the generator generates

the electricity

Electrical interview question:Why link is provided in neutral of an ac circuit and fuse in phase

of ac circuit?

Answer:Link is provided at a Neutral common point in the circuit from which various connection

are taken for the individual control circuit and so it is given in a link form to withstand high Amps.

But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is

calculated for the particular circuit (i.e load) only.So if any malfunction happen the fuse

connected in the particular control circuit alone will blow off.

Electrical interview question: what is the diff. btwn. electronic regulator and ordinary rheostat

regulator for fans?

Answer:The difference between the electronic and ordinary regulator is that in electronic reg.

power losses are less because as we decrease the speed the electronic reg. give the power

needed for that particular speed but in case of ordinary rheostat type reg. the power wastage is

same for every speed and no power is saved.In electronic regulator triac is employed for speed

control.by varying the firing angle speed is controlled but in rheostatic control resistance is

decreased by steps to achieve speed control.



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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 2 Electrical interview question:How tube light circuit is connected and how it works?

Answer:A choke is connected in one end of the tube light and a starter is in series with the

circuit. When supply is provided ,the starter will interrupt the supply cycle of AC. Due to the

sudden change of supply the chock will generate around 1000volts . This volt will capable of to

break the electrons inside the tube to make electron flow. once the current passes through the

tube the starter circuit will be out of part. now there is no change of supply causes choke voltage

normalized and act as minimize the current.

Electrical interview question:whats is MARX CIRCUIT?

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Answer:It is used with generators for charging a number of capacitor in parallel and discharging

them in series.It is used when voltage required for testing is higher than the available.

Electrical interview question:What is encoder, how it function?

Answer:An encoder is a device used to change a signal (such as a bitstream) or data into a

code. The code may serve any of a number of purposes such as compressing information for

transmission or storage, encrypting or adding redundancies to the input code, or translating

from one code to another. This is usually done by means of a programmed algorithm,especially

if any part is digital, while most analog encoding is done with analog circuitry.

Electrical interview question:What are the advantages of speed control using thyristor?

Answer:Advantages :1. Fast Switching Characterstics than Mosfet, BJT, IGBT 2. Low cost 3.

Higher Accurate.

Electrical interview question:Why Human body feel Electric shock ?? n in an Electric train

during running , We did nt feel any Shock ? why?

Answer:Unfortunately our body is a pretty good conductor of electricity, The golden rule is

Current takes the lowest resistant path if you have insulation to our feet as the circuit is not

complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a

high resistance path not much current flows through our body).The electric train is well insulated

from its electrical system.

Electrical interview question:what is the principle of motor?

Answer:Whenever a current carrying conductor is placed in an magnetic field it produce turning

or twisting movemnt is called as torque.




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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 3 Electrical interview question:why, when birds sit on transmission lines or current wires doesn't

get shock?

Answer:Its true that if birds touch the single one line (phase or neutral) they don't get electrical

shock... if birds touch 2 lines than the circuit is closed and they get electrical shock.. so if a

human touch single one line(phase) then he doesn't get shock if he is in the air (not touching -

standing on the ground if he is standing on the ground then touching the line (phase) he will get

a shock because the ground on what we standing is like line (ground bed - like neutral)। and in

the most of electric lines the neutral is grounded..so that means that human who touch the line

closes the circuit between phase and neutral.

Electrical interview question:what is meant by armature reaction?

Answer:The effect of armature flu to main flux is called armature reaction. The armature flux

may support main flux or opposes main flux.











Electrical interview question:what happen if we give 220 volts dc supply to d bulb r tube light?

Answer:Bulbs [devices] for AC are designed to operate such that it offers high impedance to

AC supply. Normally they have low resistance. When DC supply is applied, due to low

resistance, the current through lamp would be so high that it may damage the bulb element.

Electrical interview question:Which motor has high Starting Torque and Staring current DC

motor, Induction motor or Synchronous motor?

Answer:DC Series motor has high starting torque. We can not start the Induction motor and

Synchronous motors on load, but can not start the DC series motor without load.

Electrical interview question:what is ACSR cable and where we use it?

Answer:ACSR means Aluminium conductor steel reinforced, this conductor is used in

transmission & distribution.

Electrical interview question:What is vaccum currcuit breaker.define with cause and where be

use it Device?

Answer:A breaker is normally used to break a ciruit. while breaking the circuit, the contact

terminals will be separated. At the time of seperation an air gap is formed in between the

terminals. Due to existing current flow the air in the gap is ionised and results in the arc. various

mediums are used to quench this arc in respective CB's. but in VCB the medium is vaccum gas.

since the air in the CB is having vaccum pressure the arc formation is interrupted. VCB's can be

used upto 11kv.





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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 4 Electrical interview question:What will happen when power factor is leading in distribution of

power?

Answer:If their is high power factor, i.e if the power factor is close to one:

1.losses in form of heat will be reduced,

2.cable becomes less bulky and easy to carry, and very

cheap to afford, &

3. it also reduces over heating of tranformers.

Electrical interview question:whats the one main difference between UPS & inverter ? And

electrical engineering & electronics engineering ?

