DC Generators Questions and Answers

Answer: A

The emf depends on flux and speed, neither of which the load changes directly, though armature reaction weakens the flux a little and a prime mover may slow down. What falls with load is the terminal voltage, through the armature resistive drop.

Answer: B

It completes the magnetic circuit outside the poles and gives the whole machine its mechanical strength, which is why it is a substantial casting or fabrication. Cast steel has replaced cast iron because its permeability allows a smaller section.

Answer: C

Correct answer: spread the flux over a larger area of the armature surface

The pole shoe is the widened foot of the pole, and its curved face spreads the flux over a large arc of the air gap, giving a more uniform flux density under the pole and reducing the reluctance of the magnetic circuit. It also supports the field coil mechanically and, in machines fitted with them, houses the compensating winding in slots cut in its face.

Answer: D

Correct answer: laminated silicon steel

The armature rotates in the field, so its iron sees a reversing flux and would suffer heavy eddy current loss if it were solid. Building it from thin laminations, each varnished or oxide-coated, breaks up the eddy current paths, and alloying the steel with silicon raises its resistivity and lowers the hysteresis loss. The yoke, which carries steady flux, may still be solid cast steel.

Answer: A

Correct answer: carbon or a carbon composite

Carbon is used for its contact resistance, which forces the current to transfer as the brush passes from segment to segment and so assists commutation; for its self-lubricating property, which reduces wear on the commutator; and because it is softer than the copper segments, so the brush rather than the commutator is the part that wears and is replaced. Copper brushes are used only in low-voltage, very high-current machines such as platers, where the drop matters more than the commutation.

Answer: B

Correct answer: one volt per brush

The contact drop of a carbon brush is roughly one volt and, importantly, is nearly independent of the current density, so the brush loss is taken as \( I_a \) times about two volts for the pair. On a 240 V machine that is under one per cent, but on a low-voltage machine it can be a significant fraction of the output, which is why metal-graphite brushes are used there.

Answer: C

Correct answer: the number of armature coils

Each armature coil has two ends, and each commutator segment receives the finish of one coil and the start of the next, so the number of segments equals the number of coils. The number of parallel paths and the number of brushes follow from the winding type and the pole number, and are quite separate quantities - a four-pole machine may have four brush arms and several hundred segments.

Answer: D

The magnetic circuit's reluctance, dominated by the air gap, together with the ampere-turns the field supplies, fix the flux. Speed affects the emf but not the flux, which is why the two appear as separate factors in the emf equation.

Answer: A

Correct answer: doubling either the flux per pole or the speed

From \( E = P\Phi ZN/60A \), the emf is directly proportional to both the flux per pole and the speed, so doubling either doubles it. Doubling the number of parallel paths would halve it, since \( A \) is in the denominator. Armature resistance and load current do not enter the generated emf at all, though they determine the terminal voltage through the \( I_aR_a \) drop.

Answer: B

Correct answer: about half its rated emf

The emf is proportional to speed at constant flux, so half the speed gives half the emf. The relation is linear, not square-law, because it is the rate of cutting flux that generates the voltage - the quarter answer confuses this with an energy quantity. This is also why a self-excited machine has a critical speed: below it the magnetisation curve falls too low to meet the field resistance line.