Picked up a 1500w "sensor-less" compatible electric bike controller off fleabay for ~$50, so that I could try running an alternator as a motor...
The crude setup, using a common/spare alternator I had laying around, with the voltage regulator removed:
It works!
I was varying the speed via an ebike throttle in the video, just to show it was variable.
Max RPM is determined by the battery voltage, of course, and, more modestly, by the voltage being fed to the rotor/field. For example, feeding it 80v with 10v at the field, it ran at 4700rpm. With 80v and 5v on the field, it ran at 5000rpm. You can get ~3600 rpm off of 56v(48v lithium) with 5v on the field, a typical rpm for a lot of motors.
No-load efficiency falls off based on two things...the RPM of the motor, and the field voltage; the higher in either, the more power it wastes.
For example, at 80v, 10v on the field, max RPM of 4700:
13a (1040 watts) just to spin the thing!
However with 4v on the field, it's a lot more reasonable:
Of course, it wasn't 100% stable at 4v...it was fine at 5v though. One of two things comes to mind there...this well used alternator's brushes didn't want to conduct throughout the rotation at a lower voltage, or else the magnetic force from the 3 phase wingdings(the 80v side) was enough to interfere with the weaker field. I'm leaning towards the latter, since it would crap out on hard acceleration(more 3-phase current, more likely to interfere). The other option is that my adjustable power supply was the culprit and couldn't supply more current when strained suddenly.
Low RPMs barely take any current to drive the motor. As in 1-2 amps at 1200 rpm.
Efficiency? Well, if the wasted current stays the same under load (no, it won't, but I don't have much else to go on), it would be ~78% efficient at full output (max rpm and controller current). That's better than I was expecting. Without rigging up a dyno, I won't be able to be sure though.
Torque? Yessir. Not very scientific, but I tried to hold the pulley from moving by hand with a piece of rubber for friction/protection, and from a dead standstill it easily overpowered said friction, however hard I tried to stop it. And with very little current; the most I saw it draw was 4 amps at a few hundred RPM when I held on as hard as I could. Plenty of starting torque...
Anyway, I'm pretty happy with the results. Yes, I'd like it if it were more efficient at higher RPMs, and/or if it would spin considerably faster, but that's about it.
Why bother? Well...motor cost is the first reason. A 1500w brushless motor would cost north of $200, and still need the controller to work, whereas you can pick up an alternator for next to nothing. The second reason is RPM...I wanted a motor that, even if I couldn't drive it particularly fast, would survive spinning at 10,000 rpm or more. Third is size...this little guy is just under 6" across and 6" wide...a much more reasonable size to work with than the Etek motor I have. I'm willing to trade off some efficiency for those reasons.
What could it be used for? Well...for a cheap ebike with plenty of power, or for a very mild hybrid of either a car or a motorcycle comes to mind. It's small enough and light enough for those purposes, and can handle the rpms if connected to an engine. You could run whatever else you want off it instead...1500w(2 horsepower) at ~3600 rpm will run a lot of things...a lawnmower, for example...
Bla, bla, bla...if you're interested in trying to do the same, just make sure the controller you get can run "sensorless".
Peace.