Answer:uninterrupt power supply is mainly use for short time . means according to ups VA it

gives backup. ups is also two types : on line and offline . online ups having high volt and amp for

long time backup with with high dc voltage.but ups start with 12v dc with 7 amp. but inverter is

startwith 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup.







Electrical interview question:What is 2 phase motor?

Answer:A two phase motor is a motor with the the starting winding and the running winding

have a phase split. e.g;ac servo motor.where the auxiliary winding and the control winding have

a phase split of 90 degree.

Electrical interview question:Advantages of vvvf drives over non vvvf drives for EOT cranes?

Answer:1.smooth start and stop.

2.no jerking of load.

3.exact posiitoning

4.better protection for motor.

5.high/low speed selection.

6.reliability of break shoe.

7.programmable break control.

8.easy circutry

9.reduction in controls

10.increases motor life

Electrical interview question:What is the significance of vector grouping in Power

Transformers?

Answer:Every power transformer has a vector group listed by its manufacturer. Fundamentally

it tells you the information about how the windings are connected (delta or wye) and the phace

difference betweent the current and voltage. EG. DYN11 means Delta primary, Wye Secondry

and the current is at 11 o clock reffered to the voltage.

Electrical interview question:Which type of A.C motor is used in the fan (ceiling fan, exhaust

fan, padestal fan, bracket fan etc) which are find in the houses ?

Answer:Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor

start capacitor run.





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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 5 Electrical interview question:Give two basic speed control scheme of DC shunt motor?

Answer:1. By using flux control method:in this method a rheostat is connected across the field

winding to control the field current.so by changing the current the flux produced by the field

winding can be changed, and since speed is inversely proportional to flux speed can be

controlled 2.armature control method:in this method a rheostat is connected across armature

winding by varying the resistance the value of resistive drop(IaRa)can be varied,and since

speed is directly proportional to Eb-IaRa the speed can be controlled.















Electrical interview question:what is the principle of motor?

Answer:Whenever a current carrying conductor is placed in an magnetic field it produce turning

or twisting movement is called as torque.

Electrical interview question:what is meant by armature reaction?

Answer:The effect of armature flu to main flux is called armature reaction. The armature flux

may support main flux or opposes main flux.

Electrical interview question:Give two basic speed control scheme of DC shunt motor?

Answer:1. By using flux control method:in this method a rheostat is connected across the field

winding to control the field current.so by changing the current the flux produced by the field

winding can be changed, and since speed is inversely proportional to flux speed can be

controlled 2.armature control method:in this method a rheostat is connected across armature

wdg.by varying the resistance the value of resistive drop(IaRa)can be varied,and since speed is

directly proportional to Eb-IaRa the speed can be controlled.

Electrical interview question:what is the difference between synchronous generator &

asynchronous generator?

Answer:In simple, synchronous generator supply's both active and reactive power but

asynchronous generator(induction generator) supply's only active power and observe reactive

power for magnetizing.This type of generators are used in windmills.

Electrical interview question:What is the Polarization index value ? (pi value)and simple

definition of polarization index ?

Answer:Its ratio between insulation resistance(IR)i.e meggar value

for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for

motor to be in good condition it should be Greater than 2.5 .





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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 6 Electrical interview question:Why syn. generators r used for the production of electricity?

Answer:synchronous machines have capability to work on different power factor(or say

different imaginary power varying the field emf. Hence syn. generators r used for the production

of electricity.















Electrical interview question:What is the difference between synchronous generator &

asynchronous generator?

Answer:In simple, synchronous generator supply's both active and reactive power but

asynchronous generator(induction generator) supply's only active power and observe reactive

power for magnetizing.This type of generators are used in windmills.

Electrical interview question:1 ton is equal to how many watts?

Answer:1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower,

12,000 * 0.0003929 = 4.715 hp therefore 1 ton = 4.715*.746 = 3.5 KW.

Electrical interview question:why syn. generators r used for the production of electricity?

Answer:synchronous machines have capability to work on differentpower factor(or say

different imaginary pow varying the field emf. Hence syn. generators r used for the production of

electricity.

Electrical interview question:Enlist types of dc generator?

Answer:D.C.Generators are classified into two types 1)separatly exicted d.c.generator 2)self

exicted d.c.generator, which is further classified into;1)series 2)shunt and

3)compound(which is further classified into cumulative and differential).





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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 7 Electrical interview question:What is Automatic Voltage regulator(AVR)?

Answer:AVR is an abbreviation for Automatic Voltage Regulator.It is important part in

Synchronous Generators, it controls theoutput voltage of the generator by controlling its

excitation current. Thus it can control the output Reactive Power of the Generator.

Electrical interview question:What is an exciter and how does it work?

Answer:There are two types of exciters, static exciter and rotory exciter.purpose of excitor is to

supply the excitation dc voltage to the fixed poles of generator.Rotory excitor is an additional

small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to

the rotory poles through slip ring and brushes( conventional alternator). if it is an ac excitor, out

put of ac excitor is rectified by rotating diodes and supply dc to main fixed poles.ac excitor is the

ac generator whose field winding are stationary and armature rotates. initial voltage is built up

by residual magnetism.It gives the starting torque to the generator.

