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Car alternator, as a stepper motor?

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Hi :),

Well I think I'm about halfway through building my milling machine, and I can see that I'm going to run into problems obtaining stepper motor's for it, it's turned out to be rather large and all steel construction, and I think I'm going to  be needing some stepper motors for it "with some grunt ;D" there are plenty of stepper motors available to do the job, but they are rather expensive :o, and maybe this could be a cheaper alternative :-\.

Any information on converting a car alternator to be used as a stepper motor, would be greatly appreciated.


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Welcome back Dazza, you must've been bussy, this sounds very impresive, is it your design?

I don't know about using a car alternator as a stepper motor though, the problem is the phases and field coils are internally connected to a rectifier and regulator system. I don't don't whether they are sealed units or not, because you might be able to remove the electronics and then use it as a three phase stepper motor.

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Hi Dazza,

Still around, I’m glad! ;D

So it has become a big and heavy milling machine? It might work with an alternator if you rewind the stator and rip out the rectifiers, but I don’t thing it would be nearly as strong as an equally sized stepper motor though. The electro-magnetized rotor would probably be very hot since it must be fully “on” all the time to be strong enough. I doubt it would be worth the effort. The alternative is smaller but faster steppers with a gear for more torque if you can get your hands on some. This will of curse put higher demands on the driver circuit (high frequency) as well and furthermore you might want it to run faster still than possible with gears. A junkyard search might be a start for finding bigger steppers or geared steppers. Any good junkyards for this kind of stuff in the neighbourhood?  8)

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There is some discussion of alternator to stepper conversion here:
From this site:
A car stepper motor must be rewound to turn it into a stepper.
The new coils are wound around every third slot with the direction alternating on each wind.
The coils are rewound so that they form three coils which step consecutively to pull around the 7 pole stator.
Keep the centre coil the same and rewind the outer coils.
Remove the diodes and the old outer coils.
You can drive the new coils with three H bridges
The coils can be powered continuously for highest power with the direction progressing by changing the current sequence as the coils progress through the cycle.
The power is proportional to the current used.

Hope it is helpful. I have never tried this, so I cannot comment on how it works.


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Dazza, here is also an article from Don Lancaster's Hardware Hacker files:

The subject you are interested in is quite a ways down. Scroll to page 116 for the start of the alternator to stepper article.

Hope it is helpful.


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Hi, Alun, Ante and MP :)

Sorry for the late reply.

Thanks for the welcome back Alun. yes it's my own design ;), it's a make it up as you go along design ;D.

Yes I'm still around Ante 8), I just haven't had as much time to spend here lately as I would like, and it's going to be that way for awhile unfortunately :(.

Well I've had a win in obtaining some large stepper motors :D, I got a half-dozen large printers and copying machines from a junk yard, I salvaged all sorts of goodies from them and these things were dirt cheap 10$ for the lot :o, I got about 18 stepper motors large and small and some of the large ones seem to have their own drivers, I think these steppers might be strong enough to get me going if I gear them down enough, I don't mind if my mill moves slow, it's more important for now that it can cut the materials that I need and its accurate ;).

Apparently there was a guy in Sydney Australia, that was selling a video showing exactly how you convert an alternator to a stepper motor, many years ago.  I guess the reason why there isn't much information out there for converting an alternator to a stepper, is it probably doesn't work very well :-\, but still I would like to give it a go, I have some ideas on how to go about it, but I'll wait until I've got my mill up and running with the steppers I have now, at least I'll have a better idea on the steppers that would be required to drive this mill efficiently.

MP, that is very very helpful :D, thank you.

It will be awhile before I try converting an alternator to a stepper, but I definitely will give it a go after I get my mill up and running, and I'll let you know how it turns out.

Thanks for your replies, it's very much appreciated.


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Hi Dazza,

It’s necessary for a hobbyist to have a good junkyard near by! This is the only way to get a bunch of steppers for your projects as well as lots of other goodies for a reasonable amount of money. ;D  I have seen steppers hocked up in tandem to have twice the torque, maybe this is a way to go if you found pairs of equal motors. I suspect this would be a more compact construction and less power consuming than the alternator way. 8)


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Dazza why are you planning to use a stepper motor to do the cutting work with?

Steppers may be great when you want prosise movement but they're not the most efficient, normally an induction moter is used to do the real work and a stepper is used for moving things slowly at prosise speeds like the bed of the mill.

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Hi Alun, Ante :)

Yes, the steppers are to drive the X,Y and Z axis ;), I'll be using a router for milling.  One thing I have to keep in mind when trading speed for torque, is if the cutting tool is moving to slow, it can heat the material being milled, and cause damage to the cutting tool as well as what it is cutting.

Ante, among the variety of steppers I have, there are two identical steppers which I could parallel to drive the largest axis (12.0V  1.5A  1.8DEC) but then I also have one that has its own driver board attached to it(DC38V 1626.26rpm 3.0Kg-cm  3A) I think maybe once I get to the stage, when the mill is complete minus steppers, I can post some pic's of the mill and steppers, so use can maybe give me some advice on what steppers may work best for each axis, as well as the best gearing ratio to try.

Thanks again for your replies.

Dazza :)

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