Electrical Machines 1 Viva Questions and Answers

Electrical Machines 1 Viva Questions and Answers

In this post we had collected some of the most frequently asked electrical machines 1 viva questions with detailed answers. We believe that these FAQs will be useful for your practical exams and interviews as well.

[Q] What happens if DC supply is applied to the transformer?

• If DC supply is given to the transformer, the current will not change due to constant supply.
• So the mutual induction is not possible and thus the transformer will not work.
• The primary winding resistance is very small.
• For DC supply, the primary winding inductive reactance is zero.
• So the primary will draw very high current for dc supply, consequently extra heat generated and thus the transformer get damaged.
• Hence the DC supply should not be applied to the transformer.

[Q] What is the application of equivalent circuit of a single phase transformer? [Anna Univ. Dec-2005]
The equivalent circuit is the electrical model of the transformer. Once the equivalent circuit parameters are obtained, then the regulation & efficiency of the transformer at any power factor and load conditions can be obtained without actually loading the transformer.

[Q] What is synchronous speed?
For synchronous machines there exists a fixed relationship between number of poles P, frequency f and the speed of the machine. The speed of the synchronous machine for the given number of poles and the rated frequency is called the synchronous speed (Ns)
The expression for synchronous speed is
Ns = 120f/P

f = frequency
P = No of poles of the machine

[Q] What is meant by torque? or Define torque?
A turning or twisting force about an axis is defined as torque.

[Q] How can we reduce the eddy current loss in the electrical machine?

• By using laminated core construction, the eddy current loss can be reduced.
• The laminated core divides the solid iron core into thin laminations.
• The path of eddy currents is broken due to the insulating material sheets between the laminations.
• Thus the eddy currents and losses generated by them can be minimized.

[Q] In DC generators, the series field winding has low resistance while the shunt field winding has high resistance. Why?

• Always the series field winding is connected in series with armature.
• Hence it has to carry the armature current which directly gets decided by the load.
• So the current passing though series field winding is of high level.
• The voltage drop across series field winding gets added to the voltage drop across armature winding while deciding the back emf. This voltage drop must be very small.
• Hence as the current through series field winding is high, in order to keep voltage drop across it to a small value, its resistance is very low.
• The shunt field winding is directly connected across the rated supply voltage hence to limit current through it, resistance is very high.

[Q] Explain the principle of the transformer? [Anna Univ. May-2010]
The transformer works in the principle of mutual induction.
Mutual Induction:
When two coils are inductively coupled and if current in one coil is changed uniformly then an emf gets induced in the other coil.When the path is closed on the second coil, the induced emf can drive a current on it.

[Q] What is transformer?
The transformer is a static piece of apparatus by means of which an electrical power is transformed from one alternating current circuit to another with the desired change in voltage and current without any change in the frequency.

6 Responses

1. Gmichael says:

Those questions are most basic questions one electrical engineers must known it.

2. NIKHILA says:

3. Hanuma says:

I'm very thank full to you minimum basics on electricals

4. Hanuma says:

easy to remember. rly imp qns

5. jk says:

Thanks

6. WOLDE YOU says:

wow!!!!! it is GREAT!!!!!!
10Q