Rewinding a car alternator for ebike use

gomme600

1 mW
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Feb 6, 2017
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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.
 
gomme600 said:
Any information on the subject would be apreciated.
Your best first bet is to simply look at the various threads with the word "alternator" in the title, as there is probably good info in some of them. ;)

https://endless-sphere.com/forums/search.php?keywords=alternator&terms=all&author=&sc=1&sf=titleonly&sk=t&sd=d&sr=topics&st=0&ch=300&t=0&submit=Search


But if the problem is heat, then changing your gearing to better match the motor's speed to the vehicle's speed usually helps. (assuming you havent' already done that).
 
This has been tried many time by people much smarter than me. You can make an alternator into a motor, you can even re-wind it with a different diameter of wire to make it work "better" as an electric bike motor. However, you will always be struggling to get adequate performance from something that was never intended to be a DC motor.

For the amount of time, cost, and effort it would take. You will have been miles ahead by building a Lebowski axial-flux motor from scratch. he has a wonderful controller design for it too, but the motor can be run by any common controller, if you like.

If you still want to experiment, I suggest using the largest washing machine motor you can find and scouring youtube videos to cobble together a DIY controller for it.

The universal style of motors can run on DC or AC. And since they are built to use 120V AC at 3000-RPMs, then 48V DC would be about 1350-RPMs?...getting it down to the 300-RPMs of a rear wheel on a bicycle would be around a 4:1 ratio?

I'm not saying that a converted washing machine motor is the holy grail of ebikes made from free junk, just...its gotta be better than a car alternator.
 
What you now have is an asynchronous AC motor. Unlike a synchronous BLDC motor the virtual rotor (rotating magnetic field) has a slip factor where it rotates faster than the rotor does. Typically slip rates are in the 2-4% range. They will run at a synchronized speed with no load, but with load they drop efficiency when not run at the ideal slip rate, and that rate depends on the construction.

More over a 6 way IGBT switching system is going to provide a much better control of the motor, particularly when its designed with the control features better suited to an AC Asynchronous duty.

Sinamics Drivers offer some programmable units that if you do some research explain the features you need.

Alternators make great motors. Inside this Truck drive you can see the motor which is from a Wind Turbine made by Skoda in CZ. We use a 6 megawatt controller to drive pair of these on the trucks I used to design.

MT5500_Wheelmotor_10.jpg
 
I could be wrong about using a converted alternator from a car for a motor. I just haven't seen a detailed instruction on how to modify them, and how the controller for it might work (or be made).

What model of car alternator is the best candidate?

What RPMs are produced at 48V?
 
Thanks for the replies so far, I didn't think ti use the forums search function as I had already looked on google many times, seems it can miss quite a lot!
I have wondered about my gearing in the past but I'm not really sure what I need as I can't calculate the motors RPM... I hace a 25H chain with 11teeth on the motor and 80 teeth on the wheel. 7:1 ratio?

I could probably get hold of a different motor such as one from a washing machine but it would be more work getting that one on the bike and hooking it up. I feel like as I have already started with the alternator it would be best to stick with it. How much more efficient would a different motor be over an alternator?

Finally, buying a new controller is unfortunatly not possible at the moment, the one I have already cost 45€ and I don't the means to buy another one just yet...

It seems like you all have mixed feelings about using an alternator as a motor.
Would rewinding it not make a big improvement? Or would that just change at what RPM the ideal slip rate occures? Is there an ideal configuration? I suppose we would have to take into account the RPM, gear ratio and therefore the bikes speed, the design ideal slip rate and change the windings so that the ideal slip rate is close to the bikes max speed.
I don't have any idea how to do that though unfortunately...
 
You can determine the motor RPM by measuring the wheel speed, then working backwards via wheel size and your reduction ratios until you reach the motor and then you have it's RPM.
 
amberwolf said:
You can determine the motor RPM by measuring the wheel speed, then working backwards via wheel size and your reduction ratios until you reach the motor and then you have it's RPM.

Thanks, I hadn't thought of that! It will only give me the RPM while riding though and not without a load but I believe it's the former I need anyway.
 
If you put the speed sensor on the driven wheel then you can get the speed for that wheel unloaded as well--just setup the bike so it is off ground.
 
Thanks for those links, I had already seen two but the second one was very interesting althouth they seemed to have concluded that an alternator was a bad idea, shame.
I realized though that my motor is cutrently a synchronous motor.
It seems that an induction motor could be better. But that has 0 torque at 0 RPM, I could always switch between the two... I'll look into that and see if efficiency is any better. I think it will be but by how much? Rewinding still seems like it could help aswell as maybe changing my gearing although the induction motor should fix that?
Lot's to test.
 
A 3 phase induction motor with a proper drive circuit need not have zero stall torque. Essentially you want to have the slip frequency be the same at all speeds. A 50/60 Hz induction motor will typically be rated at around 5% slip at design speed, so the slip frequency is 2.5-3Hz. So at standstill you can drive it at 2.5-3Hz @ ~Vrated/20 and get full rated torque out of it, at rated current.

There are some issues with this: Motor cooling can be a problem at low motor speeds. If the motor uses any sort of cooling fan, it is designed to work at rated speed, so other cooling means will need to be provided if sustained low speed full torque operation is required.

Harmonic currents can be a gotcha, typical vfds will supply current in short pulses to produce the reduced voltage required for low speed operation. This produces extra heat and vibration that can damage the motor windings.
 
spinningmagnets said:
I could be wrong about using a converted alternator from a car for a motor. I just haven't seen a detailed instruction on how to modify them, and how the controller for it might work (or be made).

What model of car alternator is the best candidate?

What RPMs are produced at 48V?

There is an epub out there that uses a GM alternator as a motor.
DC, via a 3phase "brushless" controller, you eliminate the diodes and feed 0-6v into the slip rings. The 3 phase into the fields. Downside to this entire thing is cooling, consider that in auto use the alternator spins 3-6x the speed of the engine crank, so 10k rpm or more? They can be a brute in torque, 12v@60-150A is a lot of energy, 720-1800watts, I think you could run 24v into one with the right gear ratios, mid motor and have a neat bike. Just fan cool the shite out of it.
 
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