Actually the generator speed does not change in theory as that would get the generator out of sync and trip the protection. As load is added so is power from the prime mover and that is controlled by a governer. I had a big project where 2 generators could not be synced. they would run up and parallel but because their governors were different as soon as 1 started to speed up as you say the sync was broken and they would trip off line. A simple governor change cured it. They must absolutely run at 60 Hz and cross 0 volts at the same time 120 times per second. One generator was 600 KVA and the other was 1000 KVA. After the project was complete the smaller was rewound for 750 KVA and paralleled to two 1 mva with another 1 MVA added some 4 years later. They all share load according to their ratings but when parallel they must rotate at 60 hz which is related to the number of poles wound in the alternator so a 6 pole and a 12 pole machine would have different synchronizing speeds. The governor works to ensure the alternator speed is constant not speeding up or slowing down with load but adding or removing power according to load. Once overloaded the engine slows too and the electrical protection trips on frequency which is speed of the alternator.
Last edited by mikesh; 09/26/11 08:10 PM.
