Hi, I have a homemade ebike which uses a car alternator as the motor. It's fast and has lots of power for hill climbing. The controller is 45A @ 75.6V. The only problem is the efficiency, the alternator gets hot quickly and only gets about 45km of range on my 1.2KW battery pack (18s8p, 75.6V, 16A). This doesn't seem like much at all.
Looking at other ebikes, it seems that I could double that range with a better motor. Rinoa also built a 1.2KW pack and got over 100miles a few years ago: https://www.youtube.com/watch?v=Rjr2LKmhdVw
I was searching on google and found this on hackaday:
(http://hackaday.com/2016/08/14/alternator-becomes-motor-for-this-electric-go-kart/)
"Dax says:
August 14, 2016 at 10:12 am
Because it is optimized for low impedance for efficient output.
When the alternator is turning as a motor, the stator gets an input of AC which generates a magnetic field during the cycle. Energy is being stored in the field while the current is increasing, and it is this stored potential energy that shows up as the torque in the rotor. When the current is steady, no more energy is stored in the field, so no more energy is transferred into torque, but is only lost due to the ohmic losses of the copper wire.
The alternator’s stator windings are designed for very low impedance at the 1000-2000 RPM running speed range, which means when you input a relatively low frequency AC current to make it spin say 500 RPM, the coils don’t hold the current back. The current rises up rapidly, then the power supply saturates and cannot give any more, and the controller spends most of the cycle simply maintaining the field and wasting power.
Due to the low impedance, the motor would only run efficiently at very very high frequencies so the current would have just enough time to rise up to maximum before the cycle is over, and no power is wasted to maintain the field.
That speed would be much higher than is useful or practical and the alternator would break rapidly. To work better as a motor, you’d need to re-wind the stator with a slightly thinner wire and more of it, so the impedance at the desired speed range would limit the current instead of the controller or the battery.
In short, the alternator as a motor has parameters that would work better as a ultra-high speed motor, because in return it means that the alternator has high current capability as a generator.
The second reason is that normally BLDC motors have permanent magnets in the rotor, whereas the alternator uses extra power to energize the field coil. The field coil can be shorted out, in which case the alternator acts as an induction motor – problem being that an induction motor has zero torque at zero speed so it’s really bad at getting off the line and stalls very easily, and again the rotor coil is not optimized for induced AC so the performance is awful."
Now this is interesting information but not very precise, it seems clear that using thinner wire and more of it for the windings would increase efficiency but by how much? Does anyone have any ideas on this as I would like to try rewinding my alternator. I think that it's worth doing considering the fact that an alternator is very cheap to buy whereas a hub motor of the same power would cost quite a bit.
Thing is if it only encreases efficiency by say 5%-10% it's not worth it but any more than that then it's worth trying.
Any information on the subject would be apreciated.
Thanks,
Seb.
Looking at other ebikes, it seems that I could double that range with a better motor. Rinoa also built a 1.2KW pack and got over 100miles a few years ago: https://www.youtube.com/watch?v=Rjr2LKmhdVw
I was searching on google and found this on hackaday:
(http://hackaday.com/2016/08/14/alternator-becomes-motor-for-this-electric-go-kart/)
"Dax says:
August 14, 2016 at 10:12 am
Because it is optimized for low impedance for efficient output.
When the alternator is turning as a motor, the stator gets an input of AC which generates a magnetic field during the cycle. Energy is being stored in the field while the current is increasing, and it is this stored potential energy that shows up as the torque in the rotor. When the current is steady, no more energy is stored in the field, so no more energy is transferred into torque, but is only lost due to the ohmic losses of the copper wire.
The alternator’s stator windings are designed for very low impedance at the 1000-2000 RPM running speed range, which means when you input a relatively low frequency AC current to make it spin say 500 RPM, the coils don’t hold the current back. The current rises up rapidly, then the power supply saturates and cannot give any more, and the controller spends most of the cycle simply maintaining the field and wasting power.
Due to the low impedance, the motor would only run efficiently at very very high frequencies so the current would have just enough time to rise up to maximum before the cycle is over, and no power is wasted to maintain the field.
That speed would be much higher than is useful or practical and the alternator would break rapidly. To work better as a motor, you’d need to re-wind the stator with a slightly thinner wire and more of it, so the impedance at the desired speed range would limit the current instead of the controller or the battery.
In short, the alternator as a motor has parameters that would work better as a ultra-high speed motor, because in return it means that the alternator has high current capability as a generator.
The second reason is that normally BLDC motors have permanent magnets in the rotor, whereas the alternator uses extra power to energize the field coil. The field coil can be shorted out, in which case the alternator acts as an induction motor – problem being that an induction motor has zero torque at zero speed so it’s really bad at getting off the line and stalls very easily, and again the rotor coil is not optimized for induced AC so the performance is awful."
Now this is interesting information but not very precise, it seems clear that using thinner wire and more of it for the windings would increase efficiency but by how much? Does anyone have any ideas on this as I would like to try rewinding my alternator. I think that it's worth doing considering the fact that an alternator is very cheap to buy whereas a hub motor of the same power would cost quite a bit.
Thing is if it only encreases efficiency by say 5%-10% it's not worth it but any more than that then it's worth trying.
Any information on the subject would be apreciated.
Thanks,
Seb.