 # Basic Electrical interview questions & answers

## 1. Why we use 3-phase supply? Why are not 4 or something else?

3 phase power generating and delivering is more efficient than 2 phase which in turn is more efficient than 1 phase. Similarly, going up in the number of phases result in increased efficiency as well, i.e. 4 phase and 5 phase is more efficient than 3 phase. The increase of efficiency as the number of phases increase is attributed to that power delivery becomes more continuous as the number of phases increases and It affects the Installation cost of transmission of Transmission towers.

## 2. Why is the starting current high in a DC motor?

In DC motors,
Voltage equation is V=Eb-IaRa (V = Terminal voltage, Eb = Back emf in Motor,Ia = Armature current, Ra = Armature resistance).

At starting, Eb is zero.
Therefore, V=IaRa, Ia = V/Ra , where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.

## 3. What are the advantages of star-delta starter with induction motor?

• The main advantage of using the star delta starter is the reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online starting.

• Hence the starting current is reduced; the voltage drops during the starting of the motor in systems are reduced.

## 4. Why are Delta-Star-Transformers used for Lighting Loads?

For lighting loads, neutral conductor is must and hence the secondary must be star winding. And this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta/star transformer is used for lighting loads.

## 5. Why can series motor not be started on no-load?

Series motor cannot be started without load because of high starting torque. Series motor is used in Trains, Crane etc.

## 6. What is 2 phase motor?

A two-phase motor is a motor with 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 degrees.

## 7. what is the difference between synchronous generator & asynchronous generator?

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 is used in windmills.

## 8. What is Automatic Voltage regulator(AVR)?

AVR is an abbreviation for Automatic Voltage Regulator.It is important part in Synchronous Generators, it controls the output voltage of generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

## 9. Why the capacitors work on ac only?

Generally, capacitor gives infinite resistance to dc components(i.e., block the dc components). it allows the ac components to pass through.

## 10. What is the function of anti-pumping in circuit breaker?

When the breaker is closed at one time by close push button, the anti-pumping contactor prevents reclose the breaker by close push button after already close.

## 11. Define IDMT relay?

It is an inverse definite minimum time relay. In IDMT relay its operating is inversely proportional and also a characteristic of minimum time after which this relay operates. It is inverse in the sense, the tripping time will decrease as the magnitude of fault current increase.

## 12. What is SF6 Circuit Breaker?

SF6 is Sulpher hexa Fluoride gas… if this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.

## 13. What is the function of anti-pumping in circuit breaker?

when the breaker is close at one time by close push button he anti-pumping contactor prevent reclose the breaker by close push button after if it already close.

## 14. How many types of cooling system it transformers?

• ONAN (oil natural, air natural)
• ONAF (oil natural, air forced)
• OFAF (oil forced, air forced)
• ODWF (oil direct, water forced)
• OFAN (oil forced, air forced)

## 15. What is frantic effect?

The output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.

## 16. What are HRC fuses and where it is used?

HRC stands for “high rupturing capacity” fuse and it is used in the distribution system for electrical transformers.

## 17. What is electrical diversity factor in electrical installations?

Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of the system, to the maximum demand of the whole system, or part of the system, under consideration. Electrical diversity factor is usually more than one.

## 18. What is the difference between delta-delta, delta-star transformer?

Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i,e) generally it is used where the Voltage high & Current is low.Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.