I'm trying to understand the underlying physics with how generators work. I found this question which had some interesting answers, but it does not include all the details I'm after.

Suppose I have a generator that can output 100 watts. I connect a device that draws 50 watts. If I then connect a device that tries to draw 100 watts, what kind of energy does each device receive? More importantly, if the second device instead tries to draw 200 watts, does the 50 watt device receive less power?

My intuition says that they'll sort of "fight" for what's available and settle on an in-between value (before the generator overheats and seizes), but I'm hoping for a slightly more mathematical answer.

Edit: I understand that different systems will have different tolerances / behaviors, but is it reasonable to ask for an "average" case? If not, at least an ideal case? My assumption is that generator design is based on certain fundamentals. Approximations are acceptable; is there something that prevents us from generalizing? What I'm really interested in is understanding how we can expect such systems to function if they don't have protection mechanisms in place, and how function degrades with overload.


2 Answers 2


This is only a comment. It is not an answer because there is no general answer to your question. The answer depends on the engine that supplies mechanical power to the generator, and it depends on the devices that want the electrical power.

The power that an ideal* generator outputs is limited only by the power of the engine that turns its shaft. It is the product of the current in the circuit and the voltage between the generator's output terminals. Current is proportional to the torque provided by the engine, and voltage is proportional to the rotational speed of the shaft.

How will the "devices" behave when the generator is unable to supply the current that they expect at the voltage that they expect? The answer depends on the nature of the "devices." How will the engine behave when it is unable to supply the torque that the system demands at the speed that it demands? That depends on the nature of the engine and on whatever control system regulates its torque and/or speed output.

If you want a specific answer to your question, you will have to ask about a specific system.

* Real generators are less than 100% efficient. They turn some of the input power into heat, which can damage the machine if too much is generated. Also, they are made of physical materials that have strength limits. They can be damaged by too much torque and too much speed. The power of an ideal generator is only limited by the power of the engine that turns its shaft, but the power limit that's stamped on the ratings plate of a real generator is determined by engineering concerns rather than physics concerns.

A practical generator set—that is, an assembly consisting of a generator and an engine and their mutual control system—will protect itself from damage by tripping a circuit breaker if the current exceeds some limit, by throttling the engine to limit its speed, maybe by shutting down the engine if anything gets too hot, etc.


If you place a heavy load on a generator, the built-in speed controller will pull the throttle on the engine farther open to maintain the generator output voltage.

But if you exceed the load rating of the generator, these things will happen:

-the engine driving the generator will slow down

-more power will be dissipated in the generator windings than in the load, causing the generator windings to exceed their maximum service temperature limits

-the output voltage will sag below the voltage spec for the generator, so anything connected to the generator will malfunction (lights will get dim, motors will slow down, electronic devices will stop functioning, etc.)

Generators have fuses or circuit breakers that are designed to disconnect the load when an overload condition occurs so as to protect the windings from burning out.


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