Electrical interview question:Difference between a four point starter and three point starter?









Answer:The shunt connection in four point stater is provided separately form the line where as

in three point stater it is connected with line which is the drawback in three point stater

Electrical interview question:Why use the VCB at High Transmission System ? Why can't use

ACB?

Answer:Actually the thing is vacuum has high arc queching property compare to air because in

VCB ,the die electric strengths equal to 8 times of air . That y always vaccum used as inHT

breaker and air used as in LT .

Electrical interview question:What is the difference between surge arrestor and lightning

arrestor?

Answer:LA is installed outside and the effect of lightning is grounded,where as surge arrestor

installed inside panels comprising of resistors which consumes the energy and nullify the effect

of surge.





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SATURDAY, APRIL 9, 2011Common Electrical Interview Questions Part 8 Electrical interview question:What happens if i connect a capacitor to a generator load?

Answer:Connecting a capacitor across a generator always improves powerfactor,but it will help

depends up on the engine capacity of the alternator,other wise the alternator will be over loaded

due to the extra watts consumed due to the improvement on pf. Secondly, don't connect a

capacitor across an alternator while it is picking up or without any other load.

Electrical interview question:Why the capacitors works on ac only?

Answer:Generally capacitor gives infinite resistance to dc components(i.e., block the dc

components). it allows the ac components to pass through.

Electrical interview question:Explain the working principal of the circuit breaker?

Answer:Circuit Breaker is one which makes or breaks the circuit.It has two contacts namely

fixed contact & moving contact.under normal condition the moving contact comes in contact with

fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty

conditions(when current exceeds the rated value) an arc is produced between the fixed &

moving contacts & thereby it forms the open circuitArc is extinguished by the Arc Quenching

media like air, oil, vaccum etc.

Electrical interview question:How many types of colling system it transformers?





Answer:1. ONAN (oil natural,air natural)

2. ONAF (oil natural,air forced)

3. OFAF (oil forced,air forced)

4. ODWF (oil direct,water forced)

5. OFAN (oil forced,air forced)

Electrical interview question:What is the function of anti-pumping in circuit breaker?

Answer:when breaker is close at one time by close push button,the anti pumping contactor

prevent re close the breaker by close push button after if it already close.

Electrical interview question:what is stepper motor.what is its uses?

Answer:Stepper motor is the electrical machine which act upon input pulse applied to it. it is

one type of synchronous motor which runs in steps in either direction instead of running in

complete cycle.so, in automation parts it is used.





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Back to TOP Short questions and answers

EE1251 Electrical Machines II

1. Why almost all large size Synchronous machines are constructed with rotating

field system type?

The following are the principal advantages of the rotating field system type

construction of Synchronous machines:

· The relatively small amount of power, about 2%, required

for field system via slip-rings and brushes.

· For the same air gap dimensions, which is normally decided

by the kVA rating, more space is available in the stator part

of the machine for providing more insulation to the system

of conductors, especially for machines rated for 11kV or

above.

· Insulation to stationary system of conductors is not

subjected to mechanical stresses due to centrifugal action.

· Stationary system of conductors can easily be braced to

prevent deformation.

· It is easy to provide cooling arrangement for a stationary

system of conductors.

· Firm stationary connection between external circuit and

system of conductors enable he machine to handle large

amount of volt-ampere as high as 500MVA.

2. Write down the equation for frequency of emf induced in an Altenator.

Frequency of emf induced in an Alternator,f ,expressed in cycles per second or

Hz, is given by the following equation


F = (PN)/120 Hz,

Where P- Number of poles

N-Speed in rpm

3. How are alternators classified?

According to type of field system

· Stationary field system type

· Rotating field system type

According to shape of field system

· Salient pole type

· Smooth cylindrical type

4. Name the types of Alternator based on their rotor construction.

Alternators can be classified into the following two types according to its rotor

construction

· Smooth cylindrical type alternator

· Salient pole alternator5. Why do cylindrical Alternators operate with steam turbines?

Steam turbines are found to operate at fairly good efficiency only at high

speeds. The high speed operation of rotors tends to increase mechanical losses

and so the rotors should have a smooth external surface. Hence, smooth

cylindrical type rotors with less diameter and large axial length are used for

Synchronous generators driven by steam turbines with either 2 or 4 poles.

6. Which type of Synchronous generators are used in Hydro-electric plants and why?

As the speed of operation is low for hydro turbines use din Hydro-electric

plants, salient pole type Synchronous generators are used. These allow better

ventilation and also have other advantages over smooth cylindrical type rotor.

7. What are the advantages of salient pole type construction used for Synchronous

machines?

Advantages of salient-pole type construction are :

· They allow better ventilation

· The pole faces are so shaped that the radial air gap length

increases from the pole center to the pole tips so that the

flux distribution in the air-gap is sinusoidal in shape which

will help the machine to generate sinusoidal emf

· Due to the variable reluctance the machine develops

additional reluctance power which is independent of

excitation

8. Why is the stator core of Alternator laminated?

The stator core of Alternator is laminated to reduce eddy current loss.

