Synchronous Generators Questions and Answers

Answer: A

Armature reaction depends on both the load current and the power factor.

The armature mmf is proportional to the armature current, so its strength rises with load. Its effect on the main field depends on the power factor. At unity power factor it is mainly cross-magnetising and distorts the field. At zero lagging power factor it acts directly against the main field and is demagnetising, and at zero leading power factor it aids the field and is magnetising.

So neither the current alone nor the power factor alone determines it. The rotor speed is fixed at synchronous speed in a synchronous machine and does not alter the nature of the reaction.

Answer: B

Alternators are star connected to increase the terminal voltage.

In a star connection the line voltage is √3 times the phase voltage, so for the same winding the machine gives a higher terminal voltage. Put the other way, for a given line voltage each phase winding needs only 1/√3 of it, which reduces the insulation needed. The star point also provides a neutral for earthing and keeps third harmonic voltages out of the line voltages.

Frequency is fixed by the speed and the number of poles, not by the connection. In star the line current equals the phase current, so it does not reduce the conductor size, and the iron (magnetic) loss depends on the flux and frequency, not on the connection.

Answer: C

It will feed the same power but at a lagging power factor.

A generator connected to infinite bus bars cannot change the bus voltage or frequency. The active power it delivers is set only by the mechanical input from its prime mover, which has not changed. Raising the excitation increases its internal emf, and the only effect is to make it deliver reactive power to the bus as well.

An over-excited generator supplies lagging reactive power, so its current lags the voltage and it works at a lagging power factor. Under-excitation would give a leading power factor. To feed more power, the prime mover input would have to be increased, not the excitation.

Answer: A

Percentage regulation is \(\frac{E - V}{V} \times 100\).

Voltage regulation is the rise in terminal voltage when full load is thrown off with the speed and excitation held constant, expressed as a percentage of the rated (full-load) terminal voltage. On no load the terminal voltage equals the induced emf E, so the rise is E - V and the base is V.

The option with E - V over E uses the wrong base, the no-load voltage instead of the rated terminal voltage. V - E over V has the sign reversed, so it would give negative regulation for an ordinary lagging load, and V/E is a ratio, not a change in voltage.

Answer: C

They cannot be paralleled if their voltage ratings are different.

Before two alternators are connected in parallel, their terminal voltages must be equal, their frequencies must be the same, their phase sequence must be the same and they must be in phase at the moment of closing. Machines built for different voltages cannot meet the first condition, and connecting them would drive a large circulating current between them.

Equal frequency is a requirement for paralleling, not an obstacle to it. Different power or current ratings do not prevent parallel operation; the machines simply share the load according to their ratings and governor settings.

Answer: B

The emf closest to a sine wave comes from a distributed winding in short pitch coils.

Two design measures each suppress harmonics in the generated emf. Distributing a winding over several slots per pole per phase means the coil emfs are slightly out of phase, which reduces the harmonics much more than the fundamental. Short pitching (chording) the coils, so that they span less than a full pole pitch, can eliminate a chosen harmonic altogether, for example the 5th with a five-sixths pitch.

Using both together gives the most sinusoidal waveform. A concentrated, full pitch winding has neither benefit and gives the most distorted emf, and each of the other options uses only one of the two measures.