Synchronous Generators Questions and Answers

Answer: A

Correct answer: a cylindrical rotor with two or four poles

A steam turbine runs fast, so the alternator must have few poles - two for 3,000 rpm on a 50 Hz system, four for 1,500. At those speeds the rotor must be a long slender cylinder of forged steel with the field winding buried in slots, because a salient pole structure could not survive the centrifugal stress.

Answer: B

Correct answer: a salient pole rotor of large diameter with many poles

A water turbine runs slowly, so the alternator needs many poles to make 50 Hz - forty-eight poles at 125 rpm, for instance. Poles that many are built as separate salient structures bolted to a large-diameter spider, which the low peripheral speed permits. The shape of the machine follows directly from the speed of its prime mover.

Answer: C

Any swing of the rotor relative to the field induces currents in the bars that oppose the motion. They also carry unbalanced and negative sequence currents, which is why they are essential on a machine supplying an unbalanced load.

Answer: D

Correct answer: a shaft-mounted alternator feeding rotating rectifiers

The brushless exciter is an inside-out alternator on the same shaft: its field is stationary and its armature rotates, so the ac it generates is already on the rotor and is rectified there by diodes turning with the shaft. Nothing has to cross from the stationary frame, which removes the brushes and the maintenance they demand.

Answer: A

Low density cuts the windage loss sharply and high conductivity carries the heat away better than air, which together allow a much larger output from the same frame. Sealing the casing and excluding oxygen are the price of using it.

Answer: B

Correct answer: water removes far more heat than any gas can

Water has a far higher heat capacity and thermal conductivity than air or hydrogen, so passing it through hollow stator conductors removes heat at a rate no gas can approach. That is what allows the very largest machines to be built without their conductors overheating - at the cost of an elaborate high-purity, low-conductivity water system.

Answer: C

The distribution and pitch factors account for the winding being spread over several slots and its coils being short-pitched, both of which reduce the fundamental slightly. Omitting them overstates the emf by several per cent.

Answer: D

Correct answer: the coil emfs are displaced in phase from one another

The coils of a phase lie in successive slots, so their emfs are displaced in time phase by the slot angle and must be added as phasors rather than arithmetically. The phasor sum is necessarily less than the arithmetic sum, and the ratio is the distribution factor. The compensation is a far more sinusoidal output waveform.

Answer: A

Correct answer: the two sides of a coil are not exactly a pole pitch apart

In a short-pitched coil the two sides do not sit under corresponding points of adjacent poles, so their emfs are not exactly in opposition and the resultant is less than twice one of them. The fundamental is reduced slightly, but a chosen harmonic can be reduced almost to nothing, which is why the pitch is shortened deliberately.

Answer: B

Correct answer: mainly demagnetising

With a lagging current the armature mmf is very nearly in opposition to the main field, so it subtracts from it. The terminal voltage falls with load and the excitation must be increased to hold it - which is why a lagging power factor gives the worst voltage regulation and why regulation is quoted at a stated power factor.