08-04-2015, 08:25 AM
I want Em lab viva questions and answers
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electrical machines lab viva questions answers
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I want Em lab viva questions and answers
15-04-2017, 12:28 PM
[Q] What happens if DC is applied to the transformer?
Answer
If DC is supplied to the transformer, the current will not change due to constant supply.
So mutual induction is not possible and therefore the transformer will not work.
The resistance of the primary winding is very small.
For the DC supply, the primary coil inductive reactance is zero.
Thus, the primary will draw a very high current for the DC supply, which will generate extra heat and, therefore, the transformer will be damaged.
Therefore, the DC power must not be applied to the transformer.
[Q] What is the application of the equivalent circuit of a single-phase transformer? [Anna Univ. Dec-2005]
Answer
The equivalent circuit is the electrical model of the transformer. Once the equivalent circuit parameters are obtained, the transformer regulation and efficiency can be obtained in any power factor and load conditions without loading the transformer.
[Q] What is synchronous speed?
Answer
For synchronous machines there is a fixed relationship between the number of poles P, the frequency f and the speed of the machine. The speed of the synchronous machine for the given number of poles and the nominal frequency is called the synchronous speed (Ns)
The expression for the synchronous speed is
Ns = 120f / P
F = frequency
P = Number of poles of the machine
[Q] What is meant by pair? Or Define the pair?
Answer
A rotating or torsional force about an axis is defined as even.
[Q] How can we reduce the loss of eddy currents in the electric machine?
Answer
By using the laminated core construction, the loss of eddy currents can be reduced.
The laminated core divides the solid iron core into thin laminations.
The trajectory of parasitic currents is broken due to the sheets of insulation material between the laminations.
Thus parasitic currents and losses generated by them can be minimized.
[Q] In DC generators, the series field winding has low resistance while the branch field winding has high resistance. Why?
Answer
Always the series field winding is connected in series with the armature.
It is therefore necessary to carry the current of the armature that is decided directly by the load.
So the current passing though the field winding series is high level.
The voltage drop across the series field winding is added to the voltage drop across the armature winding while deciding the subsequent emf. This voltage drop must be very small.
Therefore, as the current through the winding field series is high, in order to keep the voltage drop across it to a small value, its resistance is very low.
Bypass field winding is connected directly through the nominal supply voltage, therefore, to limit the current through it, the resistance is very high.
[Q] Explain the principle of the transformer? [Anna Univ. May-2010]
Answer
The transformer works on the principle of mutual induction.
Mutual Induction:
When two coils are coupled inductively and if the current in one coil is uniformly changed then one emf is induced in the other coil. When the trajectory is closed in the second coil, the induced emf can conduct a current on it.
[Q] What is the transformer?
Answer
The transformer is a static piece of apparatus by means of which an electric power from one circuit of alternating current is converted to another with the desired change of voltage and current without any change in frequency.
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