DC Generators Questions and Answers

Answer: C

Conductors in slots in the pole faces, carrying the armature current in the opposite sense, cancel the armature's own field where it does most harm. It is expensive and is fitted only to machines with severe load changes.

Answer: D

Independent excitation makes the flux and therefore the emf controllable without regard to the load, which suits laboratory and Ward-Leonard use. It needs a separate supply, which is why self-excited arrangements are more usual in practice.

Answer: A

Correct answer: connected across the armature terminals

A shunt field is connected in parallel with the armature, so it sees the terminal voltage and carries a small current - typically one to three per cent of the rated current - through many turns of fine wire. Because the excitation follows the terminal voltage rather than the load current, the shunt generator holds its voltage reasonably well as load is applied, which is why it suits lighting and battery charging.

Answer: B

Few turns of heavy wire carry the load current, so the flux and the emf rise steeply with load. That characteristic made the machine useful as a booster on a distribution feeder and useless for supplying a variable load.

Answer: C

Correct answer: both a series and a shunt field winding

A compound machine carries a shunt field of many fine turns across the armature and a series field of few heavy turns in the load circuit. The shunt field sets the no-load voltage and the series field responds to load current, so the two together shape the external characteristic. Connecting the series field to assist gives a cumulative machine whose voltage is held up or even raised with load; connecting it to oppose gives a differential machine whose voltage falls steeply.

Answer: D

Choosing the series turns so that the rise from added flux exactly offsets the resistive drop gives a flat characteristic. More turns give over-compounding and a rising characteristic, which suits a long feeder with its own drop.

Answer: A

The extra series turns raise the flux faster than the resistive drop takes it away, which compensates the drop along a distribution feeder so that the voltage at the far end stays right. It is achieved by design rather than by accident.

Answer: B

Correct answer: opposing the shunt field

In a differential compound machine the series field is connected so that its magnetomotive force subtracts from that of the shunt field. As load current rises the net flux falls, the generated emf falls with it, and the terminal voltage drops steeply - giving a nearly constant-current characteristic. That is exactly what an arc welding set requires, since the arc must be struck by a short circuit and then sustained without the current running away.

Answer: C

Correct answer: generated emf against field current at constant speed

The open circuit characteristic, or magnetisation curve, is taken with the armature on open circuit and the machine driven at constant speed while the field current is varied. It shows the emf rising almost linearly at first and then bending over as the iron saturates, and it starts from a small value at zero field current, which is the residual magnetism. Everything about self-excitation - critical resistance, critical speed, the point of build-up - is read off this curve.

Answer: D

Correct answer: terminal voltage against load current

The external characteristic shows the terminal voltage as the load is varied, and it is what the user of the machine actually experiences. It falls below the open circuit value by the armature resistance drop and by the effect of armature reaction on the flux, and in a shunt machine by a third amount, since the falling terminal voltage also weakens the field. It is not to be confused with the internal characteristic, which plots the generated emf against armature current.