I have a generator operates at a rotary speed of 1800 rpm and produces up to 500 kw with 480 v, and 60 hertz. Can I decrease the speed since I am not using this whole power and maintain the same frequency? is it possible to use such as a frequency converter? Any suggestions and important considerations?
Though, maximum efficiency could be at the rated designed speed, yet speed could be set higher or lower, by sleve setting of governor used in diesel engine, which control the fuel input, for lowering and increasing as needed from set...
Without using a frequency converter (not cheap at 100s of kW) you can't easily maintain the frequency. Note also that diesels don't like running much slower than their rated speed. The power, torque and efficiency all fall away fast as they slow down. If this is not an issue, then Suggestions:
Use a gearbox between the engine and the generator;
Have the generator rewound to increase the number of poles (might not be possible)
But, if you already have the diesel and its generator, what is the real problem? It works - and if it works, DON'T FIX IT.
Rather than using the fixed-speed motor, you could use a variable speed motor to control the rpm of the rotating shaft. The use of a variable speed motor will eliminate the need for a gearbox.
Hope this helps answer your question.
Professor Yehia Khalil
Yale University
USA
On a generator, you have a prime mover (say, an engine) connected to the actual generator, which consists of either rotating coils of wire within a magnetic field, or rotating magnets surrounded by coils of wire. The number of poles (magnetic poles) and the rotational speed determine the output frequency: Freq = Engine_RPM * Number_Of_Poles / 120.
Typically, a United States portable generator runs at 3600 RPM, with 2 poles, for a design frequency of 60Hz. Larger portable generators run at 1800 RPM with 4 poles here.
That is how frequency is determined. The number of turns and the magnetic structure determine how many volts are produced at the design frequency, voltage and frequency aren't related in any fashion except for design. Again, in the States, most portable generators are wound to have a 240VAC single phase output, which is centertapped and delivered as two 120VAC hots with one neutral, but virtually any voltage can be delivered.
The current output of a generator is determined by its load, as long as the load doesn't exceed the maximum capacity of the generator's prime mover (engine) plus the conversion losses of the actual generator. Prime mover power is often rated in horsepower (US) or kilowatts (everywhere else). With no losses, a 10 horsepower engine could deliver 7457 watts continuously, or 62.1 amps at 120VAC continuously. Try to take more, and the engine will slow down (reducing both the frequency and the voltage, which will also drop the current) until you reach a point that the engine actually stalls.
You get fluctuation of frequency and voltage as the load changes because the engine cannot respond immediately to the actual load change. There are regulators controlling the engine throttle that attempt to keep the engine at a fixed (design) speed, but it takes time for the engine to respond to new commands as it has to deal with varying fuel/air mixtures and combustion which aren't instantaneous.
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