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Can ANY DC motor become a generator if torque is applied to the shaft? example: Produce-Electricity-from-a-DC-Motor

And by any, I mean the really cheap ones too. Or is there a specific type of motor (of which I do not know the name) that can work in both directions (battery to power the shaft OR torque-on-shaft to create a voltage difference between the terminals).

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Any DC motor with permanent magnets can easily be a generator. It doesn't matter whether it is brushed or not; brushless motors make great generators but you will need to add a rectifier to get a DC output.

If the motor has a separate field winding instead of a permanent magnet, you will need to energise that winding from an external DC source, e.g. a battery.

For example, a car alternator has DC slip-rings (not a commutator!) to supply a small field current to the rotor, and a three phase (usually) stator winding. To get anything out, you must energise the field winding, and then you get 3-phase AC from the stator, which must be rectified in order to obtain DC. You can control the output power (and therefore the load the alternator places on the shaft) by modulating the field current.

You can google up plenty of howtos that tell you how to make a wind generator from recycled or homemade brushless permanent-magnet motors. They're a good design specifically because of the lack of brushes, because brushes wear out, are expensive and inefficient compared to a silicon rectifier.

If it's a "series wound" DC motor, I don't think it will generate unless part of the motor chassis is residually magnetised. That style of motor (only two terminals connecting to two windings and no permanent magnets) is generally not appropriate for generation because you have no guarantee it will do anything, and no good way to control the generated power/load.

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  • \$\begingroup\$ a series wound motor has no back-emf limited top speed, and you have to exceed the top speed (for the voltage desired) to get power out of a generator, so without modification a series wound motor will never generate electricity. (except from residual magnetism) \$\endgroup\$ – Jasen Feb 8 '16 at 6:18
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After a quick read-through of that wikihow article you linked, that should work with any brushed DC motor.

NOTES:

  • Make sure you DO NOT try this with a brushless DC motor (which requires a complex drive circuit to do anything with).
  • Don't expect much power output from just putting a propeller on a motor shaft. To get any real output you'll need higher shaft speeds. Maybe try a belt & pulley system to up the RPMs if you want it to work as more than a conversation piece.
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    \$\begingroup\$ If I see a motor that says 1.5V and 4700 RPM, does that mean that if I do reach 4700RPM, the max voltage that can be output is 1.5V? \$\endgroup\$ – user2883071 Feb 8 '16 at 4:27
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    \$\begingroup\$ Actually, you'd probably end up having to spin it at about 5000 to 5300 rpm to achieve 1.5V (coil resistance & mechanical drag losses work against you both when using it as a motor & as a generator, so there's a significant rpm overlap), but your basic premise is correct there. \$\endgroup\$ – Robherc KV5ROB Feb 8 '16 at 4:31
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    \$\begingroup\$ brushless motors make excellent generators, you just need to rectify the output. A brushless motor being used as a motor does require a complex controller; if you want to use the motor as both motor and generator in the same device then you will want a two or four (depending on whether you need both directions of rotation) quadrant controller, and that costs more than a simple single-quadrant (motor-drive only) controller. \$\endgroup\$ – William Brodie-Tyrrell Feb 8 '16 at 4:38
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    \$\begingroup\$ also, a non-PM brushed DC motor will often NOT work as a generator. It's just a bunch of spinning copper windings that won't spontaneously interact with each other. \$\endgroup\$ – William Brodie-Tyrrell Feb 8 '16 at 4:40
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    \$\begingroup\$ @WilliamBrodie-Tyrrell true enough for a PM brusjless just needing rectified; however, the article OP linked was about direct sc generation using the commutation ring to rectify for you...so brusjless sould be usable for a generator, just now how the article described. user2883071 a PM motor is one with Permanent Magnets inside. 99.9% of cheap, lr R/C toy motors are going to be PM brushed DC type. I've seen many brushless DC motors for, say, high-end R/C motlrs & most computer fans; not sure I've ever come across a brushed, non PM though (well, except in high-school shop class). \$\endgroup\$ – Robherc KV5ROB Feb 8 '16 at 4:47

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