I'm trying to design a system that requires a very light weight very efficient generator to be used from converting rotational motion into DC voltage(for say charging batteries) As far as I know BLDC's are the most efficient and light weight to convert the other way around electrical power to rotational energy

What would be the best motor type to be used in this app in term of weight and conversion power?(AC, BLDC, brushed DC?)

Thanks for helping.

  • \$\begingroup\$ Just by the variety of motors: BLDC motors. \$\endgroup\$ – StainlessSteelRat Feb 18 '19 at 19:11
  • \$\begingroup\$ err, @StainlessSteelRat, aren't BLDC motors just synchronous (typically, 3 or more phases) motors with a DC-fed switch-mode power supply integrated, i.e. the only one of these that can't generaly be used as generator? \$\endgroup\$ – Marcus Müller Feb 18 '19 at 19:18
  • \$\begingroup\$ Michael: There's no generally most efficient one. It depends on size constraints, rotational speed, power, and the amount of external electronics you're willing to tolerate. So, I'm afraid: as always, "best" isn't defined without the use case... \$\endgroup\$ – Marcus Müller Feb 18 '19 at 19:19
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    \$\begingroup\$ @MarcusMüller Remove controller and drive motor mechanically and a BLDC is a pure ac generator. Add 6 diodes and it works like a car alternator. Given the variety of BLDC motors, including low rpm used in electric bikes, and they are the optimum choice as a generator. \$\endgroup\$ – StainlessSteelRat Feb 18 '19 at 19:31
  • \$\begingroup\$ heh, I like that reasoning:) \$\endgroup\$ – Marcus Müller Feb 18 '19 at 19:32

Brushless DC motors (BLDC) or permanent-magnet synchronous motors (PMSM) are pretty much the same thing and are the best to use as generators. They generate AC, so they require a rectifier on the output. There is no way to regulate the generated voltage except to regulate the speed. You will need a charge controller or other electronic power converter to regulate the voltage to the extent that it needs to be regulated.

Commutator DC motors are heavier, subject to commutator and brush wear. With wound-field DC motors, voltage can be regulated by regulating the field current, but power conversion regulator could also be used.

Another alternative is an automotive alternator. That is a wound-field synchronous generator with slip-rings. Voltage is regulated by regulating the field current with an external regulator. Rectifiers may be built in or external. Newer alternators may be permanent-magnet synchronous generators with power converter voltage regulation.

Note that some BLDC motors have built in electronic controllers that must be removed to allow them to be used as generators.

  • \$\begingroup\$ "Commutator DC motors ... can be regulated by regulating the field current". Only if they're wound-field motors, and one could build a BLDC wound-field motor (although I don't think that such things exist). There's certainly plenty of permanent-magnet brushed DC motors out there, with a variety of magnetic materials used to make the magnets. \$\endgroup\$ – TimWescott Feb 18 '19 at 19:56
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    \$\begingroup\$ Commutator motors are always wound field motors, but the field is often internally connected in series or parallel with the armature. The internal connection can often be re-connected for external access, but that may lead to inconvenient voltage requirements. A brushless motor can not have a wound field. \$\endgroup\$ – Charles Cowie Feb 18 '19 at 20:15
  • \$\begingroup\$ With all due respect, no. And no. The first no: see this video, or see half the Maxon catalog, or half the Faulhauber catalog, or half the Pittmon Motor's catalog, or pretty much the entire Mabuchi catalog. The second no: industrial synchronous AC machines are brushless motors living under an assumed name. They've been working hard in industry for the last 100 years or so. \$\endgroup\$ – TimWescott Feb 18 '19 at 20:57
  • \$\begingroup\$ I revised my answer re commutator motors. I don't think rotating exciter schemes need to be described in this context. \$\endgroup\$ – Charles Cowie Feb 19 '19 at 0:29

It's not BLDC motors so much as it's motors with rare-earth magnets. There are mechanically commutated DC motors with rare-earth magnets out there, and there are older brushless motors with other sorts of magnets.

Having said that: if you're re-purposing a motor (as opposed to engineering a generator) then a BLDC motor with rare earth magnets, followed by a synchronous rectifier, is probably going to be most efficient.

If by "light weight" you mean "around the size of a car alternator", then I would seriously consider trying -- a car alternator. Possibly a car alternator that's had everything non-essential trimmed off of it. It may or may not be as efficient as a BLDC motor of the same size -- you'd have to try.

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    \$\begingroup\$ You can't get the same efficiency or power density from an automotive alternator, the Lundell-style rotor is designed principally to be cheap, and comes with compromises in the magnetic circuit, but it would be worth looking at for ease of control and availability. Salient pole rotor alternators are available but are generally larger, high output units for specific applications. \$\endgroup\$ – Phil G Feb 18 '19 at 22:08

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