DC Generators Questions and Answers

Answer: C

The field carries the load current, so the flux and the emf rise steeply from zero as the load comes on and then flatten with saturation. It found use as a booster in series with a feeder, where that behaviour is exactly what is wanted.

Answer: D

Correct answer: battery charging and lighting loads

A shunt generator’s terminal voltage falls only slightly from no load to full load, so it is a good approximation to a constant voltage source - which is what a lighting circuit and a charging battery both need. Its drooping characteristic beyond full load is also a useful protection, since a heavy overload collapses the voltage and limits the current rather than allowing it to grow without limit.

Answer: A

Correct answer: arc welding sets

Welding needs a characteristic that falls steeply: the arc is struck by touching the electrode to the work, so the machine must survive a short circuit without excessive current, and it must then hold a roughly constant current as the arc length varies. A differential compound generator, whose series field opposes the shunt field, gives exactly that drooping, nearly constant-current behaviour.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: lap gives \( A = P \), wave gives \( A = 2 \) whatever the pole number. Statement 3 reverses the application. Lap divides the current between many paths and so suits low voltage and high current; it is the wave winding, with \( Z/2 \) conductors in series, that suits high voltage and low current.

Answer: C

Correct answer: 1, 2 and 3

All three conditions must hold together. Residual magnetism supplies the small initial emf without which nothing starts. The field resistance must be below the critical value, or the resistance line will not cut the magnetisation curve. And the speed must be above the critical value, since a low speed depresses the magnetisation curve until the same resistance line no longer meets it. Failure of any one of the three leaves the machine unexcited.

Answer: D

Correct answer: 1-Q, 2-R, 3-P

A shunt generator’s voltage falls slightly with load, through the armature drop, armature reaction and the consequent weakening of its own field. A series generator has no voltage at all at no load and rises steeply as current magnetises the field, until saturation and the resistance drop turn the curve over again. An over-compounded machine has enough series turns to make the full-load voltage exceed the no-load value, so its characteristic rises.

Answer: A

Correct answer: 1-Q, 2-R, 3-P

Interpoles are wound in series with the armature and produce a flux that induces an emf opposing the reactance voltage in the commutating coil. A compensating winding is embedded in the pole faces, also carrying armature current, and cancels the flux distortion armature reaction produces under the poles. Equalizer rings connect points of a lap winding that should be at equal potential, so that any circulating current caused by unequal pole fluxes passes through them rather than through the brushes.

Answer: B

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

Both are true and the reason is the mechanism. The field resistance line is a straight line through the origin whose slope is the field circuit resistance, and build-up settles where that line meets the magnetisation curve. If the resistance is above critical the line is steeper than the initial part of the curve, lies above it everywhere, and the only intersection is at the origin - so the machine never leaves its residual voltage.

Answer: C

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

Both are true and the reason explains the necessity. If one compound machine takes a slightly larger share of the load, the extra current strengthens its series field, raises its emf, and makes it take more still - a runaway that ends with one machine motoring the other. The equalizer bar connects the series field junctions of the two machines so that the series fields see a common current, removing the positive feedback and making the sharing stable.

Answer: D

Supply the armature and the field and the same machine develops torque, which is the basis of the back-to-back test and of regenerative braking. Minor differences in interpole design and ventilation are all that distinguish machines built for one duty or the other.