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Old 12-14-2013, 07:47 AM   #1 (permalink)
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Electric Motor Question

I noticed something in the specs for an AC motor with permanent magnets that you cannot avoid having power generated if the stator is spinning. Does this mean that with this kind of motor that you cannot free-wheel coast? Would it always be regeneratively braking?

If so, then that is another good reason to go with an AC induction motor.

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Old 12-14-2013, 10:16 AM   #2 (permalink)
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Cogging and back emf when coasting is not a problem in an ev where the energy will captured by the battery anyway. It is annoying on an E-bike if your battery went low and you had to pedal home.
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What is the efficiency of those non perm motors?
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Old 12-15-2013, 05:29 PM   #3 (permalink)
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A permanent magnet motor always generates electricity if the shaft is spinning. But, if the winding is not connected to anything, it is not generating any power. If it is not generating power, it takes no power to spin the shaft. Well, windage and bearing losses.
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Old 12-15-2013, 08:20 PM   #4 (permalink)
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The AC induction motor I linked to is rated at 95%, and that includes (I think) the inverter. The Getrag gearbox is pretty low losses as well - I've seem them at 98% or so?

Thanks for the explanation on the regen - that makes sense.
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Old 12-16-2013, 09:51 AM   #5 (permalink)
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PM motors do have cogging torque, or zero current torque, due to the interplay of the magnets. Bikes have so little regen efficiency that it usually makes sense to just install a freewheel. Some other sort of clutch (or neutral) will eliminate it too.

Reduction of cogging torque in permanent magnet machine

This is readily apparent in my prius, in order to "coast" the car, I have to feed the motors about 10 amps.


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