MCQs on Electrical Machines

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01․ The teeth in the armature of a DC machines are sometimes skewed. This is done to reduce
hysteresis loss.
eddy electric current loss.
vibrations.
copper loss.

The air gap length between the slot and teeth of the armature changes during the rotation of the rotor and this causes production of variations in torque leading to vibrations in the frame of the machine. If the armature teeth are skewed, the vibrations become limited.
02․ The number of brushes in a commutator depends on
speed of armature.
type of winding.
amount of electric current to be collected.
voltage.

The number of brushes in a commutator depends on amount of electric current to be collected as the brushes collect the DC current from the commutator after commutation process.
03․ The insulating material used between the commutator segments is normally
mica.
paper.
graphite.
insulating varnish.

The insulating material used between the commutator segments is normally mica as it has a very high dielectric stress of around 1180kV/cm.
04․ Copper brushes in DC machine are used
where low voltage and high currents are involved.
where high voltage and low current.
in both of above cases.
in none of the above cases.

Although copper brushes provide poor commutation they are used instead of carbon brushes in applications which require low voltage and high currents.
05․ The bearings used to support the rotor shafts are generally
magnetic bearings.
ball bearings.
needle bearings.
bush bearings.

The bearing used to support the rotor shafts are generally ball bearings as they reduce the frictional losses.
06․ In DC generators, the brushes remain in contact with conductors which
lie under north pole.
lie under south pole.
lie in the inter polar gaps.
there is no hard and fast rule for this.

The brushes on commutator remain in contact with conductors that lie in the inter polar region to facilitate the collection of current by commutation process.
07․ The polarity of a DC generator can be reversed by
reversing the field current.
increasing the field current.
reversing the field electric current and also the direction of rotation.
none of the above.

In a DC generator, if we reverse the field current then the polarities of voltage generated is reversed as the current flows in opposite direction.
08․ The brushes of a DC machine should be placed on the
commutator in the inter polar axis.
commutator in the polar axis.
armature midway between poles.
armature in the polar axis.

The brushes of a DC machine should be placed on the commutator in the polar axis. However, they should be given a slight shift and placed on the magnetic neutral axis to reduce the effect of armature reaction.
09․ A DC generator beyond critical resistance will generate
maximum power.
maximum voltage.
no voltage.
maximum current.

In a DC generator if field winding resistance is more than the critical resistance, the voltage build-up will not take place and the generated emf will be zero.
10․ Width of a carbon brush should be equal to
less than the width of one commutator segment.
the width of more than 3 commutator segments.
the width of 2 to 3 commutator segments.
the width of 1 to 2 commutator segments.

The width of a carbon brush is generally equal to the width of 2 to 3 commutator segments to enhance the process of commutation.
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