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When I see electrical generators specified, they usually give the fuel consumption as at "rated load". So,for example, a 5 kilowatt generator might consume 0.6 gallons per hour.

However, what happens if the load is less than 5 kilowatts? Does the generator still run at the fixed rate and any excess electricity is just shorted to ground?

Or does the engine run slower and just generator less energy? How does it know what rate to run at? Does it have to manually adjusted?

If the engine is run at less than its maximum load, what would be a typical loss of efficiency? Can someone provide an example curve that shows how a generator loses efficiency at sub-optimal loads?

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  • $\begingroup$ When you say generators, are inverter generators also part of the category. Since common inverter generators do use variable speed based on load. $\endgroup$ – Netduke Nov 16 '16 at 13:32
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Generators don't use fuel at all. You are apparently asking about a generator coupled to some sort of chemical engine, like a "genset".

Usually these engine/generator combinations have a regulator that tries to keep the speed reasonably constant. This supplies as much fuel to the engine as necessary to maintain that speed. With lower electrical load on the generator, it puts less mechanical load on the engine, which then requires less fuel to maintain the same speed.

This is no different from the cruise control in your car. It maintains 55 miles/hour, for example, on a highway as you go up and down hills. However, it opens the throttle more, and delivers more fuel to the engine, when going up hills than when going down. Your milage (miles/gallon) is quite different going up a hill than down it, even though both are at the same speed.

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  • $\begingroup$ That is a good analogy. Like going up a hill. So, I guess, the engine just needs a rotation speed detector. If it starts to slow down, it increases the fuel rate. $\endgroup$ – Wallace Park Nov 16 '16 at 14:51
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At a lower load they run at the same speed but the generator becomes easier to turn which means the throttle to the engine driving it can be reduced to save fuel. The throttle is controlled by a system designed to maintain the correct speed no matter the load.

The efficiency Vs load curve is going to be generator specific.

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If you are talking about gas electric generators designed to supply household ac power, there are two basic types. These generators must supply a reasonably accurate 60hz frequency power, and there are two basic ways to achieve that.

most commonly the generator is wound to produce 60hz at a specific speed and the generator must be run at that constant speed regardless of load (as noted the throttle is closed to reduce air/fuel under reduced load.)

the alternate is to use an electronic device called an inverter to regulate the ac frequency. This permits the actual generator to be run at variable speed. (This is typical of small camping type generators). Inverter type gensets do indeed drop speed to a minimal level under light load. Note this has as much to do with making them quiet as saving fuel. Note also the inverter itself has an efficiency cost to it so these units may be less fuel efficient at high load than comparable fixed speed units.

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Most home gasoline-engine generators have a "2-pole" rotor which is spun at 3600 RPM by a governor on the engine. This results in a pair of 60 Hz sine-wave volatges coming out of the stator winding...180 degrees out-of-phase with each other. The losses in a typical gasoline engine at 3600 RPM are quite significant. If you happen to have an automobile with a tachometer, you might note that the RPM rarely goes over 3600. If the electrical load on the generator increases, the rotor current is increased to maintain 120 volts (normal line voltage). That puts a higher load on the gasoline engine and the governor opens the throttle to maintain speed. Very simple and inexpensive.

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Most generators will attempt to maintain a constant sped/frequency regardless of load. As far as the engine is concerned the load is the torque on the alternator which is itself dependant on the current being drawn.

Engine RPM is controlled by some sort of electronic or mechanical regulator ie some sort of feedback control system, which detects engine speed and adjusts the throttle to attempt to maintain a preset rpm.

In practice this regulation is imperfect and there will often be a some lag with the engine speed slowing noticeably when a large load is applied, for example starting a large electric motor.

Note also that the power generated is proportional to the electrical impedance across the alternator ie the current that what ever is connected to it is trying to draw so less load simply means that the engine needs to deliver less torque to maintain the same rpm. For example if the engine is running with nothing connected to the electrical output then no electrical power is generated and the engine only has to overcome its own internal friction, as soon as a load is connected this creates a back emf in the alternator which imposes a torque on the engine which needs to be matched by opening the throttle to maintain a constant speed.

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  • $\begingroup$ This is interesting, I had no idea that there was actually a mechanical force imposed by higher load, but this makes a lot of sense. A motor literally operating backwards, basically. $\endgroup$ – trpt4him Nov 24 '18 at 17:24

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