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Solution To
"Synchronous Generators"


Generator #2 will continue to rotate in the same direction as before. However, the generator now acts as a motor, and its current reverses.


Current now flows into Generator #2 to keep the rotor spinning in the same direction as before. Generator #2 becomes a user of power (i.e. a load to the power system albeit a motor operating at no load since nothing is now attached to the rotor) instead of a supplier of power.

Generator #1 remains unchanged, still supplying power to the bus. However, this generator must carry the full system load and its current increases and more torque must be provided by its prime mover.

It is interesting to note that if the prime mover were disconnected suddenly, all the load would immediately shift to Generator #1. This shifted load would normally cause Generator #1 to slow down, and if it is too large a load, the generator would fall completely out of synchronization.

What one normally wishes, is for both generators to share the load according to their MVA ratings and for both generators to maintain the same synchronous 60 Hz speed. If it is necessary to remove Generator #2 from service, you would want to shift the load from one generator to the other in small increments.

Shifting of load is done by reducing the mechanical torque on the generator to be removed from service, and increasing the torque on the generator that is expected to pick up the load. This must be done while still regulating the synchronous speed of both generators.

When the load is shifting from Generator #2 to Generator #1, Generator #2 would want to speed up. By lowering the mechanical torque to Generator #2's rotor, the synchronous speed is maintained at 60 Hz and the load is thus decreased.

Meantime, the load that is shifted to Generator #1 would try to slow Generator #1 down. More mechanical torque has to be supplied to Generator #1's rotor to maintain the synchonous speed.

Remember: It is the mechanical torque of the prime mover that determines how much wattage each generator supplies to the load. When load increases, more torque must be supplied to the generator by the prime mover in order to maintain the 60 Hz speed of the generator.

(Note: The amount of vars the generator produces is determined by the strength of the excitation field that is supplied to the rotor's windings!).

The speed of the generator's rotor determines the frequency of the voltage sine wave (which in the USA is of course 60 Hz or 60 cycles per second). The rotor will actually spin at either 3600 RPM or 1800 RPM (depending on the number of poles with which it is designed). This rotational speed must be carefully monitored and the generator's governor (or whatever control system is used) must adjust the mechanical torque to ensure that these speeds are strictly adhered to.

In fact, most clocks use the frequency of the power system to regulate their timing. If the power frequency decreases (i.e. the generator slows down) the clocks will also slow down. Speeding up the generator, speeds up the clocks.

In practice, the generator's speed is constantly changing ever so slightly, going above and below the 60 Hz benchmark. The cumulative effect of the over speeds and the under speeds must average out to 60 Hz over time. This way a clock's time is maintained over a long period.

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