2-15
ARMATURE REACTION is the distortion of the main field in a motor by the armature field. This
causes the neutral plane to be shifted in the direction opposite to that of armature rotation. Interpoles and
compensating windings are used to reduce the effect of armature reaction on motor operation.
STARTING RESISTORS are necessary since the dc resistance of a motor armature is very low.
Excessive current will flow when dc voltage is first applied unless current is limited in some way. Adding
resistance in series with the armature windings reduces initial current. It may then be removed after
counter emf has been built up.
ANSWERS TO QUESTIONS Q1. THROUGH Q16.
A1.
Direction of armature current, and direction of magnetic flux in field.
A2.
Direction of conductor movement (rotation), direction of flux, and the direction of current flow.
A3.
There are no differences.
A4.
Generator action.
A5.
Speed.
A6.
The device to be driven by the motor.
A7.
It must have a load connected to avoid damage from excess speed.
A8.
High torque (turning force) at low speed.
A9.
It maintains a constant speed under varying loads.
A10.
Only outside of coils cut flux (inefficient).
A11.
By winding the armature in a way that places the entire coil where it is exposed to maximum
flux.
A12.
By reversing either field or armature connections.