9. How does electrical degree differ from mechanical degree?

Mechanical degree is the unit for accounting the angle between two points

based on their mechanical or physical placement.

Electrical degree is used to account the angle between two points in rotating

electrical machines. Since all electrical machines operate with the help of

magnetic fields, the electrical degree is accounted with reference to the

magnetic field. 180 electrical degree is accounted as the angle between

adjacent North and South poles.

10. What is the relation between electrical degree and mechanical degree?

(OHFWULFDO GHJUHH H DQG PHFKDQLFDO GHJUHH P DUH UHODWHG WR RQH DQRWKHU E\

the number of poles P, the electrical machine has, as given by the following

equation

H 3

P

11. What is distributed winding?When coil-sides belonging to each phase are housed or distributed in more

than one slot under each pole region then the winding is called distributed

winding

A full pitch coil has width of coil otherwise called coil-span as 180º

:KHUH - angle between adjacent slots in electrical degree and x=

1,2,3…

12. Why is short pitch winding preferred over full-pitch winding ?

Advantages

· Waveform of the emf can be approximately made to a sine

wave and distorting harmonics can be reduced or totally

eliminated.

· Conductor material , copper , is saved in the back and front

end connections due to less coil-span.

· Fractional slot winding with fractional number of

slots/phase can be used which in turn reduces the tooth

ripples.

· Mechanical strength of the coil is increased.

13. Write down the formula for distribution factor.

Kd = sin(m

RU .dn = sin(mn

P sin(n

m= number of slots/ pole/ phase

DQJOH EHWZHHQ DGMDFHQW VORWV LQ HOHFWULFDO GHJUHH

n = order of harmonic

14. Define winding factor.

The winding factor Kd is defined as the ratio of phasor addition of emf

induced in all the coils belonging to each phase winding to their arithmetic

addition.

15. Why are Alternators rated in kVA and not in kW?

The continuous power rating of any machine is generally defined as the power the

machine or apparatus can deliver for a continuous period so that the losses

incurred in the machine gives rise to a steady temperature rise not exceeding the

limit prescribed by the insulation class.

Apart from the constant loss incurred in Alternators is the copper loss, occurring

in the 3 –phase winding which depends on I

2

R, the square of the current delivered

by the generator. As the current is directly related to apparent – power delivered

by the generator , the Alternators have only their apparent power in

VA/kVA/MVA as their power rating.

16. What are the causes of changes in voltage in Alternators when loaded?

Variations in terminal voltage in Alternators on load condition are due to the

following three causes:

· Voltage variation due to the resistance of the winding, R· Voltage variation due to the leakage reactance of the

winding, Xt

· Voltage variation due to the armature reaction effect, Xa

17. What is meant by armature reaction in Alternators?

The interaction between flux set up by the current carrying armature

FRQGXFWRUV a and the main fieOG IOX[ m is defined as the armature reaction.

18. What do you mean by synchronous reactance?

Synchronous reactance X s= (Xl + Xa)

The value of leakage reactance Xl is constant for a machine based on its

construction. Xa depends on saturating condition of the machine. It is the

addition of Xa , which represent the armature reaction effect between two

synchronously acting magnetic fields that makes the total reactance Xa to be

called syncheornous reactance.

19. What is meant by synchronous impedance of an Alternator?

The complex addition of resistance, R and synchronous reactance , jXs can be

represented together by a single complex impedance Zs called synchronous

impedance.

In complex form Zs = (R + jXs )

In polar form Zs = | Zs | <

Where | Zs | = ¥5

2

+ X

2

s )

And WDQ

-1

(Xs /R)

20. What is meant by load angle of an Alternator?

The phase angle introduced between the induced emf phasor, E and terminal

voltage phasor , U during the load condition of an Alternator is called load

angle.

21. Upon what factors does the load angle depend?

7KH PDJQLWXGH RI ORDG DQJOH LQFUHDVHV ZLWK LQFUHDVH LQ ORDG)XUWKHU WKH ORDG

angle is positive during generator operation and negative during motor

operation.

22. An Alternator is found to have its terminal voltage on load condition more than

that on no load. What is the nature of the load connected?

The nature of the load is of leading power factor , load consisting of resistance

and capacitive reactance.

23. Define the term voltage regulation of Alternator.

The voltage regulation of an Alternator is defined as the change in terminal

voltage from no-load to load condition expressed as a fraction or percentage of

terminal voltage at load condition ; the speed and excitation conditions

remaining same.Voltage regulation in percentage , URP = [(|E|-|U|)/|U| ]x 100

24. What is the necessity for predetermination of voltage regulation?

Most of the Alternators are manufactured with large power rating , hundreds

of kW or MW, and also with large voltage rating upto 33kV. For Alternators

of such power and voltage ratings conducting load test is not possible. Hence

other indirect methods of testing are used and the performance like voltage

regulation then can be predetermined at any desired load currents and power

factors.

25. Name the various methods for predetermining the voltage regulation of 3-phase

Alternator.

