MCQs on Electrical Machines

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01․ In a DC generator if field winding attains the critical resistance
machine will generate maximum voltage.
machine will generate maximum power.
field winding will burn.
the voltage generated will be zero.

In a DC generator if field winding attains the critical resistance, the voltage build-up will not take place and the generated emf will be zero.
02․ In a DC machine, the angle between the stator and rotor fields is
0o.
90o.
180o.
dependent upon the load.

The angle between the stator and rotor fields is zero to generate maximum torque.
03․ The purpose of providing dummy coils in a generator is
to enhance flux density.
to amplify voltage.
to provide mechanical balance for the rotor.
to reduce eddy current.

In DC machine there are some coils in armature winding called dummy coils which are electrically not connected so there is no any current in these coils, so these coils don't give any contribution in power. These coils are there only to provide mechanical balance.
04․ Lamination of core are generally made of
cast iron.
carbon.
silicon steel.
stainless steel.

Because of magnetic losses, rotor is built using armature core which in turn is made with many low-hysteresis silicon steel laminations. These laminations are stacked together to create cylindrical structure. Thus hysteresis and eddy current loss are reduced.
05․ Poles of DC machine are often laminated to
reduce pulsation loss.
reduce iron weight.
dissipate more heat.
reduce armature reaction.

The pulsation losses are rotor surface losses and eddy-current losses in the rotor bars and therefore the poles are laminated to reduce them.
06․ The ventilating ducts in most of DC machine are
radial.
radial as well as longitudinal.
longitudinal.
none of these.

Both axial and longitudinal ventilation systems are used in DC machines.
07․ In a commutator
copper is harder than mica.
mica and copper are equally hard.
mica is harder than copper.
none of above.

Mica in commutator is stronger than copper to protect cascading failure of commutator and brush assembly.
08․ In DC machine, the armature windings are placed on the rotor because of necessity for
electromechanical energy conversion.
generation of voltage.
commutation.
development of torque.

The armature windings are placed on the rotor in order to facilitate the process of commutation.
09․ What are slot wedges in a DC machine made of?
Mild steel.
Cast iron.
Silicon steel.
Fibre.

Slot wedges are used to reduce the air gap flux to reduce armature reaction. So they are made up of silicon steel.
10․ The armature of DC machine is made of silicon steel lamination to
reduce hysteresis loss only.
reduce eddy current loss only.
increase the permeability.
reduce both eddy current and hysteresis loss.

Because of magnetic losses, armature core is made with many low-hysteresis silicon steel laminations. These laminations are stacked together to create cylindrical structure. Thus hysteresis and eddy current loss are reduced.
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