Electrical Interview Question

Electrical Interview Question

1.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.

2. 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 electromagnetic flux 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 isthe operation of PT but measuring voltage.

3. 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.

4. 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).

5.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.

6. 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.

7. Difference between a four point starter and three point starters?

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

8.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 .

9. What is the difference between surge arrestor and lightning arrestor?

Answer: A 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.

10. 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 cannot start the Induction motor and Synchronous motors on load, but cannot start the DC series motor without load.

11. What is ACSR cable and where we use it?

Answer: ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.

 12. What is vacuum circuit 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.

13. 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).

14. 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

15.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.

16.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.

17.Give two basic speed control scheme of DC shunt motor?

Answer: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.

18)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.

19)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.

20)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.

21)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.

22)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 .

23)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.

24)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.

9)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.

10)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 pow varying the field emf. Hence syn. generators r used for the production of electricity.

11)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).

1)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.

2)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.

3)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.

4)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.

7)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 verycheap to afford, &
3. It also reduces over heating of tranformers.

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

9)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.

10)Advantages of vvvf drives over non vvvf drives for EOT cranes?
Answer:

1.      Smooth start and stop.

2.      No jerking of load.

3.      Exact positioning

4.      Better protection for motor.

5.      High/low speed selection.

6.      Reliability of break shoe.

7.      Programmable break control.

8.      Easy circuitry

9.      Reduction in controls

10.  increases motor life

11) 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 phase difference between the current and voltage. EG. DYN11 means Delta primary, Wye Secondary and the current is at 11 o clock referred to the voltage.

12) Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal 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.

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 days, magnetic traction is also used for bullet trains. Basically dc motors are used for electric traction systems.

2) How can you start-up the 40w tube light 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.

3) 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 components (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).

4) 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

5) 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 connections 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 happens the fuse connected in the particular control circuit alone will blow off.

6) 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 rheostat control resistance is decreased by steps to achieve speed control.

7) 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.

8) Whats is MARX CIRCUIT?

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.

9) What is encoder, how it function?

Answer: An encoder is a device used to change a signal (such as a bit stream) 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.

10) What are the advantages of speed control using thyristor?
Answer: Advantages:

1. Fast Switching Characteristics than Mosfet, BJT, IGBT

2. Low cost 3. Higher Accurate.

11) Why Human body feel Electric shock?? N in an Electric train during running, we did not 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.

12) What is the principle of motor?

Answer: Whenever a current carrying conductor is placed in a magnetic field it produce turning or twisting movement is called as torque.