Synchronous Generators Questions and Answers

Answer: C

Correct answer: mainly magnetising

A leading current puts the armature mmf nearly in line with the main field, so it adds to it and the terminal voltage rises with load. The regulation may then be negative - the terminal voltage on load exceeding that at no load - which is one of the reasons a lightly loaded cable network can overexcite a generator.

Answer: D

The armature field sits across the main one and shifts the resultant without much changing its magnitude, though saturation makes the net effect slightly demagnetising. Lagging and leading loads add the axial components to that.

Answer: A

Treating armature reaction as a reactance lets it be lumped with the leakage reactance into one element, which is what makes the simple phasor diagram possible. Its per unit value usually exceeds unity in a large machine.

Answer: B

Correct answer: gives a pessimistic result because it ignores saturation

The emf method takes the synchronous reactance from the unsaturated open circuit line, which overstates it, because in the real machine saturation reduces the flux that the armature mmf can add. Overstating the reactance overstates the voltage drop, so the calculated regulation is worse than the machine actually achieves.

Answer: C

Correct answer: gives an optimistic result, understating the regulation

The mmf method works with field ampere-turns read from the saturated open circuit characteristic, which errs in the opposite direction from the emf method and gives a regulation better than the truth. The two methods therefore bracket the real value, and the Potier method was developed to fall between them.

Answer: D

A zero power factor test at rated current, plotted with the open circuit characteristic, gives the Potier triangle from which the leakage reactance and the armature reaction ampere-turns can be read separately. That separation is what makes it more accurate than either simpler method.

Answer: A

Correct answer: terminal voltage against field current at rated speed

The open circuit characteristic is the machine’s magnetisation curve: driven at rated speed with no load, the terminal voltage is recorded as the field current is raised. It rises linearly at first along the air gap line, then bends over as the iron saturates, and it is that bend which makes the choice of regulation method matter.

Answer: B

It is essentially a straight line because the machine is unsaturated at short circuit, the flux being held down by the heavy demagnetising armature reaction. Its slope with the open circuit curve gives the synchronous impedance.

Answer: C

It is the field current for rated voltage on open circuit divided by that for rated current on short circuit, which comes to the reciprocal of the reactance. A larger ratio means a stiffer machine with better regulation and a larger fault current.

Answer: D

Correct answer: good voltage regulation and a large fault current

The short circuit ratio is the reciprocal of the per unit synchronous reactance, so a large ratio means a small reactance. A small reactance means a small voltage drop on load, hence good regulation - and, by the same token, little to limit the current into a fault. The two consequences are inseparable, and machine design trades one against the other.