# Converting a PMDC brushed motor to wound-field



## Amberwolf (May 29, 2009)

I guess I didn't explicity state the reasoning behind wanting to do this, other than the heat problem, but here's some further thoughts generated by a conversation on my blog post regarding this:

http://electricle.blogspot.com/2009/08/pmdc-to-wound-field-motor-hmmm.html



> I know it will take more power to run the windings, but I also know that with wound-field motors in larger EVs, it's possible to do more control of the motor this way. (and that the heat buildup in motors that large tends to destroy any magnets they would have had, sort of forcing their hand in generally using non-PM motors to begin with).
> 
> Perhaps "finer" control was not the right word.
> 
> ...


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## Amberwolf (May 29, 2009)

Nobody at all? Guess it was dumber idea than I thought.


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## major (Apr 4, 2008)

Amberwolf said:


> Nobody at all? Guess it was dumber idea than I thought.


Hi Amber,

Dumb idea? Probably 

The right way is to model the magnetic circuit and calculate the magnetization characteristic starting with the B-H curve for the steel. Or you could do something similar to what I instructed todd to do with his SepEx motor. See http://www.diyelectriccar.com/forums/showpost.php?p=130543&postcount=11 

A guy named steiner took that procedure and posted up his result on post #29 of that thread. He did a pretty nice job of it.

The permanent magnets give you a constant flux. This flux times the speed generates the back EMF (Eg). This same flux times the current produces the torque. So to get a wound field to perform like the PM, you need to get the same flux. In the wound field, the flux is a function of the mmf, or ampere-turns (AT) on the field coil.

Flux is a pretty difficult concept and hard to measure. So what is done is to use the fact that it is proportional to voltage for a given armature at a fixed RPM. That is why I had these guys measure the armature voltage as a function of field amps, which is a magnetization curve.

So, for you, let's pick a RPM to use. Keep it the same for all tests. Let's choose 1000 RPM. Drive the PM motor at 1000 RPM. Measure the no-load armature voltage. Let's pretend it is 15 volts.

Now put the wound field on the armature. Drive it at 1000 RPM. Excite the field with a variable supply and adjust it so the armature is 15 volts. That would be the field current required to match the PM motor performance. Let's say that is 2 amps for that particular field. While you're at it, vary the field current and take the curve for future reference.

So now you know you need 2 amps in that field. Maybe that took 40 volts. You might want to run it at 24 volts. So you could rewind the coils to get the same AT at 24 V as you did at 40 V. You need to adjust the turns per coil (T/C) and the wire size. You basically want to keep the same total amount of copper in the coil, so if you increase the wire size by 40/24, reduce the turns by 24/40. Simple, right?

That was for a shunt or SepEx type field. If you want a series field, different procedure, so let's not go there.

After you get the coil turns figured out to give you the correct excitation (AT), you'll need to look at the thermal suitability. The new application of that field set might be unsuitable. But you have limited space. So, I'd suggest, once you have the wire size and T/C figured out, just add extra turns of that wire until you run out of space. This is how to reduce coil watts with shunt coils.

Oh yeah, make sure your brushes are on neutral for all these tests.

If that doesn't confuse the hell out of you, you might have a chance of getting it to work. 

Regards,

major


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## Amberwolf (May 29, 2009)

major said:


> Dumb idea? Probably


I didn't really think it would be "easy".  Just hoping someone had done it before, and already discovered if it was a really bad idea or not.  So far, every time I think I am pioneering something on my Electricle projects, then post about it on the blog, then later on someone replies to the post saying that they could never get it to work, or it broke in a few miles, or was really inefficient, or something. Of course, that's *after* I have already started or even completed something. :roll:

(even when I do searches for things, spending hours or even days, I often dont' yet know the right word to look for, and so I don't end up finding the gazillion projects already around for it!)



> <snip>A guy named steiner took that procedure and posted up his result on post #29 of that thread. He did a pretty nice job of it.<snip>


Thanks! I'll be reading that thread next. 

I really appreciate your help with this, and when eventually I get enough data and time to try this, I'll post my results up here. 



> <snip>If that doesn't confuse the hell out of you, you might have a chance of getting it to work.


Actually, it makes good sense to me, although I am sure there are details I will need to learn about the physical winding processes that I haven't yet learned from experiments in rewinding burned motors (most of which were failures).  

BTW, you're probably the one person I would have really wanted to answer this thread, if I could only get one reply, but also couldn't have expected to do so, for such an ambiguous and questionable idea.


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## coulombKid (Jan 10, 2009)

Amberwolf said:


> I didn't really think it would be "easy".  Just hoping someone had done it before, and already discovered if it was a really bad idea or not.  So far, every time I think I am pioneering something on my Electricle projects, then post about it on the blog, then later on someone replies to the post saying that they could never get it to work, or it broke in a few miles, or was really inefficient, or something. Of course, that's *after* I have already started or even completed something. :roll:
> 
> (even when I do searches for things, spending hours or even days, I often dont' yet know the right word to look for, and so I don't end up finding the gazillion projects already around for it!)
> 
> ...


I have an excellent book on windings. It was purchased as a reprint from Lindsay books. The original was published in 1917.


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## Amberwolf (May 29, 2009)

coulombKid said:


> I have an excellent book on windings. It was purchased as a reprint from Lindsay books. The original was published in 1917.


What's the title? If I'm lucky, one of the libraries here might have access to it.
________
Washington dispensaries


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