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Question is pretty much in the title. I know it's definitely possible to use certain DC brushless motor as a generator, but is it always the case?

Thanks a lot in advance.

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  • \$\begingroup\$ What different types of BLDC do you know? \$\endgroup\$ – Eugene Sh. Aug 4 '17 at 17:49
  • \$\begingroup\$ generate what? ac or dc? \$\endgroup\$ – user3528438 Aug 4 '17 at 17:56
  • \$\begingroup\$ @EugeneSh. No I mean could some motors have some added features specially added to stop the motor from being used as a generator or something similar... \$\endgroup\$ – Cyril Gliner Aug 4 '17 at 18:06
  • \$\begingroup\$ @user3528438 I want to use the motor in combination with a speed controller to recharge a battery (I'm working on a regenerative breaking project), I hope this answers your question \$\endgroup\$ – Cyril Gliner Aug 4 '17 at 18:08
  • \$\begingroup\$ Of course there could be some extras on the motors, and only the imagination of the manufacturer is the limit. It can be mechanical restriction, such as a one-way gear (worm gear). It can be electrical, like a bunch of diodes or similar, allowing one way power transmission only. \$\endgroup\$ – Eugene Sh. Aug 4 '17 at 18:27
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Permanent magnet BLDC motors can be used as generators.

There is a type of brushless motor called a Reluctance Motor, which has soft-iron core, so will generate nominally zero voltage when spun.

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    \$\begingroup\$ An easy way to check if any unknown motor would work as a generator or not would be to rotate the shaft with the terminals open, then short the terminals and rotate the shaft again and compare if it's more difficult to rotate with shorted terminals or not. \$\endgroup\$ – Harry Svensson Aug 5 '17 at 6:25
  • \$\begingroup\$ Actually, a switched reluctance motor can be used as a generator. It just needs a little encouragement to start. \$\endgroup\$ – Jon Dec 20 '18 at 10:11
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Brushless DC motors are usually motors that have permanent magnet rotors. It would be extremely unusual to find any other type of motor described as a brushless DC motor. All such motors can be used as generators, but some designs are easier to use as generators than others. A major example of a difficult motor is a BLDC fan motor found in a computer. Those have electronic circuitry built into them that must be removed or disconnected in order to use the motor as a generator. You might find some other design described as a BLDC motor that would be difficult to use as a generator, but most of them only require the shaft to be turned to produce AC at the terminals and a rectifier added if you want DC.

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  • \$\begingroup\$ Exactly. A simple permanent magnet DC motor, like the ones that used to be common in battery-operated toys and model kits, work as generators without trouble. But a DC brushless motor has MOSFETs controlled by a microcircuit to switch power to the stator coils. The microcircuit can only be powered from the outside. \$\endgroup\$ – Jamie Hanrahan Dec 6 '18 at 6:20
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Two examples where it is desirable for a motor to function as a generator are:

  1. Regenerative braking in a vehicle so at least some of the energy is recovered rather than being totally wasted in a mechanical friction brake.

  2. Using the generator function to hold the motor in a stopped condition by switching a resistor or a short across its terminals.

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