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Sunday, August 23, 2009

PMDC to Wound Field Motor? Hmmm....

I asked this question over on the DIYElectricCar forums, where there are a LOT of very knowledgeable and helpful people, and I'll post the gist of the answers I get in a later post. I've already done some searching around the web before asking, but my google-fu is not working well these last days, and hasn't come up with very much relevant info, and really nothing about what I am considering trying to do. Many things about doing the opposite (which I've already done myself), just not this. That might be a hint that it's not worth doing. :-)

I'm sure this is possible to do, but I'm not sure how I'd go about calculating the windings' pattern, number of turns, or gauge of wire to be used.

Essentially, I have those smallish PMDC brushed motors in the form of the wheelchair/powerchair motors. The way I'm using them, they get pretty hot. Part of that is certainly because of the enclosed design of the motor, which I have some solutions for, as discussed in the last post.

Part of it might be, so I've read here and there, because of the PMs themselves, and possibly because of the flux ring (cylinder, actually) around them that constitutes the heavy thick main body of the motor. For a few reasons, including that, I'm considering an experiment in converting one of them to wound-field instead of PM. It would theoretically also give me finer control over the power used in the motor, and it's response to various situations.

The motors are radial-flux, and have PMs about 4" long with an outer radius of around 3", a bit more than 1/4" thick. They're not rare-earths. I don't have any real idea of the magnet strength, but it is fairly high, but not nearly what you'd see in a modern harddisk servo magnet.

What I'd like to do is replace the PMs with a set of field windings for the stator. I actually already have a number of motors around here that are of similar but not identical diameters/radii, which have such windings, but I suspect I could not simply move those windings over to this one and use as-is. (though I *have* taken the entire cylindrical PM assembly from other motors and tested them with some success on former wound-field AC motors to turn them into PMDC motors for experiments). Most of the ones I have were drills and the like, run from 115VAC at a few amps max.

One is from a 115VAC weedeater by Ryobi, and it's windings are the best size match for the powerchair motor armature radius, but still not an exact match. It might be possible for me to heat and bend the Ryobi field windings to more closely match the radius, but it may also damage the insulation on the wire, so I'm curious to know if it is worth trying that. The field windings appear ok, but the armature windings are heat-damaged from overlong use of it in our Phoenix summers. :( That makes it a good donor candidate because I don't think I'll ever rewind it's armature. :)

Now we find out: Is this yet another of my crazy and stupid ideas, or is it just crazy?


  1. If the bearings in the motors are good ball or roller type, it could help to do that to the fields. Then cogging would disappear when the windings are unpowered.

    That trouble indicates to me that I got a fortunate find with the Pittman motors that I have, since they do not seem to suffer from the maladies that your motors do. I wish that I could send you one.

  2. There is no noticeable cogging with these PM motors, because of the gearboxes on them. And they have just two poles, and two magnets. The only real resistance the system has is due to the gearbox design, as it is a worm gear built into the shaft that drives the other gears perpendicular to them, to give a right-angle gearbox output.

    There's a clutch pin on it that rotates to push the main output shaft gear out of engagement with the rest of the gearbox, but it can't be engaged or disengaged while spinning (it'd strip the gears). It's usually used on the wheelchair so the rider can push it manually if the batteries die, or so a helper can push to save battery power, etc.

    The problem I have isn't cogging, but heat buildup, and some sort of cooling is probably going to be necessary to continue abusing the motor like this without destroying it. :)

    I'm thinking of the field windings not only to perhaps help this but also to prevent loss of torque due to heat damaging the magnets (which would no longer be there) but also to give finer control of the motor's power output.

  3. My impression is that it won't help enough to be worthwhile to wind the fields with wire. It will lower efficiency, though aince those windings will dissipate power. The fineness of the control is easy to vary just through PWM on a PM motor. Mine has very fine control. It can vary continuously from zero torque to full torque.

  4. 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.

    Just using PWM can vary the control very finely even with PM motors, but there is a maximum torque that can be achieved at any particular voltage (or PWM duty cycle) based on the field magnet strength.

    If the field can be made stronger by a higher current whenever it is necessary (such as at startup), more torque can be generated.

    That's really what I meant; I should have written it out to start with. :)

  5. You've given a great clarification. Thanks! :)

  6. Now if I could come up with a great and simple way to *do* it. ;)

  7. I forgot to update this with the results of the question I asked over at DIY Electric Car forums.

    A knowledgeable expert there named Major gave a good concise reply that basically tells me if I'm willing to deal with a fairly complex experimental approach to figuring out what the field windings will have to be, then yes, it is possible to do. It might not end up being worth having done, however.

    I'll just link here to the thread I asked the question in and his answer, and a thread he refers to there that has a method by which to find some of the info I'd need.


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