I'm familiar with how an electric motor works, and also familiar with what stall current and free current refers to in an electric motor. I'm also somehow familiar with the notions of energy being transformed from electromagnetic to kinetic and to heat.

I've also reviewed questions like these:

What I'm yet to fully understand is why exactly, from a physics point of view, does the current increase when the motor is stalled, and more intriguingly why does the current drop when the motor is moving.

I'm guessing that the coil resistance increases/decreases when the motor is moving/stalled, but I can't grasp the idea of why does that happen and what the explanation at an electric/electromagnetic level is.

I guess another way to put it's why isn't current always flowing at it's maximum in an electric motor and actually drops when the motor is moving?



1 Answer 1


When a motor moves it also acts as a generator and the current trough the windings is given by the difference of the external voltage and the induced voltage. When the motor stands still, though, the generated voltage is zero and the windings will draw the max. current they can based on their DC resistance.

In other words, the faster the motor runs, the higher a voltage it generates and the smaller the difference becomes. There are, of course, electrical and mechanical losses, so there has to be a finite amount of power that is being supplied by the power source, which means that the motor never quite reaches the rpms at which the generated voltage would be exactly the same as the supply voltage.

  • $\begingroup$ The second paragraph is an edit. I think edits as substantial as this are more properly made in a comment. $\endgroup$
    – garyp
    Commented Jun 27, 2015 at 19:33
  • $\begingroup$ yes, I asked CuriousOne to add his comments as an answer. He only added his first comment, but I found the his second one also very useful so I add it. Is that ok? $\endgroup$ Commented Jun 27, 2015 at 19:35
  • $\begingroup$ @garyp. I am OK with it. The OP asked if he could add my second comment to the answer and I approved it. I will leave it at this trivial answer. If someone wants to write out the theory of an electrical engine with details, they are welcome, one can certainly fill entire textbooks with the details! $\endgroup$
    – CuriousOne
    Commented Jun 27, 2015 at 19:36

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