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Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
I'm playing with a 3 phase ac motor (aka a brushless model airplane motor) used as an alternator.
Curiously, while I often hear claims about these permanent magnet brushless motors being 90% efficient, I'm measuring only ~30% electrical efficiency as a generator. Is this typical? I tested with only a single load (8ohms); does the efficiency vary with load for a given alternator?
Some of my losses come from my silicon rectifier, dropping .9V or so. Is there a better way, perhaps some arrangement of transistors acting as my electronic Maxwell's Daemon?
EDIT: why the hell did I post this in Projects?! hold please...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi
I never tried such a thing, but I don't think it should be that inefficient. At least not at power levels airplane motors work. If you pushed like 300W into it and dissipated 200, motor (or diodes) wouldn't last long.
How did you measure power throughput?
For low voltages, schottky diodes are preferable over normal silicon diodes. Synchronous rectifier ('maxwell's demon ) even better, although building a controller for it is a bit tricky on a generator.
Maxwell's demon also wins the advantage in multiphase system because mosfet's dissipation goes up with square of current.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Are you running it at the correct speed?
As to the varying with load, there will definantly be a 'sweet spot' where the i/v curve will give a maximum power. The easiest way to find that is to just use a bunch of different resistors and see how much power you have to put into the generator (presumably through another motor) to get a disired output power.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
I'm not sure if it applies to motor/ generators but if this were some other sort of power source then the maximum power you could get out of it would be the power it delivered to a load with the same resistance as the effective source resistance as the generator. You could do that by measuring the power into lots of reistors and seeing which does the best job. The other way is to measure the open circuit voltage and then find a resistor that drops the voltage to half that value. My best guess is that 8 ohms is a bit big.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If you are lucky, the controller that came with the motor will act as a synchronous rectifier. You just need to drive the motor with the controller as normal, and then use your prime mover to spin it a little faster than it normally wants to go. Depending on the design and firmware of the ESC, it may start to regenerate.
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
Part of the problem is that my prime mover is such a grumpy little turd I'm trying to tune them both simultaneously. Without an absolute referrence for shaft power theres a certain amount of trial and error and guesswork involved.
Anyway, I probably have about 50W of shaft power available. My confidence in this number is +/-15%.
My motor/alternator has a kv of 1080, and coil resistance of 0.19 ohms. As a motor, this is designed for 11.1V and 12k RPM or so.
Mashing the two together, I get 13V measured across 8ohms, after my rectifier (21W).
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
lol we are trying to do the same thing, but with a much larger motor. Testing the regenative breaking codes on a new motor controller. We are getting power from the motor controller when we spin the motor in one direction and tell the controller to go in reverse. its weird...
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
I think you really need to try running that thing into a lower resistance load. A bunch of 6 volt light bulbs might be an easy cheap solution for an experimental rig (though you need to measure I and V because R is nothing like constant.)
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Alternator efficiency / 3phase recitifier
) even better, although building a controller for it is a bit tricky on a generator.
