Test: Effect of Brush Shift & Compensation Windings - Electrical Engineering (EE) MCQ

The brushes are placed along magnetic neutral axi so that in ideal conditions commutations take place without sparking. 

For the generator, the electromagnetic torque produced is in the opposite direction and so the induced current. So to eliminate the sparking the brushes should be shifted in the forward direction of rotation.

The armature reaction flux at the trailing edge weakens the flux and at the leading edge is strengthens the flux in the dc motor.

Since the time taken is more than the prescribed time limit for the commutation, it is under commutation.

• GNA: Stands for geometrical neutral axis, and is the axis that bisects the angle between the center lines of adjacent poles.

• MNA: Stands for magnetic neutral axis, and is the axis that is perpendicular to the mean direction of the flux passing through the center of the armature.

 In a practical dc machine, the coil undergoing commutation lies along GNA so as the brushes and the coils which actually get short circuited lies along mna. This lead to sparking as a voltage difference is created at the coils.

 The interaction between the field excitation and armature supply create the mmf and the interconnected behaviour creates the emf.

The static emf induced inside gets reduced due to the armature reaction in large machines when there is no compensating winding present.

Compensating conductors per pole face is Z/A*Ia.

AT/pole for the compensating winding = 0.7*(Z*I/2P)
= 0.7*(860*100/2*10)
= 3010 AT.

All the options are valid for the interpoles except a because they are connected in series not in parallel.

All the given measures are used to eliminate the effect of armature reaction and hence improves commutation also.

By reversing the current direction in the MNA the current can be limited in the coils to avoid sparking.

The armature reaction is initiated by the loaded conditions of the machine because an unloaded dc machine does not armature flux to get distorted.

The compensating winding functions such that to nullify the cross magnetization effects in the armature.

In the saturation region the flux will be constant.