Electrical engineering Interview questions and answers

Electrical - Interview questions and answers

1.What is meant by regenerative breaking?

When the supply is cut off for a running motor,it still continue running due to inertia.In order to stop it quickly we place a load across the armature winding and the motors should have maintained continuous field supply. So that back emf voltage is made to apply across the resistor and due to load motor stops quickly.This type of breaking is called regenerative breaking.

2.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?

The motor has max load current compare to that of transformer because the motor consumes real power.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.

3.What is power factor?Power factor should be high or low? why?

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 apparent power.It has to be ideally 1.

4.Why series motors cannot be started on no load?

Series motors cannot be started without load because of high starting torque. Series motor are used in trains,cranes etc…

5.What is the difference between isolator and Circuit breaker?

Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits.

It is manually operated and does not contain any solenoid unlike circuit breaker. it should not be operated while it is having load. First the load on it must be made zero and then it can safely operated.

Circuit breaker is onload automatic device used for breaking the circuit in case of abnormal conditions like short circuit,overload etc…

It has three specification 1st is rated current and 2nd is short circuit breaking capacity and 3rd is instantaneous tripping current

6.What are the different type of transformer earthing ? What is the function of transformer earthing?

The two types of earthing are familiar as equipment earthing and system earthing. In equipment earthing body of the equipment should be earthed to safeguard the human beings.

System earthing : In this neutral of the supply source should be grounded. With this, in case of unbalanced loading neutral will not be shifted.

So that unbalanced voltages will not arise.We can protect the equipment also. With size of the equipment and selection of relying system earthing will be further classified into directly earthed,Impedance earthing,resistive.

7.What is SF6 Circuit breaker?

SF6 is sulpher hexa flouride gas.If the gas is used as arc quenching medium in a circuit breaker SF6 CB.

8.What is frantic effect?

Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.

9.What is the difference between MCB and MCCB where it can be used?

MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit.

MCCB molded case circuit breaker and is thermal operated for over loaded and magnetic operation for instant trip in short circuit condition.

Under voltage and under frequency may be in inbuilt. Normally it is used where normal current is more than 100A.

  1. Define IDMT relay?

It is an inverse definite minimum time relay.In IDMT relay its operating is inversely proportional and also a characteristics 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.

11.Why delta star transformers are 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 transformers is used for lighting loads.

12.What are the transformer losses?

Transformer losses - Transformer losses have two sources copper loss and magnetic loss.Copper losses are caused by the resistance of wire.

Magnetic losses are caused by eddy current and hysterisis in the core. Copper loss is a constant after the coil has been wound and therefor a measurable loss.

Hysterisis loss is a constant for particular voltage and current. Eddy current loss,however,is different for each frequency passed through the transformer.

13.What are the advantages of star delta starter with induction motor?

The main advantage of using the start delta starter is reduction of current during the starting of motor. Starting current is reduced to 3-4 times of current of direct online starting.

Hence the starting current is reduced,the voltage drop during the starting of motor in system are reduced.