DC Machines Questions and Answers for Experienced people on “Ways to Reduce Effects of Armature Reaction – 2”.
1. Axis undergo shifting as a result of armature reaction, can be balanced by ______
a) Increase in armature current
b) Decrease in armature current
c) Introducing interpoles
d) Removing interpoles
Answer: c
Clarification: Apart from distortion of the resultant flux density wave, its MNA also gets shifted from its GNA by a small angle α so that the brushes placed in GNA are no longer in MNA as is the case in the absence of armature current, due to armature reaction. This effect is countered by the interpoles placed in GNA.
2. The choice of average coil voltage determines the minimum number of commutator segments for its design.
a) True
b) False
Answer: a
Clarification: The maximum allowable voltage between adjacent segments is 30–40 V, limiting the average voltage between them to much less than this figure. The choice of the average coil voltage determines the minimum number of commutator segments for its design, to avoid any flashover and ultimately short circuit.
3. Compensating winding will provide incomplete neutralization ____________
a) Under pole region
b) In interpolar region
c) Everywhere
d) Complete neutralization
Answer: b
Clarification: The compensating winding neutralizes the armature mmf directly under the pole while in the interpolar region, there is incomplete neutralization. Further, the effect of the resultant armature mmf in interpolar region is rendered insignificant because of large interpolar gap.
4. Cross-magnetizing effect of armature reaction can be reduced by __________
a) Removing saturation in teeth and pole-shoe
b) Making smooth pole shoes
c) Introducing saturation in teeth and pole-shoe
d) Cannot be determined
Answer: c
Clarification: The cross-magnetizing effect of the armature reaction can be reduced by making the main field ampere-turns larger compared to the armature ampere-turns such that the main field mmf exerts predominant control over the air-gap flux. This is achieved by introducing saturation in the teeth and pole-shoe.
5. Cross-magnetizing effect of armature reaction can be reduced by __________
a) Removing saturation in teeth and pole-shoe
b) Making smooth pole shoes
c) Chamfering the pole shoes
d) Cannot be determined
Answer: c
Clarification: By chamfering the pole-shoes which increases the air-gap at the pole tips. This method increases the reluctance to the path of main flux in a DC machine but its influence on the cross-flux is much greater. This is because the cross flux has to cross the air-gap twice.
6. To counter the effect of shift in MNA due to armature reaction, which of the following component can be shifted?
a) Poles
b) Commutator
c) Brushes
d) Cannot be determined
Answer: c
Clarification: To counter the effect of shift in MNA due to armature reaction, the brushes could be shifted. A small brush shift in appropriate direction, in the direction of rotation for generator and in opposite direction for motor, also helps in commutation.
7. Calculate the number of conductors on each pole piece required in a compensating winding for a 6-pole lap-wound dc armature containing 286 conductors. The compensating winding carries full armature current. Assume ratio of pole arc/ pole pitch = 0.7.
a) 6
b) 8
c) 9
d) 7