Auto alternator as AC motor

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portablestew's picture
Last seen: 14 years 11 months ago
Joined: Friday, January 5, 2007 - 19:39
Points: 9
Auto alternator as AC motor

So I picked up a couple alternators at the local pick-a-part (damn core charge, lol). Now I want to build a controller to drive an alternator as a stepper motor.
As you can see here and here, standard automotive alternators can be easily converted into AC synchronous motors and/or stepper motors. Apparently it's even a relatively common practice to do so, when cheap heavy-duty stepper motors are needed.
I'm just going to use a traditional design for a stepper driver. That is ... computer generated waveforms used to control power to each of the alternator's 3 coils, and freewheel diodes across each of the motor coils. There's a bunch of details that concern me, though. As the motorspins, will current be generated by the non-powered coils? With normal (small) stepper motors, this isn't a problem ... but with an alternator, could generated current possibly overheat my freewheel diode (or damage anything else)? I guess I could do low power tests with cheap components just to see how it works.
Also, just to keep things simple and cheap, I plan on having only 2 states for each coil: positive and disconnected. For now, the added complexity of allowing current in both directions simply isn't worth it (I'd need large, solid-state H-bridges).
Eventually, if it works out, I might throw the whole system into an old 1970 honda scooter frame and roll around on all 2 of its horsepower.

Fechter's picture
Last seen: 13 years 11 months ago
Joined: Friday, November 17, 2006 - 07:01
Points: 199
I don't think you'll have

I don't think you'll have any problems from back EMF. Are you going to use permanent magnets on the rotor? That's probably the best way. Otherwise, you might be able to make a new rotor from a chunk on steel and make a variable reluctance motor.

I think you'd get the best performance by going with permanent magets on the rotor and a 3 phase brushless motor controller. You can get a chinese brushless controller for under $100.

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