Synchronous Generators Questions and Answers

Answer: C

Correct answer: the phase voltage is then lower and the neutral is available

Star connection gives a phase voltage \( 1/\sqrt{3} \) of the line value, so the winding insulation need only withstand the smaller figure. It also brings out a neutral, which can be earthed - usually through a resistance - giving a reference for the system and a path for earth fault protection to detect.

Answer: D

Correct answer: appears between line and neutral but not between lines

Third harmonics in the three phases are all in phase with one another, so in a line-to-line measurement they cancel exactly and the output is free of them. Measured from line to neutral they are present. This is one of the practical advantages of the star connection, and in delta they would instead circulate as current round the closed loop.

Answer: A

Correct answer: suppress a chosen harmonic in the output waveform

Choosing the coil span sets the pitch factor for every harmonic independently, and a span of five-sixths of a pole pitch reduces the fifth and seventh very substantially while costing the fundamental only about four per cent. Trading that small loss for a much cleaner waveform is almost always worth it.

Answer: B

Correct answer: The speed of its prime mover and the system frequency

\[ P = \frac{120f}{N}. \] The frequency is fixed by the system and the speed by the turbine, so the pole number is not a free choice at all - it follows from the two. This is why steam-driven machines have two or four poles and water-driven machines have dozens.

Answer: C

Correct answer: about 98 to 99 per cent

A large turbo-alternator is among the most efficient machines ever built, losing only one or two per cent in copper, iron, windage and excitation. At several hundred megawatts even that fraction is several megawatts of heat, which is precisely why hydrogen and direct water cooling are needed to carry it away.

Answer: D

The rotor field is supplied rather than induced, so there is no slip-dependent rotor loss of the kind an induction machine suffers. That absence is a large part of why the synchronous machine is the more efficient of the two.