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