Generator Excitation

Excitation is switched at near to rated speed only as it gives the indication of incoming voltage at the acceleration stage itself and that is to be matched with the running voltage for synchronizing.

But in your case 560 rpm is not near to 750 rpm. So I dont know the reason for your session.

Excitation at low speed builds up some voltage required

However, Overexcitation will only cause generator to perform at lagging PF, underexcite will cause the generator to perfom at leading PF.

Lagging or leading ower factor operation of the generator comes into picture when the m/c is loaded. But why should be the excitation switched ON at near to rated speed only?

ok Sir... Let me go back to the fundamentals... The Induce emf in alternator depends on the speed of the rotor, and the excitation current. suppose if the speed of the rotor is brought upto synchronous speed or near Ns.. and then we turn on the full excitation. would'nt the flux linkages with stator windings be better and hence the induced emf..

You are right about overfluxing at low speeds. Most Automatic voltage regulators I know have a function called Under-frequency Roll-off. The point at which the AVR goes off on the speed curve during shut-down is the same as that at which it comes on during start-up. This is to allow the terminal voltage to decay fast enough during shutdowns (safety feature). Usually this is 80% of rated speed.

Secondly, consider cases where a load has not been isolated during start-up. What would happen if the voltage was to build up together with speed? During start-up the electromagnetic processes in the generator are in a highly dynamic state and may either collapse or lead to overshooting desired values. That is a calculation I am not willing to delve into. I think the important thing is during shut-down, terminal voltage has to decay at a fast rate. The relay to switch on/ off is one and has that one setting.

Though the exitation due to residual mag is there to develope some voltage for controls but not for load which should only be switched on when the Frequency is in the region where system can capture to synch to the specified frequency*** i.e. 50 /60 Hz. In your case it may be due to the manufacturers' option for any reason you should look into.

*** The Gen-sets being used on power-net need to be synchronised with main source.

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560rpm is not near to 750rpm to be accepted as near to rated speed.Lets say for example, 730rpm or 740rpm can be said to be near to rated speed of 750rpm.

When generators are excited,it is to start the gen at its rated speed,but when it is loaded,the speed drops slightly within tolerance range.The generator should run on no-load or close to rated speed,so that it maintains its rated voltage output within tolerance as well as its frequency when loads are switched on.Some load may drop the voltage so low to affect other loads as well as the gen set.

Pls read further on generator excitation.

Patrick Whowha