Armature Reaction and Commutation Questions and Answers

Answer: D

It has two jobs: neutralising the armature field where the coil is commutating, and then producing a further flux to induce an emf opposing the reactance voltage. Being in series with the armature, both scale with the load.

Answer: A

Correct answer: the main pole ahead in the direction of rotation in a generator

The interpole must produce a flux that induces an emf opposing the reactance voltage in the commutating coil, and for a generator that requires the polarity of the main pole next in the direction of rotation. In a motor the rule reverses, which is why a machine that has been reconnected must have its interpole connections checked.

Answer: B

Correct answer: midway between adjacent main poles

The geometric neutral axis is fixed by the physical arrangement of the poles and does not move. The magnetic neutral axis coincides with it only at no load; under load armature reaction shifts the magnetic axis away, and the difference between the two is the whole commutation problem.

Answer: C

Correct answer: the resultant flux density in the air gap is zero

A coil crossing this axis has no emf induced by the main field, which is where its current can be reversed most easily. Under load the axis shifts - forward in a generator, backward in a motor - so the brushes, which are fixed, are no longer at the right place unless something restores it.

Answer: D

Equal ampere-turns are added at one tip and subtracted at the other, but the added ones act on iron already near saturation and produce less flux than the removed ones took away. That is the origin of the demagnetising effect in a machine with no brush shift.

Answer: A

In a lightly saturated machine the distortion is large because the iron responds readily, and the flux redistributes freely. A heavily saturated circuit resists the redistribution, though it then shows a larger net demagnetising effect.

Answer: B

Correct answer: a coil is short-circuited by a brush

The brush spans more than one segment, so for a brief interval it connects both ends of one coil together. During that interval the coil’s current must reverse completely, and the whole difficulty of commutation lies in getting it to do so before the short circuit is broken.

Answer: C

Correct answer: reverses uniformly through the commutation period

Linear commutation is the ideal: the current changes at a constant rate and reaches its full reversed value exactly as contact is broken, so no energy remains in the coil’s inductance and no arc is drawn. Real commutation lags behind this because of the reactance voltage the reversal itself induces.

Answer: D

Correct answer: the trailing edge of the brush

When commutation is delayed, current is still flowing in the coil at the instant the segment leaves the brush, and the remaining energy in the coil’s inductance is released as an arc. It happens at the trailing edge because that is where contact is finally broken.

Answer: A

Correct answer: the leading edge of the brush

Too strong an interpole reverses the current early, so a reverse current is already established while the brush is still making contact. The excess must then discharge at the leading edge. It is the less common fault, since interpoles are usually set weak rather than strong, but it points to the opposite adjustment.