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Maybe this is a silly question, but why does, say, a gasoline-powered AC generator have to use more gas depending on the load?

Let's say I have a 120VAC generator and either a 1A or a 10A load, and assume it can handle 1200W without issue. For 10A, the generator uses more gas and works harder compared to 1A.

Is this because the 10A is passively causing a magnetic field that makes the generator shaft physically more difficult to spin? If so, why does the magnetic field oppose the generators movement if the current is flowing in the direction the generator is trying to make it go to begin with? (Sorry, I know that is probably silly but I have a very limited understanding of electricity and magnetism.)

Or is it because the circuitry in the generator is actively sensing voltage drop due to higher load, and increasing the throttle to maintain a constant output voltage? Sort of like I'd imagine a water pump would have to work harder to maintain a constant pressure in a system with a leak in it or with flow. (Related: If so, is this type of sensing necessary for a generator to limit the voltage to low loads, or is it just a bonus feature to increase efficiency by not running at full throttle all the time?)

Or is it some combination of both? Or are both of those somehow the same thing? (It seems like it must be a little bit of the former; because the generator stalls under too high loads, which I guess means something is resisting the spin - unless the generator just actively shuts itself down?)

What is the process from "higher load" => "higher fuel usage"?

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  • $\begingroup$ Being placed on a bot net has me finding ancient information on 8 year old questions but I'm going to throw my 4 cents in anyways. In simple, less scienmathical terms. More demand for power = more throttle being mechanically automated = more gas being used. $\endgroup$ Commented Apr 15, 2023 at 16:15

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From a simple energy conversion perspective, and assuming the output voltage is constant, increasing the current by a factor of 10 means the generator must supply 10 times more electric power (electric power is the product of voltage and current).

Since the generator only converts mechanical power to electric power, the motor driving the generator shaft must supply at least 10 times more power in order for the generator to supply 10 times more power.

This is fundamental and inescapable.

Your suspicion that the generator is more difficult to turn when the current increases is correct.

I have an old Lifecycle exercise bike. When I pedal, I'm turning the shaft of a standard automotive alternator. The control circuitry adjusts how difficult it is to pedal by changing the load connected to the alternator - the more current supplied to the load, the more difficult it is to pedal the 'bike'.

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  • $\begingroup$ Isn't this pretty relevant: en.wikipedia.org/wiki/Counter-electromotive_force ? $\endgroup$ Commented Jun 19, 2014 at 21:56
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    $\begingroup$ @BrandonEnright, that's a truly awful article. $\endgroup$ Commented Jun 19, 2014 at 22:32
  • $\begingroup$ The OP was asking WHY the generator is more difficult to turn. His first guess about the generated current generating a field that opposes the stator's field is nearly correct, but his more detailed question regarding that is ingnored in this reply. $\endgroup$
    – Robert M.
    Commented May 16, 2023 at 14:11
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First you have to understand this simple principles when you pass a magnet on a coil or rather rotate a magnet near a coil that coil will create an opposite polarity of that magnet. What do you know about unlike poles they attract each other. Second part of the question why does a generator require more energy when load is increased. When current flows out of the coil in this case it creates an opposite polarity to that of the shaft and the more current flows out the coils the more powerful the magnet you create at the coils. In you case 10ampare a more stronger field than 1amp are .so you need more torque to rotate the shaft for the 10ampare

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The generator produces an output voltage due to the electromotive force, emf, created by rotating an internal coil in a magnetic field (Faraday induced emf). The induced emf depends on the speed of rotation of the coil. The generator is designed to maintain (nearly) constant output voltage within the limits of the external load it can handle. With increased load, the generator delivers more current (increased load torque). Internal to the generator, with increased current more internal torque is required to maintain constant speed of rotation of the coil to maintain constant emf and constant output voltage. To supply the increased internal torque to balance the load torque, more power is required to rotate the coil, and this requires using more gas.

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