The following are the three methods which are used to predetermine the

voltage regulation of smooth cylindrical type Alternators

· Synchronous impedance / EMF method

· Ampere-turn / MMF method

· Potier / ZPF method

26. How synchronous impedance is calculated from OCC and SCC?

Synchronous impedance is calculated from OCC and SCC as

|Z | s = E0/Isc(for same If)

A compromised value of Zs is normally estimated by taking the ratio of (E0/Isc)

at normal field current Ifn. A normal field current Ifn is one which gives rated voltage

Ur on open circuit.

|Zs| = U /r Iscn

27. What are the advantages and disadvantages of estimating the voltage regulation of

an Alternator by EMF method?

Advantages:

· Simple no load tests (for obtaining OCC and SCC) are to be

conducted

· Calculation procedure is much simpler

Disadvantages:

· The value of voltage regulation obtained by this method is

always higher than the actual value

28. Why is the synchronous impedance method of estimating voltage regulation

considered as pessimistic method?

Compared to other methods, the value of voltage regulation obtained by the

synchronous impedance method is always higher than the actual value and

therefore this method is called the pessimistic method.

29. In what way does the ampere-turn method differ from synchronous impedance

method?The ampere-turn /MMF method is the converse of the EMF method in the

sense that instead of having the phasor addition of various voltage

drops/EMFs, here the phasor addition of MMF required for the voltage drops

are carried out. Further the effect of saturation is also taken care of.

30. What are the test data required for predetermining the voltage regulation of an

Alternator by MMF method?

Data required for MMF method are :

· Effective resistance per phase of the 3-phase winding R

· Open circuit characteristic (OCC) at rated speed/frequency

· Short circuit characteristic (SCC) at rated speed/frequency

31. Why is the MMF method of estimating the voltage regulation considered as the

optimistic method?

Compared to the EMF method, MMF method, involves more number of

complex calculation steps. Further the OCC is referred twice and SCC is

referred once while predetermining the voltage regulation for each load

condition. Reference of OCC takes care of saturation effect. As this method

require more effort, the final result is very close to the actual value. Hence this

method is called optimistic method.

32. State the condition to be satisfied before connecting two alternators in parallel

The following are the three conditions to be satisfied by synchronizing the

additional Alternator with the existing one or the common bus-bars.

· The terminal voltage magnitude of the incoming Alternator

must be made equal to the existing Alternator or the bus-bar

voltage magnitude.

· The phase sequence of the incoming Alternator voltage

must be similar to the bus-bar voltage.

· The frequency of the incoming Alternator voltage must be

the same as the bus-bar voltage.

33. How do the synchronizing lamps indicate the correctness of phase sequence

between existing and incoming Alternators?

The correctness of the phase sequence can be checked by looking at the three

sets of lamps connected across the 3-pole of the synchronizing switch. If the

lamps grow bright and dark in unison it is an indication of the correctness of

the phase sequence. If on the other hand, they become bright and dark one

after the other, connections to any two machine terminals have to be

interchanged after shutting down the machine.

34. What are the advantages and disadvantages of three dark lamps method of

synchronizing?

Advantages:

· The synchronous switch using lamps is inexpensive· Checking for correctness of the phase sequence can be

obtained in a simple manner which is essential especially

when the Alternator is connected for the first time or for

fresh operation after disconnection

Disadvantages:

· The rate of flickering of the lamps only indicates the

frequency difference between the bus-bar and the incoming

Alternator. The frequency of the incoming Alternator in

relation to the bus-bar frequency is not available.

35. How synchronoscope is used for synchronizing Alternators?

Synchronoscope can be used for permanently connected Alternators where the

correctness of phase sequence is already checked by other means.

Synchronoscope is capable of rotating in both directions. The rate of rotation

of the pointer indicates the amount of frequency difference between the

Alternators. The direction of rotation indicates whether incoming Alternator

frequency is higher or lower than the existing Alternator. The TPST switch is

closed to synchronise the incoming Alternator when the pointer faces the top

thick line marking.

36. Why synchronous generators are to be constructed with more synchronous

reactance and negligible resistance?

The presence of more resistance in the Synchronous generators will resist or

oppose their synchronous operation. More reactance in the generators can

cause good reaction between the two and help the generators to remain in

synchronism in spite of any disturbance occurring in any one of the generators.

37. List the factors that affect the load sharing in parallel operating generators?

The total active and reactive power delivered to the load, connected across the

common bus-bars, are shared among Synchronous generators, operating in

parallel, based on the following three factors

· Prime-mover characteristic/input

· Excitation level and

· Percentage synchronous impedance and its R/X ratio

38. How does the change in prime mover input affect the load sharing?

An increase in prime-mover input to a particular generator causes the activepower shared by it to increase and a corresponding decrease in active-power

shared by other generators. The change in reactive power sharing is less

appreciable. The frequency of the bus-bar voltage will also subjected to slight

increase in value.

39. How does change in excitation affects the load sharing?

The decrease in excitation in one generator causes the reactive power shared

by it to decrease and a corresponding increase in reactive-power shared byother generators. The change in active-power sharing is less appreciable.

There will be a slight decrease in terminal voltage magnitude also.

40. What steps are to be taken before disconnecting one Alternator from parallel

operation?

