# Car Alternator As E-Bike Motor



## Tesseract (Sep 27, 2008)

Sounds like a great idea, and yet hardly anyone repurposes alternators as motors, so the question to ask first is why? The main reason is because the typical alternator is a "claw pole, wound-field synchronous" design that is exceptionally cheap to manufacturer, but only 55-65% efficient.

Another reason is you will need to do major surgery on it to use it as a motor, removing the field regulator and rectifier assembly, then bringing the leads for the field and stator out to terminal studs somehow. If you want decent torque down to 0 rpm you will also need to install either Hall effect sensors at the right locations or an absolute encoder on the shaft (and since there is just one shaft that makes mating the alternator/motor to a drivetrain more difficult).

Finally, you will need to supply the field with either a constant current, which will make the alternator/motor act like a PM type BLDC motor, or, better, with a current that is proportional to desired torque, which will let you extend the so-called "constant power speed range" greatly (via "field weakening").

As for "regenerative braking", once the diode assembly is removed from the alternator it is up to the inverter to control that. And as for the alternator being rated for 117A, that is almost certainly for a specific duty cycle and time frame, and not a continuous rating. It will likely be able to take much more current than that for brief periods (ie - seconds) but the continuous rating is likely to be much lower, particularly because of the inefficient claw pole design.

In short, the typical automotive alternator just isn't worth the time/effort it would take to use as a motor, especially given the low power levels involved.


----------



## korneld (Sep 14, 2013)

Tesseract said:


> Another reason is you will need to do major surgery on it to use it as a motor, removing the field regulator and rectifier assembly, then bringing the leads for the field and stator out to terminal studs somehow.


Firstly, thanks for all the great info in your reply. 

As I said, I have great access to those terminals and I don't even have to remove regulator/rectifier assembly to get to them. All I had to do is to remove an aluminum shroud that, I guess, is supposed to protect it from rocks bouncing up, etc. 

I measured the voltage coming off them (this is the one in my car), while the engine was running. They gave me 7 volts each. The thing is, there is a 4th one and I don't know enough yet to figure out what this is. What is that? 

I could also use a link or two to resources where I could learn how to turn my DC current to AC in DIY fashion and run my 3-phase motor and what to hook where.

Thanks again.


----------



## PStechPaul (May 1, 2012)

The easiest way to get three phase power is a VFD motor controller, and you can get low power versions up to about 2 HP for less than $100. They are made much like a switching power supply so there is an internal DC bus which you can supply with DC from batteries, but it usually needs at least 200V.

You can also get (or build) a static phase converter using inductors, capacitors, and resistors, but you need an AC source for that. And if you are really adventurous, you can build a simple VFD using a PIC and three phase motor module.
http://www.mouser.com/ProductDetail...36-3023B/?qs=sGAEpiMZZMv/bGM7XKYHK9eAjLvP22F2
http://www.mouser.com/ProductDetail...V455A1/?qs=sGAEpiMZZMu%2b6ulL5WffptwDkBdtHKs0
http://www.mouser.com/ProductDetail...30CH60CT/?qs=sGAEpiMZZMsywdCd57IrWdEk7mqi0BMQ
http://www.mouser.com/ProductDetail...=sGAEpiMZZMuqBwn8WqcFUigEgEsKWM9qUWWD7GKXE2k=

But as Tesseract said, car alternators are built cheaply and are rather inefficient, so they are not the best choice for any serious power, as a generator or motor. You can get a 2 HP 3 phase AC motor for less than $100 and it will be much easier to use and more efficient. If the alternator used as a motor on YouTube draws 9 amps at 48 volts, that is about 450 watts just for no load, whereas a similar size AC motor would draw only 100-200 watts. 

Here are some links that might help:

http://auto.howstuffworks.com/alternator.htm
http://www.allaboutcircuits.com/vol_6/chpt_4/8.html
http://en.wikipedia.org/wiki/Alternator
http://in.answers.yahoo.com/question/index?qid=20100531050522AAIpNAi


----------

