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Discussion Starter #1
Apologies as the question is not necessarily EV-related, but this is one of the most knowledgeable forums.

I am tinkering with BLDC motors to potentially use in a series-hybrid model train or go kart or something fun like that. Two identical BLDC motors will basically replace the transmission in a crude manner.

I want to attach one motor to a small engine as a generator and another identical motor will be the traction motor.
The generator will be connected directly to the traction motor and speed will be controlled with the small engine throttle since this does not need to be precise or efficient, it is just for fun.

I also want to use an ebike controller connected to the generator motor to use it as a starter for the small engine, and potentially as 'boost' during low speeds or high loads.

My question is, when I want to disconnect the traction motor from the generator motor, or the controller from the generator motor, is it sufficient to only put a contactor on one of the three phase wires?
Or do the other two wires still stay 'live' while the generator motor is spinning and thus I should put contactors on all three to prevent any drain or potential damage?

I am only familiar with brushed DC where one contactor is sufficient on the positive wire, but I am still a bit clueless when it comes to brushless motors.

Thank you.

4,906 Posts
You didn't mention a controller for the traction motor - you need that. To disconnect the generator from the motor, you would presumably just disconnect the DC link between the controllers; I don't see any need to disconnect the 3-phase AC conductors at any point.

engine mechanically drives
generator which pushes 3-phase AC power to
generator controller which produces DC power to
traction motor controller which produces 3-phase AC power for
traction motor.

If I understand the plan correctly, like a train and unlike a typical hybrid car, there is no energy storage (battery) involved in normal operation.

For starting,
a battery provides DC power to
generator controller which produces 3-phase AC power for
generator/motor which mechanically drives

You can't just connect the 3-phase power conductors of the generator and motor, because that would only work when the two are turning the same speed... and the whole point is to allow the engine to run at a suitable speed unrelated to the wheel speed (and thus traction motor speed). The 3-phase power with a synchronous motor (including permanent magnet motors called "brushless DC") is not only at the same frequency as the motor rotation, it needs to be in phase with the motor.

To more directly answer the original question, with two of three phase wires connected between a 3-phase power source and a motor, the motor would run like a three-cylinder engine which is misfiring on two cylinders - if it could run at all - but it would definitely be powered.
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