Regulating generator voltage

      Dealing with voltage control and wiring for generators. January 31, 2001

Question
I have just installed a used 375 KVA generator at my shop. Will I need to do something to control the voltage? The tag says it will produce 480 volts/350 amps at 1800 RPMs. It is hooked right to the breaker cabinet now. Can I wire it right to my moulder? I installed a governor on the engine and set it at 1800.

Forum Responses
The engine governor should maintain the voltage at the rated speed. The generator should already have a voltage regulator in the control circuit. You can install an over-under voltage sensor.



You cannot run the generator together with the local net, just separated (unless you always provide a synchronizing). Be sure that the main is always switched off when you are going to run the generator.


It depends on the way the generator is set up to operate. Get a cheap voltage meter and read the voltage output. The governor is designed to automatically speed up the generator, or slow it down, depending on load demand. Be careful with the test leads and don't get shocked.


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Generators are controlled two ways at the same time. The voltage is controlled electrically via the power sent to the rotating field. The frequency, cycles per second, Hertz (usually) 60hz. is determinded by the rotational speed of the prime mover (engine). 1800 rpm is this case. The governor controls the engine speed and is supposed to hold it to 1800 rpm or 60 hz regardless of the load. The fuel consumption goes up the more load you apply to the generator but the speed is suppposed to remain constant.



Always consult a qualified electrician. Previous comments about a disconnect and the governor are usable information. You obviously don't want to feed into the electrical distribution line of the local utility, so a disconnect from main panel is appropriate. The local utility will have requirements for this type of back-up generator setup. Also, conact the utility to insure their system can handle your new load - if you are at the end of a circuit, you could experience poor voltage problems.


Comment from contributor D:
If connecting a generator to a mains fuse board then you must use a mechanically and electrically interlocked double contactor to switch over automatically or a double pole two position switch. In this case if you run the generator during a power failure there is no chance of voltage appearing on the lines out of the building and electrocuting the people re-connecting the power. It is always advisable to use a voltage regulator where you have sensitive devices such as computers and electronics.



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