I have a DC motor connected to a roller. The roller is rotated by a load. If I don't power supply the motor, it will free spin. Then I would like to turn on the motor only on certain times, to help the spinning. Could this be done or it damages the motor?



You can use the motor as you describe, without damaging the motor. The load spinning the motor will generate a voltage at the motor terminals (it will act as a generator) so make sure that whatever it is connected to will withstand this voltage - if there is a load on the motor it will act as a brake.

  • \$\begingroup\$ Ok I see, thanks. What do you mean with "if there is a load on the motor it will act as a brake"? \$\endgroup\$ – user53026 Nov 6 '15 at 10:15
  • \$\begingroup\$ It will be harder to turn the motor if it is supplying current. Motors act as generators when they are used like this, so if you put a load on the generator, the power the load uses must come from somewhere, hence it is harder to turn the motor when it has a load. Try spinning a DC motor without its terminals connected, then try again with its terminals shorted - the difference is quite surprising. \$\endgroup\$ – Icy Nov 6 '15 at 10:20
  • \$\begingroup\$ ok, I have to perform some tests... thank you Icy! \$\endgroup\$ – user53026 Nov 6 '15 at 12:55

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