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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Background:
im experimenting with a tiny 40nx glow engine, as many of you know, for multirotor purposes, as we can only fly for 8 minutes with a useful payload. potential users like police and fire would consider this a disadvantage. I think practicality begins at 30 to 45 minutes, and preferable around 1 hour. now there are many machines capable of using Lion laptop batteries, custom fabricated for flight. these are even better than my LiPo batteries, but these machines are 95% battery and 5% machine, with no real payload. Question: so, im wondering, when i double up the use of the generator as a starter motor, will the Back EMF be distorted?
the basic function is as follows, alcohol -> piston <-> starter/generator <-ESC/diode bridge switch. -> power out
if the 3-phase driver (ESC) is turning the motor to start the engine, when the engine starts it jumps in rpm, would this kill diodes, mosets, and confuse the microcontroller using the back EMF for timing? thi switching from starter to generator is the killer, the rest easy enough for anybody figure out.... my brain hurts...
[ESC = electronic speed control = can be expensive... ]
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Assuming these motors are permanent magnet rotors the emf should match the rotor position regardless so the control circuit shouldn't loose track of the phase. But the problem might be with the PWM part that might inherently keep the phases grounded for the most part (low duty cycle PWM) if it thinks the speed is over the setpoint, so that it will try to absorb all the energy until it thinks it has slowed to the correct speed, sounds like that would end up in a smoking controller or motor. Could you spin it up to high then immediately set a low speed to see if it slows down like it's freewheeling or suddenly. Can these motors provide enough torque to start the engine? Guessing thay could. Also how efficient are these motors/generators
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Electra wrote ...
if it thinks the speed is over the setpoint, so that it will try to absorb all the energy until it thinks it has slowed to the correct speed, sounds like that would end up in a smoking controller or motor.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I imagine if you used a RC car ESC with regen braking, it would work out OK. You just have to send a brake command to the controller when the engine starts. As far as the ESC is concerned, it is just braking a really big and heavy RC car down a hill that never seems to end. :)
Grounding the phases during the PWM cycle is OK, in fact it's how regen braking works. Energy builds up in the inductance of the short circuited winding, then returns to the battery when the FETs turn off. It's basically a boost converter, allowing the motor to return energy to the battery even when its back EMF is smaller than the battery voltage.
The only way it might kill anything is if the engine were allowed to rev to a very high speed and that generated more voltage than the ESC could handle. But I don't think that will be a problem. Even if the ESC's control electronics shut down completely, the body diodes of the FETs would still rectify the motor output and feed it back to the battery, so the voltage would be clamped and the engine would get some sort of load.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ok steve ... ill try this, i was thinking of beefing up the Schottky bridge and ESC transsistor bank with TVS's just to prevent occasonal V spike fro killing them all out at once.
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3 phase electric motor/generator hybrid, Back EMF issues.