The following steps are to be taken before disconnecting one Alternator from

parallel operation

· The prime-mover input of the outgoing generator has to be

decreased and that of other generators has to be increased

and by this the entire active-power delivered by the

outgoing generator is transferred to other generators.

· The excitation of the outgoing generator has to be decreased

and that of other generators have to be increased and by this

the entire reactive-power delivered by the outgoing

generator is transferred to other generators.

· After ensuring the current delivered by the outgoing

generator is zero, it has to be disconnected from parallel

operation.

41. What is meant by infinite bus-bars?

The source or supply lines with non-variable voltage and frequency are called

infinite bus-bars. The source lines are said to have zero source impedance and

infinite rotational inertia.

42. How does increase in excitation of the Alternator connected to infinite bus-bars

affect this operation?

Increase in excitation level of the synchronous generator will effectively

increase the reactive component of the current supplied by the generator and

hence the active power delivered.

43. In what respect does a 1-phase Induction motor differ from a 3-phase Induction

motor?

Construction wise a plain 1-phase Induction motor is more or less similar to a

3-phase squirrel-cage Induction motor except that its stator is provided with

only 1-phase winding.

44. What are the inherent characteristics of plain 1-phase Induction motor ?

A plain 1-phase Induction motor is not used in practice due to the following

inherent characteristics

· A plain 1-phase Induction motor does not have any starting torque

· However, if the rotor is initially given a starting torque, by some means,

the motor can pick up its speed in a direction at which the initial torque is

given and deliver the required output.45. Name the two different theories with which principle of 1-phase induction motors

are explained.

The two different theories are

· Double revolving field theory

· Cross field theory

46. State double revolving field theory.

Double revolving theory, formulated by Ferrari, states that a single pulsating

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PDJQHWLF ILHOGV RI P

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synchronous speed proportional to the frequency of the pulsating field.

47. Name any four types of 1-phase induction motors.

Based on the method of starting arrangement provided, the 1-phase Induction

motors are classified as follows

(i) Split-phase motor

(ii) Capacitor start motor

(iii) Capacitor start and run motor

(iv) Shaded pole motor

(v) Repulsion start Induction run motor

48. Why are centrifugal switches provided on many 1-phase Induction motors?

Centrifugal switches are provided on many 1-phase Induction motors to

disconnect the starting / auxiliary winding from the supply when the motor

reaches about 70% of its synchronous speed.

49. How is the direction of a capacitor start Induction motor be reversed?

The direction of rotation can be reversed by interchanging the terminals of

either the main winding or the starting winding.

50. State the principle of 3 phase IM?

While starting, rotor conductors are stationary and they cut the revolving magnetic

field and so an emf is induced in them by electromagnetic induction. This induced

emf produces a current if the circuit is closed. This current opposes the cause by

Lenz’s law and hence the rotor starts revolving in the same direction as that of the

magnetic field.

51. Induction motor can run at synchronous speed ? True or false? Explain .

No, if the speed of induction motor is Ns then the relative speed between the

rotating flux and the rotor will be zero and so no torque is produced.

52. An induction motor is generally analogous to ?

It is analogous to a winding rotating transformer with its secondary circuit closed’

.53. Can the starting torque of of a slip ring induction motor being increased?

Yes. It can be increased by adding resistances to the rotor.

54. What would happen if a 3 phase induction motor is switched on with one phase

disconnected?

The motor is likely to burn .

55. What happens if the air gap flux density in an induction motor increases?

The increase in air gap flux increases iron loss and hence efficiency decreases.

56. State the advantages of skewing?

It reduces humming and hence quite running of motor is achieved.

It reduces magnetic locking of the stator and rotor.

67 State the condition at which the starting torque developed in a slip-ring induction

motor is maximum.

When R2=X2

68 What are the effects of increasing rotor resistance on starting current

and starting torque?

The additional external resistance reduces the rotor current and hence

the current drawn from the supply.

It improves the starting torque developed by improving the power factor in high

proportion to the decrease in rotor current.

68 What is slip of an induction motor?

The slip speed expressed as the ratio of synchronous speed is defined as slip.

Percentage slip S=Ns-N/Ns*100

69 How the magnitude of rotor emf is related to the slip in an I M?

Rotor circuit emf per phase E2r=SE2

70 How the frequency of rotor emf is related to the slip in an I M?

Frequency of rotor emf/current fr=Sfs

71 What is the normal value of slip of an I M operating at full load?

3 - 5%

72 Why is not possible for the rotor speed of an I M to be equal to the speed of its

rotating magnetic field?The machine will not be able to develop any mechanical torque to run as a

motor.

73 State the condition at which the torque developed in a 3 phase induction motor is

maximum.

When R2=SX2

74. What are the advantages of slip-ring I M over cage I M?

(i) Rotor circuit is accessible for external connection.

(ii) By adding external resistance to the rotor circuit the starting current is

reduced with the added advantage of improving starting torque.

(iii) Additional speed control methods can be employed with the

accessibility in the rotor circuit.

75. What are the losses occurring in an I M and on what factors do they depend?

Magnetic losses Wi

Electrical losses Wcu

Mechanical losses Wm

For I M operating in normal condition (with constant voltage and frequency)

magnetic and mechanical losses remain constant whereas electrical losses

vary in square proportion to the current.

