DC Motors Questions and Answers

Answer: B

Correct answer: with the machine at rest and the field disconnected

The measurement is a simple direct-current voltmeter-ammeter test on the stationary armature, with the field open so that no emf can be generated and nothing turns. Several rotor positions are used and the results averaged, since brush position and contact vary. Running the machine would introduce a back emf and make the measurement meaningless.

Answer: C

Correct answer: copper resistance increases with temperature

Copper’s resistivity rises by about 0.4 per cent per degree, so an armature that has warmed from 20 to 100 degrees has a resistance nearly a third higher. This is why armature resistance is quoted at a reference temperature and corrected to the working one, and why a cold-measured value used in a hot-running calculation understates the losses.

Answer: D

Correct answer: in series with the armature

Interpoles must produce a flux proportional to the current being commutated, and the only way to guarantee that at every load is to carry the armature current itself. Being series-connected, they compensate automatically as the load varies, which is exactly what brush shifting could not do. Their windings are of few turns of heavy conductor for the same reason.

Answer: A

Interpoles deal with the commutating zone, but the distortion under the main poles remains, which matters on machines with rapid load changes. A pole-face winding addresses that and is fitted where the duty justifies its cost.

Answer: B

Correct answer: is correct for only one load and one direction

The magnetic neutral axis shifts with the armature current, so the brush position that gives good commutation at one load is wrong at another; and since the shift is in the direction of rotation for a generator and against it for a motor, a setting correct for one direction is wrong for the other. Interpoles, carrying the armature current, correct automatically at every load and in both directions, which is why they superseded brush shifting entirely.

Answer: C

Chatter breaks the current path momentarily and each break draws an arc, which burns both the brush and the commutator. Correct spring pressure, a true commutator surface and a suitable brush grade all guard against it.

Answer: D

Correct answer: true, clean and lightly glazed

A commutator in good condition is concentric, free of high or low segments, and carries a thin brown glaze of copper oxide and carbon that reduces friction and wear. Polishing it back to bare copper removes that film and accelerates wear; grease is fatal, since it insulates and collects carbon dust; and roughening it would destroy the contact. The mica between segments is undercut so that it does not stand proud as the copper wears.

Answer: A

Correct answer: sparking from intermittent contact

Too little spring pressure lets the brush lift momentarily as it rides over the commutator, and each break in contact draws an arc, which burns the brush and the commutator surface. Too much pressure has the opposite effect, causing rapid mechanical wear and excessive friction loss. Pressure is therefore specified within a narrow band and checked at each inspection.

Answer: B

Correct answer: full load and low speed

Full load means full armature current and full copper loss, while low speed on a self-ventilated machine means little cooling air, so the heat generated is greatest at the moment the cooling is least. This combination governs the rating of any machine expected to work at reduced speed, and it is why such duties call for forced ventilation or a derated torque.

Answer: C

Correct answer: loaded above its rating for the permitted period

A continuous rating is a thermal limit: it is the load the machine can carry indefinitely without exceeding its permitted temperature rise. On short-time duty the machine is run for a period and then allowed to cool, so it may be loaded well above that figure provided the temperature does not reach the limit within the period. The permissible overload depends on the duty cycle and the machine’s thermal time constant.