Armature Reaction and Commutation Questions and Answers

Answer: C

Reactance voltage grows with the current to be reversed, and rapid load changes add the difficulty of a commutating flux that must track them. The first needs only carbon brushes, the second interpoles and the third pole-face compensation as well.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. The reactance voltage is proportional to the armature current, so the correction must be proportional to it too. Connecting the interpoles in series with the armature makes their flux follow the load automatically, at every load and at every moment, with nothing to adjust.

Answer: A

A shift that is right for one direction is doubly wrong for the other, since it then adds to the error instead of correcting it. Interpoles reverse with the current and are correct in both, which is why they are universal.

Answer: B

Equal ampere-turns are added and subtracted at the two tips, but the added ones act on iron nearer saturation and produce less flux than the subtracted ones removed. The effect is small but is what makes the field regulator necessary.

Answer: C

A shorter period means a faster current reversal and therefore a larger emf of self-induction to overcome. That is one reason high-speed dc machines are limited in size and why the commutator eventually sets the ceiling.

Answer: D

Reactance voltage grows with current and speed, and beyond a certain product no arrangement of interpoles and brush grades will commutate cleanly. That ceiling is a large part of why very large variable-speed drives are ac rather than dc.

Answer: A

Because the armature field revolves in step with the main one in a synchronous machine, its effect can be lumped with the leakage reactance into a single synchronous reactance. The dc machine cannot be treated so neatly, its field being fixed in space.

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.