Generator speed, and power output. Frequency

Question

When you have a sudden change in load, I understand that you can expect to see sudden, voltage/current spikes. I also understand that you can expect to see a generator speed up or slow down for a short duration as this load change occurs. So I understand that as the generator changes rpm, the frequency output of the generator must change, even if it is for a short moment. During this short moment while frequency, and VI both increase is where I'm a bit curious. I've been trying to find some kind of formula that explains how frequency and power or current or voltage are related, I'm a second year electrical apprentice, so try and use terms I would understand! Thanks :)

Answer 1

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 center tapped 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 (actually VA for non-resistive loads) 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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As a clarification to other discussions here:

For a purely resistive load, halving the voltage would halve the current, and result in one quarter the power consumed. You can't say that just cutting the voltage in half cuts the power consumed in half. With some devices, that may be true, but it entirely depends on the load.

Answer 2

A few things happen on sudden load change. (in this order)

1. Load is switched. There is suddenly more/less demand, lowering/raising the voltage of the generator.
2. The automatic voltage regulator will change the magnetic field strength of the rotor.
3. The governor (speed regulator) will respond to the changed load conditions.

The effects you could theoretically model are:
- The magnetic effects.
- The PID behavior in the voltage regulator.
- The PID behavior in the governor.

It is though, vastly more complicated when AC theory walks in. Different loads change the behavior of everything. The magnetic properties, the voltage regulator and the motor governor.
Beyond the scope of an answer here.

Answer 3

Base on my experience RPM hunting or up and low frequency cause by the following factors:

1. The unit fuel lines are clogged, no fuel enough to sustain in the injection pump or the fuel is dirty.
2. Defective Speed Controller (EFC)Engine fuel Control the voltage is enough to open the actuator where the fuel enter in the injection pump.

Answer 4

If a certain power unit is in the national grid system the only reference where you have to increase or decrease the power unit load is its grid frequency.As the grid frequency decreases The power unit RPM decreases , so as its voltage. When the frequency increases the RPM increases as well and so is the voltage.At this event load MW was not yet changed.To correct such increment and decrement, power unit load MW must be increased or decreased to supply the demand.In other words MW must be increased to correct the frequency difference from 60Hz.Example if frequency is 59.95hz MW is to be increased if frequency is 60.05hz Mw must be decreased.For the voltage AVR will increase or decrease the voltage changes by varying the excitation voltage to keep it at the reference voltage say 21KV it has to keep it at 21KV.

Answer 5

So if the engine RPM slows down the freq will also be reduced. The Voltage regulation will try to keep the voltage at operating voltage but the current draw from the load would increase due to lower freq of operation.

Answer 6

Sorry but the answer you have been given is not true. The power draw of a device is not determined by its voltage. The current would be twice as high at half the voltage, but the power, V*I is the same.