# MCQs on Electrical Machines

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01․ Which of the following is/are the advantages of carbon brush over the copper brush?
They are not hard as copper brush
They are self lubricating in nature
In case of any sparking they will be less damaged than copper brushes
All of the above

Advantages of carbon brush over the copper brushes are as follows:
1. They are not hard as copper brushes.
2. They are self lubricating in nature which ensure excellent mechanical conditions with rotating commutator.
3. In case of any sparking they will be less damaged than copper brushes.
But they have less current density than copper brushes.

02․ The size of inter poles is small as compared to main poles. What is the reason?
In order to not to get saturated.
In order to get saturated.
In order to get more flux.
All of the above.

Inter poles are small poles and they are tapered in shape, intentionally designed with larger air gap than the main pole, so as not to get saturated when the load current flows through the inter polar windings.

03․ The functions of inter poles are
nullify reactance voltage and improve the commutation.
reduce cross magnetization effect and improve commutation
both 1 and 2
none of the above

Interpoles serves two functions 1. It produce a counter emf in the coil undergoing commutation because of its opposite polarity and nullify the reactance voltage produced during commutation. This will improve the commutation. 2. It reduces the cross magnetization effect of armature reaction and impoves commutation.

04․ Which of the following statements is/are correct ?
Inter pole winding will act in interpolar region.
Compensating winding will act under the pole.
Both A and B.
None of the above.

Compensating winding will act under the pole to reduce armature flux and demagnetizing effect of armature reaction. Inter pole winding will act in inter polar region to improve commutation where compensating can't work.

05․ In DC machine shape of main field flux distribution is?
triangular
flat tapped or trapezoidal
saddle shape
peaky in nature

The main field flux distribution is flat tapped or trapezoidal in shape.

06․ In DC machine shape of armature MMF wave is?
Triangular and directed towards brush axis.
Triangular and directed towards main pole.
Saddle shape.
Peaky in nature.

The shape of armature MMF is triangular in nature and directed towards brush axis.

07․ In DC machines shape of net flux density is?
sine wave
flat tapped or trapezoidal
saddle shape
peaky in nature

In DC machine the net flux density due to armature reaction is peaky in nature.

08․ In DC machines internal characteristics are plotted between
Eg vs load current
Vt vs Ia
Eg vs Ia
Vt vs load current

To know the application the characteristics should be known. It is basically variation of terminal voltage with load. Characteristics are divided in to 1. No load 2. load. Load characteristics are again divided in to 1. Internal characteristics (Eg vs Ia) 2. External characteristics (Vt vs load current)

09․ Series generators are used in which of the following applications?
Air crafts.
Arc welding.
Used as boosters in dc distribution or transmission.
All of the above.

Series generators have rising voltage characteristics, which are not at all suitable for ordinary power supplies but they are used as boosters in dc distribution or transmission to compensate the voltage drop.

10․ Which of the following generators are used in arc welding?
shunt generators
series generators
cumulative compound generators
differential compound generators

To limit the welding current which resembles short circuit, DC differential compound generators are used.

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