Electrical Interview Questions and Answers

1) What is the difference between a single pole and a double pole circuit breaker?

Single-pole breakers have only one hot and one neutral wire. When an overload occurs in a single-pole breaker's circuit, only that breaker trips.

In double-pole breakers, two hot wires are linked by a single neutral wire. That is, if any of the poles' hot wires shorts out, both trip.

2) What Is the Difference Between Fuse And Breaker?

When the current exceeds the fuse's limit, a metal wire called the fuse link or element melts. It operates automatically and is a one-time use item that must be replaced.

A circuit breaker is an electro mechanical switch that opens the circuit in the event of an over current or short-circuit. It works both automatically and manually and can be used again simply resetting the lever.

3) What is the difference between circuit breaker & Isolator?

A circuit breaker, like a fuse, is a protective electromechanical device used to control the flow of current. In the event of a fault condition such as a short circuit or overload, it automatically breaks the circuit. It can also break the circuit manually. It is an ON-load and OFF-load device, which means it can operate in both ON and OFF supply conditions.

Isolator: An isolator is a mechanical switch used in substations to isolate or disconnect electricity supplies. It is an off-load device, which means it operates when the power supply is turned off.

4) Why Motor rated in kW instead of kVA?

The transformer is rated in KVA since its PF (Power Factor) is affected by the nature of the loads. However, the motor has a fixed power factor, i.e. the motor has a defined power factor (P.F) and the rating is stated on the motor in kW or HP. To put it another way, the motor only uses active electricity and provides mechanical power in HP or kW at the motor shaft, hence the motor rating in Watts.

5) What is the definition of generator & motor?

A machine that transforms electrical energy to mechanical energy is known as an electric motor. A motor's operation is based on a current-carrying conductor that experiences force when kept in a magnetic field. A machine that transforms mechanical energy to electrical energy is known as an electric generator. The generator's operation is based on electromagnetic induction.

6) What is a motor starter?

A motor starter is a device that connects in series with the motor to reduce the starting current (which, under normal conditions, could damage the winding) and gradually increase current after starting the motor (in other words, start or stop the motor) and offer overload safety.

7) What are the different methods for starting an induction motor?

The following methods are used to start an induction motor:

DOL: direct online starter

Star delta starter

Auto transformer starter

Resistance starter

Series reactor starter

8) What is the difference between a generator and an alternator?

Both the alternator and generator operate on the basis of Faraday's law of electromagnetic induction. An alternator is a device that converts mechanical energy into (alternating current) electrical energy. It always generates alternating current. Alternators have a high efficiency. A Generator is a mechanical device that converts mechanical energy into either alternating current  or direct current  electrical energy. It can produce alternating or direct current.Generators are thought to be less efficient. The primary distinction between an alternator and a generator is that in an alternator, the armature is stationary while the field spins, but in a generator, the armature rotates while the field remains stationary.

9) What Are The Advantages Of Star¬ Delta Starter With Induction Motor?

The following are the primary benefits of using a star delta starter:

To reduce the starting current required for the induction motor, which is 6 to 7 times higher than full load current and can damage the motor's windings.

To eliminate the voltage drop problem caused by the high starting current results in a voltage drop down the consumer line, which may cause harm to other electrical equipment.

Its operation is quite simple.

This beginning kit is rather inexpensive.

It has a strong torque-to-current ratio.

 

10) Why is the starting current high in dc motor?

There is no back emf in a dc motor. The resistance of the circuit controls the armature current when the motor starts. The armature has a low resistance, and when the entire voltage is provided to the motor while it is at rest, the armature current becomes extremely high, causing damage to the motor's components.

11) What is an induction motor's slip?

Slip is the percentage difference between an induction motor's synchronous speed Ns and rotor speed N. It is represented by the letter S. The induction motor's rotor speed is always smaller than its synchronous speed.

12) Why can't a series motor be started on no-load?

No load should never be applied to a series motor. Because there is no mechanical load on the series motor, the current is low, and the counter-EMF produced by the field winding is weak, therefore the armature must revolve faster to produce enough counter-EMF to balance the supply voltage. Over speeding might cause harm to the motor.

13) Explain the principle of Induction Motor.

To make a DC motor rotate, we must provide double excitation. We provide one supply to the stator and another to the rotor in a DC motor via a brush arrangement. However, because we only provide one supply in an induction motor, it is interesting to learn how an induction motor operates.

It is simple, from the name itself we can understand that here, the induction process is involved. When we turn on the power to the stator winding, a magnetic flux is created in the stator as a result of the current flowing through the coil. The rotor winding is constructed in such a way that each coil becomes short-circuited.

The flux from the stator short-circuits the rotor's short-circuited coil. According to Faraday's law of electromagnetic induction, when the rotor coils are short-circuited, current begins to flow through the rotor coil. When current goes through the rotor coils, additional flux is generated in the rotor.

There are now two fluxes, one stator flux and one rotor flux. The rotor flux will be slower than the stator flux. As a result, the rotor will experience torque, causing it to revolve in the direction of the revolving magnetic field. This is how single-phase and three-phase induction motors work.

14) What is the difference between a Four Point Starter and a Three Point Starter?

The starter which consist three terminals is known as the three point starter. The three-point starter's terminals are the armature, field, and line. The no-voltage coil (NVC) is linked in series with the field winding in the three-point starter.

The starter with four terminals is known as a four point starter. Along with the armature, field, and line terminals, a four-point starter has an extra terminal that connects the no voltage coil in parallel with the shunt field winding. The no-voltage winding is linked in parallel with the field winding in a four-point starter.

The three-point and four-point starters are built similarly. When the speed of the motor varies with a three-point starter, however, current flows through the field coil, affecting the no-voltage coil. The four-point starter is intended to alleviate the difficulty.

15) What Is Meant By Regenerative Braking?

When the motor's speed surpasses the synchronous speed, regenerative braking occurs. This baking method is known as regenerative braking because the motor acts as a generator and the supply receives power from the load, i.e. motors. The rotor must rotate faster than synchronous speed for regenerative braking to occur; only then will the motor operate as a generator, and the direction of current flow through the Circuit & torque direction reverse and braking occurs. The primary disadvantage of this sort of braking is that the motor must run at super synchronous speed, which may cause mechanical and electrical damage to the motor; however, regenerative braking can be performed at sub synchronous speed if a variable frequency source is provided.