Armature Reaction and Commutation Questions and Answers

Answer: A

Correct answer: \( I_aZ/2AP \) for \( Z \) conductors and \( A \) paths

Each of the \( Z \) conductors carries \( I_a/A \), and \( Z/2 \) turns are formed from them, spread over \( P \) poles: \[ \mathrm{AT\ per\ pole} = \frac{I_a Z}{2AP}. \] The result is proportional to the load current, which is why armature reaction grows with load and why the interpoles correcting it must carry that same current.

Answer: B

The armature field then sits at right angles to the main one and distorts it without opposing it, though saturation makes the net effect slightly demagnetising. Shifting the brushes introduces an axial component deliberately.

Answer: C

Correct answer: \( AT \times 2\theta/180 \) for a total of \( AT \) per pole

A shift of \( \theta \) mechanical degrees brings the conductors within \( 2\theta \) of the pole axis into direct opposition to the main field, and those occupy the fraction \( 2\theta/180 \) of a pole pitch. Only that fraction demagnetises; the rest continues to act across the field.

Answer: D

Correct answer: the total less the demagnetising component

The total ampere-turns are unchanged by the shift - the same current flows in the same conductors. What the shift does is reclassify some of them as demagnetising, so the cross-magnetising remainder is whatever is left. The two components always sum to the total.

Answer: A

Correct answer: 2,500

For a lap winding the number of paths equals the number of poles, so \( A = P = 4 \): \[ \frac{I_a Z}{2AP} = \frac{200 \times 400}{2 \times 4 \times 4} = \frac{80{,}000}{32} = 2500. \] A wave winding would have \( A = 2 \) regardless of the pole count, which doubles the figure - so the winding type must be established before the formula is applied.

Answer: B

Correct answer: 250

\[ AT_d = AT \times \frac{2\theta}{180} = 2500 \times \frac{18}{180} = 250. \] So a nine-degree shift converts a tenth of the armature ampere-turns into demagnetising ones, leaving 2,250 still cross-magnetising. That is the cost of using a brush shift for commutation, and it is why interpoles are preferred.

Answer: C

Only the part of the armature field lying under the pole arc has to be cancelled, so the winding carries the armature current through a number of turns proportioned to the pole arc. Being in series, the correction follows the load automatically.

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.