01․ The teeth in the armature of a DC machines are sometimes skewed. This is done to reduce
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
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
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
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
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
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
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
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
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
The width of a carbon brush is generally equal to the width of 2 to 3 commutator segments to enhance the process of commutation.