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To better explain what I am asking in the title, please watch at 18 seconds into this video: https://www.youtube.com/watch?v=6kgzrXFSDwA

Those are of course brushed DC motors.

I know that BLDC motors require more complex controls to spin up as opposed to just being hooked up to a DC source. However, I am curious if two BLDC motors would behave in the same manner as long as the two motors are identical, thus assumedly having matching phases. Then U would simply be connected to U, V to V, and W to W.

If it is indeed possible, would it also be possible to reverse the direction of the powered motor? How would the U, V, and W terminals need to be reconnected?

To clarify, this is not for any ‘free energy’ purposes, I am fully aware that there would be efficiency losses.

Thank you in advance for any help.

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    \$\begingroup\$ I have done this with BLDC's. Connect the phases directly together. If you spin one motor, the other one will spin a bit, but sometimes it misses a step. If you swap phases, it will spin in the opposite direction. But it doesn't work very well, at least in my experience. It might be possible to design a pair of non-identical BLDC motors so that one of them is a master and the other is a slave, and achieve better results. The other option would be to connect the two identical BLDC's to the same DC bus, and have separate controllers so that the "generator" can spin a bit faster than the "motor." \$\endgroup\$ – mkeith Nov 13 '17 at 4:10
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    \$\begingroup\$ I think the reason it doesn't work very well is that the back EMF of a BLDC running as a generator is a bit lower than the back EMF of a BLDC used as a motor, due to voltage drop in the equivalent series resistance of the motor windings. \$\endgroup\$ – mkeith Nov 13 '17 at 4:11
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    \$\begingroup\$ Are you sure you aren't looking for a synchro or resolver type system here? It sounds like that's what you're creating in essence... \$\endgroup\$ – ThreePhaseEel Nov 13 '17 at 4:22
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    \$\begingroup\$ You can run a BLDC motor that way, but then the driven motor is not really running as a BLDC motor but a PM synchronous motor, which has very low starting torque (i.e. there's no commutation circuit to maintain the proper separation between D and Q axes to produce proper torque under all conditions.) \$\endgroup\$ – John D Nov 13 '17 at 4:46
  • \$\begingroup\$ I've done this with stepper motors and it works so long as the input is fast enough to overcome cogging and any mechanical load on the output. at low speeds the internal resistance of the motors eats all your power so there's very little torque, \$\endgroup\$ – Jasen Nov 13 '17 at 5:11
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Yes. One BLDC motor can be used as a generator to drive another.

However, this is only really useful for a quick test or demonstration of a motor. The generating motor only produces voltage while it's being turned. If the generator is turned too slowly, it may not generate enough voltage to overcome the friction or load of the motor. If the motors are of different sizes, use the larger one for the generator.

If you connect the three wires of the motor at random to the generator, then the motor will turn one way or the other. You can switch the direction by swapping any two phases, it doesn't matter which.

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