Re-winding coil in stepper motor.

[Harris Creek Central]

I have a "ASTROSYN" Stepper motor, it has 8 wires, of which I can find pairs for three sets. the forth set shows no continuity. Presuming this means I have one dead coil, has anyone in this group done any rewinding of a coil. Perhaps there is another group more into rewinding that you can direct me to. I'll see if I can attach a picture. (looks like it worked.) So this is the little motor I am talking about. I had an older brother that re-wound motors, but he is now gone on to his Heavenly mansion. I have had many different motors apart, and done cleaning etc, but I have never attempted any re-winding as yet. Thanks for your suggestions, and have a Very Merry CHRISTMAS.

Harvey
in Kelowna, BC.

 

[Tony Wells]

One of our members, Alan Douglas, knows a good deal about electric motors, having worked in a motor shop. I don't know the depth of his knowledge on steppers, but you might PM him, or he may see this thread and comment. I haven't wound any motors other than those experimentals as a kid, and those don't count except to learn theory.

 

[jgedde]

I'd venture to say one of the lead wires is broken just inside the case or where the lead wire is soldered to the magnet wire internally. It's very rare for there to be an open magnet wire in the stator core. You'll usually get a shorted turn or a short to case if the the motor has been overheated.

This former is likely fixable. When you take the motor apart, be careful not to chip any of the magnets. Some suitable shim stock inserted between the rotor and stator will help prevent this.

If the problem isn't a broken connection just get a new motor. Most stepper motors follow a standard NEMA or ISO form factor so a drop in replacement should be relatively easy to find.

Why throw it out? Here's why: Rewinding a stepper motor is not trivial and WAY more complicated than rewinding a typical induction, 3-phase or DC brush motor. PLus, you'll likely find a suitable replacement for not a lot of dough.

Basically it involves winding the correct number of turns on a specific number of coils (one coil for each slot in the motor's stator) of the right gauge wire on what's called a stack fixture. The stack is basically a bunch of discs and bushings. Each disc has a small groove in the OD to allow the wire to pass across the disc for the next coil. Between each disc, a coil for each slot is wound.

When you've wound all of the coils, the stack assembly is taken apart, leaving you with several individual coils connected by a wire. Picture a bunch of cursive 'o's strung together. Except, of course, each 'o' has many turns.

This coil set is then inserted into the stator's lamination stack one coil at a time. Since you have an 8 wire motor, you'd have to wind and insert 200 coils (if it's a 1.8 degree stepper) and install them into 50 slots. Then the finished stator needs to be impregnated with a resin to prevent the wires from rubbing against each other (because of the Lorentz force).

John

 

[Harris Creek Central]

Thanks for such an in depth run down. I will see if I can take this apart and look for a broken wire. The motor was purchased through our Princess Auto Store.. Yes, it was very expensive, I don't thiink I paid any more than $5.00 for it.. The re-winding idea came more as a challenge, than a neccesity. At 75 yrs. of age, I still have a lot to learn and I am still adding to my knowledge evry day. Today I just learned that maybe taking on rewinding a coil in a stepper motor was biting off more than I ever realized. Now a broken wire still presents a challenge I will be looking into..

I think I bought a couple of these about ten years ago. I'll have to check and see which of those secret places I put the second motor. You know the place you put something so you will never forget where it is.....Oh Yeh!. I can forget half an hour after I put it away..

Thanks again for your information. Wishing you and all the members of this forum a VERY MERRY CHRISTMAS, and a Happy New Year.

Harvey
in Kelowna, BC.

 

[jgedde]

Harvey,

You're very welcome. There is another option... Just don't use four of the 8 coils (including the bad one). You'll give up torque but she'll run...

MERRY CHRISTMAS to you too!

John

 

[Alan Douglas]

That pretty much covers the process. It isn't something you can normally do at home. Perhaps the hardest part is the dipping and baking of the finished stator; you can't get the varnish in small lots, it's very expensive, and the stator has to be immersed to fill all voids. Not to mention where you would bake it overnight.

If you don't dip and bake, the motor will last a day or so before the wires chafe through and short.

With the exception of replacing a start winding in a single-phase motor, you can't just replace one coil, because all of them overlap one another. You have to do the whole thing. As mentioned, though, your particular problem may be a break at an external connection, which you could find and fix.

 

[DMS]

Another thing to add. My understanding is that hybrid type stepper motors (the most common type) are magnetized in place (after assembly) because the rotor cannot sustain the magnetic forces outside the assembly. In other words, if you remove the rotor from the housing, it may work when re-assembled, but at a significantly lower level of performance.