76. What care should be taken at the time of construction to reduce eddy current

losses in I M?

Make the resistance of the core body as large as possible.This is

achieved by laminating the stator core,stacked and revetted at right angles to

the path of eddy current.The laminations are insulated from each other by thin

coat of varnish.

77. Why is there not appreciable magnetic losses in the rotor core of Induction

motors?

Although the rotor core is also subjected to magnetic flux reversals and since

the frequency of flux reversals in the rotor, fr = Sfs, is very small, the iron loss

incurred in the rotor core is negligibly small.

78. What is meant by synchronous watt?

With the power input to the motor Pi

, after the losses in the stator winding.

Wcu1 and stator core, Wi

, are met with, the remaining power is transferred to

the rotor by the rotating magnetic field as power input to the rotor Pir

Pir = Pi –wau1 - Wi

The power input to the rotor Pir is transferred from the stator

to the rotor by rotating magnetic field which rotates at

synchronous speed Ns. Torque Td is developed in the rotor asa result of Pir and the equation for Pir can alternatively be

expressed as

Pir 1 /s 60). Td W

Or Td = Pir Syn. W

79. How does the shaft torque differ from the torque developed in 3-phase Induction

motor?

The mechanical power developed Pd causes the rotor to rotate at a speed Nr

due to the torque Td developed in the rotor . Therefore, equation for Pr can be

written as

Pd 1rTd / 60

The remaining power, after the mechanical losses Wm are met with, available

in the shaft as mechanical power output Po

Po = Pd – Wm

The mechanical power output Po, which is less than Pd is available in the shaft

running at a speed of Nr and with a shaft torque T. Therefore the shaft torque

(T) is slightly less than the torque developed Td,

Pd 1rT / 60

Wm = Pd – Po > 1r(Td – T)] / 60

80. Name the tests to be conducted for predetermining the performance of 3-phase

induction machine.

(a) No load test

(b) Blocked rotor test

81. What are the informations obtained from no-load test in a 3-phase I M?

(i) No –load input current per phase,Io

(ii) No load powerfactor and hence no load phase angle

(iii) Iron and mechanical losses together

(iv) elements of equivalent circuit shunt branch

82What are the informations obtained from blocked rotor test in a 3-phase I M?

(i)Blocked rotor input current per phase at normal voltage

(ii) Blocked rotor power factor and hence phase angle

(iii) Total resistance and leakage reactance per phase of the motor as

refered to the stator

83 What is circle diagram of an I M?

When an I M operates on constant voltage and constant frequency

source,the loci of stator current phasor is found to fall on a circle.This circle

diagram is used to predict the performance of the machine at different loading

conditions as well as mode of operation.

84 What are the advantages and disadvantages of circle diagram method of

predetermining the performance of 3 –phase I M?

The prediction can be carried out when any of the following information

is availableThe input line current., the input power factor, The active power input, The

reactive power input, The apparent power input, The output power , The slip

of operation, The torque developed, The equivalent rotor current per phase,

Maximum output power, Maximum torque developed.

The only disadvantage is, being a geometrical solution, errors made during

measurements will affect the accuracy of the result.

85 What are the advantages and disadvantages of direct load test for 3 –phase I M?

Advantages

Direct measurement of input and output parameters yield accurate

results

Aside from the usual performance other performances like mechanical

vibration, noise etc can be studied.

By operating the motor at full load for a continuous period, the final steady

temperature can be measured.

Disadvantages

Testing involves large amount of power and the input energy and the entire

energy delivered is wasted

Loading arrangement cannot be provided for motors of large power rating

86 State the characteristic features of synchronous motor.

a. the motor is not inherently self starting

b. The speed of operation is always in synchronous with the supply frequency

irrespective of load conditions

c. The motor is capable of operating at any power factor.

87 In what way synchronous motor is different from other motors?

All dc and ac motors work on the same principle. Synchronous motor operates

due to magnetic locking taking place between stator and rotor magnetic fields.

88 Name any two methods of starting a synchronous motors

· By an extra 3 phase cage induction motor

· By providing damper winding in pole phases

· By operating the pilot excitor as a dc motor

89 What is the effect on speed if the load is increased on a 3 phase synchronous

motor?

The speed of operation remains constant from no load to maximum load in

the motor operating at constant frequency bus bars.

90 Why a synchronous motor is a constant speed motor?

Synchronous motor work on the principle of force developed due to the

magnetic attraction established between the rotating magnetic field and the

main pole feed. Since the speed of rotating magnetic field is directly

proportional to frequency the motor operates at constant speed.91 What is the phasor relation between induced emf and terminal voltage of a 3 phase

synchronous motor?

The rotating magnetic field is initially established by the prime source of

supply V. The main field then causes an emf e to get induced in the 3

phase winding. Hence when the machine operates as a synchronous motor

the emf phasor always lags the terminal voltage phasor by the load/torque

DQJOH

92 At what load angle is power developed in a synchronous motor becomes its

maximum value ?

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93 What are V and inverted V curves of synchronous motor ?

