Armature Reaction and Commutation Questions and Answers

Answer: B

Correct answer: the effect of the armature's own field on the main field

The armature current produces its own magnetomotive force, and that field adds to the main field on one side of each pole and subtracts on the other. The result is a distorted and slightly weakened resultant field with its neutral axis displaced - which is why the effect matters for both the generated voltage and the commutation.

Answer: C

Correct answer: the reversal of current in a coil as it passes the brushes

As a coil moves from one side of a brush to the other its connection to the external circuit reverses, so the current in it must reverse too. Doing that cleanly, in the brief interval the brush short-circuits the coil, is what the interpoles, the brush grade and the brush position all exist to achieve.

Answer: D

Correct answer: hard-drawn copper segments separated by insulation

The segments are hard-drawn copper for wear resistance and conductivity, separated by mica or a similar insulant. The mica is undercut slightly below the copper, so that as the copper wears the insulation does not stand proud and lift the brushes - a routine but essential piece of maintenance.

Answer: A

Correct answer: providing a large number of segments

The voltage between adjacent segments is the total armature voltage divided among them, so more segments means less between any two. Keeping that figure low - conventionally below about 30 volts - is what prevents flashover across the commutator, and it is one of the constraints setting the size of a dc machine.

Answer: B

A surge of current distorts the flux badly and raises the voltage between segments, and an arc drawn at a brush can then track right round the commutator. Fast overcurrent protection and pole-face compensation both guard against it.

Answer: C

The interpoles look after the commutating zone and the pole-face winding looks after the distortion under the poles, and a mill or a hoist drive needs both. Together they nearly remove armature reaction as a practical concern.

Answer: D

A coil short-circuited by a brush while sitting in the main flux would carry a large circulating current, so it must be commutated where the flux is zero. That is why the neutral axis matters and why its shift with load is such a nuisance.