DC Generators Questions and Answers

Answer: A

With no field current the flux falls to whatever residual magnetism remains and the emf with it. The inductive kick as the field current is interrupted can be large, which is why field circuits are opened through a discharge resistance.

Answer: B

Many turns on iron store a great deal of energy, and interrupting that current without a path induces a voltage high enough to break down the insulation. The resistance is switched in before the field circuit is opened.

Answer: C

Copper and iron losses heat the machine, and the insulation and the commutator surface set the limit on how hot it may get. Commutation quality can bite first on a machine of large current, which is why very large dc machines are rare.

Answer: D

Correct answer: more turns of finer wire in its armature winding

For a given power, a higher voltage means a proportionately smaller current. The winding must therefore develop more volts, which needs more conductors in series, and it carries less current, so each conductor can be of smaller cross-section. Hence more turns of finer wire - and, for the same reason, a wave winding rather than a lap winding, since wave puts \( Z/2 \) conductors in series against lap’s \( Z/P \).

Answer: A

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

The commutating zone is the angular region in which a brush spans two adjacent segments, short-circuiting the coil between them while its current reverses. It lies on the magnetic neutral axis, where the main field flux is nominally zero, so that no unwanted emf is induced during the reversal. Interpoles are placed there precisely to induce a controlled emf that cancels the reactance voltage and drives the reversal to completion.

Answer: B

Correct answer: a sudden increase in the load current

The reactance voltage that must be overcome during commutation is proportional to the rate of change of current in the coil, and hence to the current being reversed. A sudden load increase raises that current and so the reactance voltage, and the interpoles - though series-connected and therefore self-compensating in the steady state - cannot follow an abrupt change instantly. Interpoles and more segments both improve commutation, and lower speed reduces the reactance voltage.

Answer: C

The neutral axis shifts forward in one direction and backward in the other, so a fixed brush shift is right for one direction and doubly wrong for the other. Interpoles reverse with the current and are correct in both directions.

Answer: D

Correct answer: Slip loss in the rotor circuit

Slip is an induction machine notion: it is the relative motion between the rotating field and the rotor that induces the rotor current, and the associated \( sP_g \) is dissipated in the rotor. A dc machine has no slip and no induced rotor current of that kind - its field is stationary and its armature is fed through the commutator. The other three are all genuine dc machine losses.

Answer: A

Armature reaction redistributes the flux and drives additional eddy currents in the iron and the conductors, and the total is difficult to compute. It is usually allowed for as a percentage of output rather than calculated from first principles.

Answer: B

Correct answer: 90 to 94 per cent

A large dc machine converts most of what it receives, the losses being a few per cent of rating: iron loss, friction and windage, the armature and field copper losses and the brush drop. Small machines do worse, since the constant losses are a larger fraction of a small output. No machine reaches 100 per cent, since the losses cannot be eliminated, and the maximum occurs where the variable loss has grown to equal the constant loss.