# Top Electrical Engineering Interview Question and answers part-3

In this post, we will discuss the Top Electrical Engineering Interview Question and Answers. these Question and Answers may be helpful in your interviews.

##### Top Electrical Engineering Interview Question and answers part-3

Question: why we prefer VFD (variable frequency drive) in place of star delta starter?

Answer: We all know that for synchronous motors, starting current and torque is very high, this may lead to burning the winding of the motor when we will start the motor using star delta starter.

In VFD, we control the frequency. The current is directly proportional to frequency so a motor can be started at low starting current.

Another advantage of using VFD over star delta starter is that we can run the motor at any rpm, this saves the power.

Question: is there any mechanical connection between the field winding and armature winding of a DC Generator?

Answer: DC generators are of two types:

Self Excited DC Generators.

Separately Excited DC Generators

The mechanical connection between the field winding and armature winding of a dc generator depends on the type of DC generator.
In separately excited DC generators, field windings and armature windings are not connected.

Self Excited DC Generators are further categorized into two types

Series Excited DC Generators

Shunt Excited Generators

In Series Excited DC Generators, field  windings and armature windings  are connected in series

In Shunt Excited DC Generators, field windings and armature windings are connected in parallel.

Question: What is AVR and describe its working?

Answer:  AVR is an Automatic Voltage Regulator. AVR is a device that is designed to automatically control, adjust or maintain a constant voltage level.  An AVR (Automatic voltage regulator) is a feedback control system.

An AVR works on the principle of detection of the error. AVR takes alternator output voltage as feedback and compares this output to the set point and generates an error signal that is used to adjust the excitation of the alternator and maintain terminal voltage constant from no load to full load.

AVR makes the terminal voltage constant by adjusting the excitation of the Alternator.

The AVR Panels will have 3 modes.

PF or VAR mode

Auto mode

Manual mode or current feedback mode

The alternator consists of rotor winding and stator winding. The output terminal voltage is taken from the stator winding.

The rotor of the alternator is coupled to the turbine.

Question. what is the difference between earthing and neutral?

Answer: The Earthing is connecting the body of any electrical equipment to the earth pit. When any fault occurs between the body and the circuit, the current flows to the ground through earthing. So earthing is basically a protection of electrical equipment.

Whereas, the neutral is defined as a closed path of an electrical circuit having zero potential. if we connect an electrical equipment to the phase only but keep the other end open without connecting the neutral then the circuit is not completed. So in this condition, the electrical equipment does not function. Once the neutral is connected to the circuit, the circuit is completed and the electrical equipment starts work.

Question: N=120f/p. what is 120?

Here N= rpm (revolution per minute) of a motor.

f= frequency

p= no. Of poles

f=1/T    here,         T = time period for one full cycle.
since 1 revolution is completed in T seconds.
So, in 1-second 1/T revolutions or f full waves are formed.

One South Pole and one North Pole produce a cycle.

So p=2.

If there is P no. of poles, then P/2 no. of full waves will be formed in one full revolution of the field.

Now, if n=revolution of the field in 1 second,
then f= (P/2)*n
or n=2f/p.

When we take the rotation of the field in one minute
N= (60*2*f)/P

(Where N= revolution per minute)
or  N=120f/P.

Question: Why we use transformers, what happens if we will not use the transformer?

Answer: transformer is a static device.  the transformer is used to transfer current, voltage, power without changing its frequency.

The transformer is categorized into two types: step up and step down and is used to increase or decrease the voltage.

If we transform the power without transformer – on increasing the distance of transmission the voltage rating will be down.

Question : Define CT & PT?

Answer: CT is the current transformer and is used to measure the current and PT is the Potential Transformer and is used to measure the voltage.

CT: The line current is too high, and it is very difficult to measure the line current directly. So we use the current transformer. CT (Current Transformer) decreases the high value of current into a fractional value which is easy to measure by the instrument.

The line whose value is to measure is connected directly to The primary of the current transformer. The secondary of the current transformer will be connected to the meter or ammeter. Since CT decreases the high value of current into fractional value so it is easy to measure the value by an ammeter or a meter.

PT:  PT reduces the high voltage values into the small voltage values. Primary of the Potential transformer is connected directly to the line whose voltage is to measure.  The secondary of the Potential transformer is connected to a voltmeter or meter. A voltmeter can easily measure the value.

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