If you take as an example the alternator on a car. Say the engine is set to idle at 1000 rpm. The resistance of the engine itself and any accessories it is driving require a certain amount of gas to maintain 1000 rpm. Now assume the alternator at idle speed can provide 1000 watts of power, but keeping the battery charged and running the engine is only pulling 100 watts. (I have no idea if these numbers are realistic. I'm just using it as an example.) Then if you use the alternator to to power a 900 W device, so now you're pulling the full 1000 W potential, does the mechanical resistance of the alternator increase, thus requiring slightly more gas to maintain 1000 rpm?

If so, what causes this? The electricity is being generated by passing coils through a constant magnetic field. The potential output is limited, I assume, by the rotational speed. Does it matter how much current is load it actually has at that speed? Or is any excess potential simply wasted.

This might be a duplicate but I couldn't find an answer that made sense to me.

  • 1
    \$\begingroup\$ When I switch on my rear window heater or the fan from 0 to max while the motor is in idle, I can notice a slight drop in RPM. But the ECU compensates this within a second or so. \$\endgroup\$
    – sweber
    Commented Mar 16, 2016 at 19:50
  • \$\begingroup\$ There were (perhaps even are still) some cars that had 3 cylinders with small displacement that were unable to keep up with the electrical load and charge the battery at idle. I presume they were fitted with alternators that wouldn't stall the engine at idle, but as a result were not able to produce the energy required to run all the electrics and also keep the battery topped up. Apparently, if you left it idling the battery would slowly lose a charge. \$\endgroup\$
    – user65586
    Commented Mar 16, 2016 at 20:15
  • \$\begingroup\$ Also, perhaps of interest: Does electrical usage contribute to fuel consumption? \$\endgroup\$
    – user65586
    Commented Mar 16, 2016 at 20:49

5 Answers 5


Do you own a car? It's a very easy experiment, just turn everything electric on and you can "feel" the engine working harder.

The reason for this is the magnetic field every current creates. Under no load you have the spinning magnetic field from the rotor, which creates a voltage in the stator.

If there is a load a current starts to flow which creates a magnetic field in the reverse direction. This hinders the rotors movement, so you have to put in more mechanical power to keep the rpm constant.

  • \$\begingroup\$ I can turn everything electrical on in my car and I don't notice any change. However your last paragraph is what I was getting at. The current flow does create a magnetic resistance to the rotor. I was figuring that there had to be more resistance but I couldn't figure out where it was coming from \$\endgroup\$
    – Tom
    Commented Mar 16, 2016 at 20:25
  • 2
    \$\begingroup\$ maybe your engine is more powerful than mine ;) \$\endgroup\$
    – Christian
    Commented Mar 16, 2016 at 20:29
  • \$\begingroup\$ It's a pretty big engine. The a/c makes a big difference but that's not running off the electrical system. \$\endgroup\$
    – Tom
    Commented Mar 16, 2016 at 21:06
  • \$\begingroup\$ @Tom back emf :) \$\endgroup\$ Commented Mar 17, 2016 at 8:19


All the electrical output of the generator has to be provided as mechanical input, plus a bit more for losses.

One horse power is roughly 750 watts. However, car alternators are relatively inefficient as generators go (the input mechanical power is 'free', and there's plenty of cooling air) so 900/1000 watts output will need around 2HP from the engine.

If you have any 3v model motors kicking around, then a good experiment is to spin one, while connecting/separating the two wires together to change the load on it. The resistance offered to being spun increases quite dramatically when the motor is loaded.


This is the basic principle of generator. The energy is converted from mechanical power into electrical power, it is the opposite of motor mode where electrical power is converted into mechanical. Lots of people think that you can spin the alternator and it produces electricity without influence to the mechanical driver, but it is not true. When the alternator produces power it has to take it from the engine. The engine is then controlled at idle speed by governor that adds some more gas to mantain the idle RPMs.


If you have a small permanent magnet DC motor laying around, you can demonstrate the loading effect yourself easily.

1) Leave the motor completely disconnected from anything and spin the shaft with your fingers.

2) Short out the armature winding, and with it shorted, spin the shaft again.


The answer is Yes, Yes, Yes. (I'm a rusty electrical engineer) Anything else would be a violation of 2nd law of thermodynamics.

Let's start with a simple magnetic moving across a coil. If you then put a load on the coil, such as a resistance, then swiping the magnetic will attempt to induce a current, which will be limited by resistance, and as it does so, the voltage will rise across the coil. (It takes time for charges to pass through a resistance, and a voltage is created as the charges accumulate) This is an inductive spike.

If you swipe a magnet so that an emf is generated in the coil and you have the ends of the coil unattached (open), then you will attempt to induce a current, but there will be no path for the current to flow, and a large separation of charge creates a much higher voltage spike from your coil.

If you swipe a magnet so that an emf is generated in the coil and you have the ends of the coil shorted together (!), then as much current as can be induced by the changing magnetic field is allowed to do so. Note that voltage will be LOW, current will be HIGH. However, the high current induced in the coil ALSO produces a magnetic field, in the opposite direction as to the magnetic field change due to the swipe. In other words, if a magnetic is swiped, whatever current is allowed to flow in the coil will resist the swipe. This is how regenerative braking works. (Think stepper motors, or fancy electric cars)

In a car, this system works as a whole. The generator attempts to induce a current, but the current is resisted by the internal resistance of the battery, etc. In effect, an electrical system will only draw as much current as it needs (you can think of it as current draw is determined by the load)

Your car is "governed", meaning that it will use less gas or more gas in order to maintain its idle setting.

What that means is, if the load on the generator is greater, it will be able to induce a greater current to meet that need, at the expense of an opposite field by that current, meaning the engine will be loaded down and the gas usage will rise.

Now, you mentioned alternators ~~ these are a slightly different beast in that they don't have permanent magnets. Instead, the regulators in them shunt current through their field coils to create a magnetic field. In essence, though, the same result occurs.

The only effect that the regulator has (for both generators and alternators) is that there will be an upper limit on the amount of load that can be "seen" by the generator/alternator, in order to protect the windings from melting, and to maintain a more consistent system voltage.

An alternator which has no load on it will free spin, as the regulator shunts no current through the field windings.

A generator which has no load will --free spin-- [Edit: It won't free spin, because the permanent magnetic can still induce eddy currents in the core of the armature which resist the motion, etc.] , but generate high open circuit voltages. That gets handled with bleeder circuitry in the regulator.


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