I have a large DC Motor I ripped out of a treadmill and I'm curious if the back emf generated from manually rotating the motor is a DC output or an AC output. Is it AC only when it's from the power grid and other sources treated as DC? How can I always tell?

For context, the motor is a 2.65Hp, 21.4A, P.M.D.C Motor

Thank you!


For Permanent Magnet DC motor, even if you rotate it with the constant rate the back EMF you'll get externally from the motor connector pins ought to be more similar to a rectified AC.

enter image description here

However, I wouldn't expect it to be exactly sinosoidal.

  • $\begingroup$ Is there a reason why it would look like this? From my understanding, the back EMF is proportional to the speed of the motor shaft and so, at steady-state where the speed of the motor is constant, wouldn't the back EMF approach a steady-state value and hence be a DC voltage? $\endgroup$
    – Pelumi
    Mar 8 at 17:52
  • $\begingroup$ The reason I expect it to be so (I've never actually measured this type of motor, although I have done measurement on wind turbines), is that the windings revolve in a don't go through a constant magnetic field, so there is this sinusoidal curve. Also the rectificiation is due to the commutators. $\endgroup$
    – NMech
    Mar 10 at 6:28
  • $\begingroup$ That's actually quite helpful and that checks out with me logically. I have seen that some of the large motors have a permanent magnet in them and with the rotation of the rotor, the generated voltage should behave as you say. Thank you very much! $\endgroup$
    – Pelumi
    Mar 14 at 4:37

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