The variation of magnitude of line current with respect to the field current

is called V curve . The variation of power factor with respect to the field

current is called inverted V curve.

94 What happens when the field current of a synchronous motor is increased beyond

the normal value at constant input?

Increase in emf causes the motor to have reactive current in the leading

direction. The additional leading reactive current causes the magnitude of

line current, accompanied by the decrease in power factor.

95 Distinguish between synchronous phase modifier and synchronous condenser

A synchronous motor used to change the power factor or power factor in

the supply lines is called synchronous phase modifier.

A synchronous motor operated at no load with over excitation condition to

draw large leading reactive current and power is called a synchronous

condenser.

96 How the synchronous motor can be used as s synchronous condenser?

Synchronous motor is operated on over excitation so as to draw leading

reactive current and power from the supply lines. This compensates the

lagging current and power requirement of the load making the system

power factor to become unity. The motor does the job of capacitors and

hence called as synchronous condenser.

97 What type of single phase induction motor would you use for the following

applications?

(i) Ceiling fan (ii) Wet grinder

Ceiling fan – capacitor start and run motorWet grinder – capacitor start motor

98 After servicing a single phase fan it was found to run in reverse direction. What

could be the reason?

The connection to the starting/ auxiliary winding would have reversed.

99 What will be the direction of rotation of a shaded pole single phase induction

motor?

The motor rotates in the direction specified by the unshaded to shaded

region in the pole phase

100What is the property of a single phase single winding induction motor?

It has zero starting torque

101Which winding in a double cage induction motor has high resistance and low

inductance?

Outer cage winding

Long answer Questions

1. State the requirements for paralleling of alternators.

2. A two pole,50 Hz, 3-phase, turbo alternator is excited to generate the bus-bar

voltage of 11 kV on no load.The machine is star connected and the short

circuit current for this excitation is 1000A. Calculate the synchronizing power

per degree of mechanical displacement of the rotor and the corresponding

synchronizing torque.

3. A 3300V, 3 phase star connected alternator has a full load current of 100A.On

short circuit a field current of 5A was necessary to produce full-load

current.The e m f on open circuit for the same excitation was 900V.The

armature resistance was 0.8 ohm/phase. Determine the full load voltage

regulation for (i) 0.8 p f lagging (ii) 0.8 p f leading.

4. Explain the construction and principle of operation of 3-phase alternator

5. Develop the equivalent circuit of three phase I M

6. Explain the various techniques of speed control of 3-phase I M

7. A 3-phase ,star connected, 16 pole alternaror has 192 slots with 8 conductors

per slot, coil span=160 electrical degrees, speed of alternator=375 rpm, flux

per pole=55mWb.Calculate the line and phase values of EMF generated.

8. The following data refers to a 20pole,460V,60Hz, 3-phase I M :

R1=2 ohm,X1=1 ohm, R2’ =3 ohm, X2’ =7 ohm. When the motor is tested on

no load, it is observed that it takes 3.9A and the total core loss is 450W.By

using an approximate equivalent circuit at 5% slip, calculate

(i) Rotor current (ii) Supply current and pf (iii) Gross load torque

(iv) draw the equivalent circuit.9. Explain about crawling and cogging

10. Describe any two methods of determining the voltage regulation of 3-phase

alternator

11. Explain the operation of single phase induction motor on the basis of double

field revolving theory.

12. Explain the operation of the types of stepper motors.

13. A 3 MVA , 50Hz, 11 kV, 3-phase star connected alternator supplies 100A at

zero p f leading. The line voltage is 12370V.When the load is removed, the

line voltage is 1100V.Find the regulation at full load,0.8 pf lagging.

Ra=0,4ohm/phase.

14. Bring out the characteristics of two alternators working in parallel. What is the

effect of change in excitation on load sharing.

15. Derive the equation for torque developed by an I M. Draw the tipical torqueslip curve and deduce the condition for maximum torque.

16. Write a note on Hysterisis motor

17. Write short notes on (i)A C series motor (ii) Reluctance motor

18. How do you determine the direct axis and quadrature axis reactances of a

salient pole alternator?

19. A three phase star connected alternator has direct axis synchronous reactance

of 0,7p u and quadrature axis reactance of 0,5 p u,If the generator delivers

kVA at rated voltage at full load and lagging, find the percentage

regulation.Resistance drop at full load is 0,017 p u.

20. Two alternators are connected in parallel, what happens when we (i) increase

the excitation of one machine (ii) increase the steam supply of one machine.

21. Two similar 3000kVA synchronous generators work in parallel.The governor

of first machine is such that frequency drops uniformly from 50Hz on no load

to 48 Hz on full load.The corresponding speed drop of second machine is from

50Hz to 47.5Hz.determine the following

(i) How will the two machines share a load 5000kW at full load

(ii) What is the maximum load at u p f that can be delivered without

overloading either of the two machines.

22. Explain why synchronous motor is not self starting.

23. Explain the various schemes of starting of 3 phase I M

24. Describe how 3-phase supply produces a rotating magnetic field of constant

value at constant speed with vector diagrams.

25. Derive the EMF Equation of 3 phase alternator .Define distribution factor and

coil span factor?s

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