# Electrical engineering interview questions answers

#### Electrical engineering interview questions answers

Question 1. Explain different losses in a transformer?

Iron Losses

Eddy Current Losses

Stray Losses

Ohmic Losses or Copper Losses

Hysteresis Losses

Dielectric Losses

Question 2. Explain the types of DC Motors.

Series DC Motors

In series motors, armature windings and field windings are connected in series.

A DC series motor converts electrical energy to mechanical energy.

The DC series motor works on the principle of a simple electromagnetic law which states that when a magnetic field is created around current carrying conductor and interacts with an external field, rotational motion is generated.

The main components of a DC series motor are the stator, armature (rotor), field windings, axle, brushes, and commutator.

Shunt DC Motors

In shunt motors, armature windings and field windings are connected in parallel or shunt. A Shunt DC motor operates on direct current. The shunt motor is able to maintain a constant speed with any load.

When a voltage is applied to the motor, a current produced in the armature and generates a strong magnetic field, in this way this produced field interacts with the magnetic field and makes the armature rotates.

Permanent magnet DC Motors

As the name indicates, the permanent magnet motor uses a permanent magnet to create field flux. The poles of permanent magnet DC motor are made of permanent magnet.

Working Principle:

When a current carrying conductor comes inside a magnetic field, a mechanical force will be experienced by the conductor and the direction of the force is governed by Fleming’s left-hand rule.

This type of DC motor provides great starting torque and has good speed regulation, but the torque is limited so these types of motors are typically used on low horsepower applications. Like- toys, computer disk drive, small robots etc.

Compound DC Motors

As the name indicates, the DC compound motor is a combination of the series motor and the shunt motor.

In the compound DC motor, a series field winding is connected in series with the armature and a shunt field is in parallel with the armature. This combination of the series field winding and the shunt field winding allows the motor to have the torque characteristics of the series motor and the regulated speed characteristics of the shunt motor.

Question 3. Name the various kinds of cables for transmission?

Answer: Following are the cables used for transmission

High tension cables- High tension cables can transmit voltage up to 23KV (23000 Volts)

Low-tension cables- Low-tension cables can transmit voltage up to 1 KV (1000 Volts)

Super- tension cables- Super- tension cables can transmit voltage up to 132 KV

Question 4. What are the reason and disadvantages of low power factor?

the reason of low power factor-

The reason for the low power factor is because of the inductive load. The current in the inductive load lag behind the voltage. So the power factor is lagging. The inductive loads responsible for low power factors are- welding equipment, three-phase induction motors, lamps etc.

Low power factor causes a large voltage drop, this results in the poor voltage regulation hence extra regulation equipment is required keeping the voltage drop in the prescribed range.

Larger transformers and generators are required.

Higher current is required by the equipment, so power consumption and hence the economic cost of the equipment increases.

At low power factor, the current is high, so copper losses are high in the system hence the system efficiency is reduced.

Question 5. What are the methods of improving power factor?

Answer: the methods for improving power factor are as follows-

Static Capacitors:

The static capacitors are installed parallel to the equipment or system running on low power factor to improve the power factor. Using capacitors, the current leads the voltage and hence the inductive load reduces. The capacitor bank is used to improve the power factor.

#### Synchronous Condenser

The synchronous condenser is a 3-phase synchronous motor which is overexcited. This motor operates under leading power factor and draws a leading current and hence is used to improve the power factor of an equipment or system.

In this method, an AC exciter also known as a phase advancer is used to improve the power factor.

The phase advancer is only used for induction motors because its stator winding draws exciting current that lags 90 degrees behind the voltage and results in the low power factor. Exciting ampere-turns are provided by the use of an external source to get rid of this problem.

Question 6. Explain real power, reactive power, and apparent power for an AC circuit?

Answer: Real power: Real power is the product of current, voltage and the power factor (the cosine angle between current and voltage). Real power is expressed in W or KW.

P= VIcosØ

Apparent power: Apparent is the product of current and voltage. Apparent power is expressed in VA or KVA

P= VI

Reactive power: Reactive power is the product of current, voltage and the sine angle between current and voltage. Reactive power is expressed in VAR or kVAR

P=VIsinØ

Question 7. What are the advantages of speed control using a thyristor?

Answer:  Following are the advantages of using a thyristor for speed control-

High Accuracy

Low cost

Fast Switching than BJT, IGBT, and MOSFET.

Question 8. What is the full form of ACSR cable and where it is used?

Answer: ASCR is Aluminium Conductor Steel reinforced. ACSR cable is used in transmission and distribution.

Question 9. 1 ton equals to how many watts?

Question 10. Why we use the VCB (Vacuum Circuit breaker) instead of ACB (Air Circuit Breaker) at high transmission system?

Answer: The vacuum has the high arc quenching property compared to the air. In the VCB, the dielectric strength is 8 times of air. So the vacuum used in the HT breakers instead of the air.

Question 11. Why series motors cannot be started at no-load?

Answer: Series motors cannot be started at no-load due to high starting torque.

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