universal motors

[darryl]

After all the talk about electric motors today, I went and found a couple universal motors I have kicking around. One was marked 'seized', so I took it apart, forced the armature out, then did a belt sand around the armature to remove the melted plastic. The windings didn't look burnt in any way, so I put it back together and it runs perfectly.

So there it is running from my dc power supply at about 20 volts. It's probably turning at a good 2000 rpm or so, not that it matters. Then I'm wondering how much of the voltage is used by the field and how much remains for the armature- turns out there's about 1 volt across the field and the rest across the armature ( these are in series as in every universal motor I've had apart).

I wonder if the same will be true running it on ac? Probably not, but my main thought here is if I turned it into a permanent magnet field, what voltage would the armature then need in order to turn at some appropriate rpm? After I get back from dinner I think I'll hook it up to ac and see if I can get some ideas. The other option is to separate the wiring and power the field and armature separately. I would first try to power the field to about the level that permanent magnets would give, then see what armature voltage and rpm figures would be like. I'd like to try to use these armatures as is, since they are well balanced already and have a large number of comm segments, making them suitable for 'high' voltage operation. With luck, maybe I can get a motor that will run well from rectified 110 vac, and with rpms down in the low k range.

I know that neodymium magnets aren't cheap, but there would not be many needed, only about $30 worth I'm estimating, and the result could be a motor that's nearly ideal for things like Lane is working on, where you want small size, good torque, and induction motor-like rpms.

All comments welcome.

[darryl]

Well, I separated the wiring and started by powering just the armature. The motor took off, but without a lot of torque. As it warmed up, the rpm increased. Then I applied some field current. At first it slowed the motor a bit and increased the armature current draw. Then I reversed the connections and got the same slowing, but much reduced armature current draw. All as expected so far, and a fairly dramatic way to show how the strength of the field affects things. If I had been running it on a higher voltage, the armature would have tried to overspeed and also get hot because there would be no back emf to control it.

Now I'm going to try applying the largest field current I can without smoking things. I expect the armature current to drop to what might be considered a no-load value, and the rpms to drop also. I should have pretty good torque without the rpms dropping too much, like a pm motor.

And that's what happened. Now I need a second power supply so I can ramp up the field current and look for a point where the armature current begins to rise again.

[darryl]

And now I find that I can't apply enough field current to cause the armature idle current to rise. The best I can give it is 6.5 amps and 14 volts. Doesn't take long to heat up the windings at that level.

Now I need to devise a way to measure the strength of the field, or at least compare it to a magnet. I should be able to get a meaningful result by sticking a neo magnet to the inside of the field and apply the current. If the generated field is the same polarity as the face of the magnet stuck to the metal, then when the field strength rises to the same level as the magnet provides, the magnet should suddenly jump off, twist around and re-stick.

I'll also do a comparison test with a screwdriver blade. If I don't get as much attraction as I can with a ferrite motor magnet, then I'll know that I'm just not getting enough current through the field winding to reach that level of magnetization.

I do know that with 12v applied to the armature, and the maximum field current that I can supply, I'm getting about 300 rpm. That means that with 120 volts to the armature I'll be getting about 3000 rpm, so at least I know I'll be in the range I'm hoping for without rewinding the armature.

[Deja Vu]

A few pictures of your experiments would be great here. ...just trying to imagine.

[MaxHeadRoom]

To avoid re-inventing the wheel some, this link may help?
http://www.cnczone.com/forums/genera...nt_torque.html
Max.

[darryl]

Thanks for the link, it was interesting to read through it. I think the difference in what I'm doing is that I plan to replace the field entirely with a permanent magnet structure. Then I don't have all that heating and power loss to energize the field. It obviously makes it a dc motor, so armature current has to be dc.

My initial question was whether the armature could be used as is, or if the number of turns on it would be found to be unsuitable to get the rpms I want with the voltage that would be easily available, which is bridge rectified 110vac. What I have found is that I can be right in the ballpark.

Some of the motor heating is eliminated by using permanent magnets instead of field current, but armature heat can't be dissipated as quickly at the slower rpm. It will be another experiment to see if there is extra heating at unloaded condition because of the field magnetism always being strong- but this would now be your basic PM dc motor, and they are well documented. Some of them could run all day and not get more than warm.

At any rate, not having to rewind the armature will be a big bonus, and is the turning point in whether the conversion is practical. If I decide to build one of these, it will simply be a mechanical project and will have predictable results.

[MaxHeadRoom]

Unless it is for the academic experience, I would think you would be better off picking up a Ex-Treadmill DC motor?
You are not going to get the rpm probably of the Universal motor because of the fixed strength field?
Max.

[darryl]

Almost every treadmill motor is larger than what this would be. Otherwise I'd use one- I actually do use a few treadmill motors on my machinery.

I'm looking at getting a decent amount of power in a smaller package here- and something that will run from 110vac with little else required.

You are right, the rpms are less with this 'mod'- precisely what I wanted as well.

If I go ahead with this project, my only real problem is going to be getting some 2 inch long, 1/8 thick, by 1/4 inch wide neodymium magnets delivered to my home without paying exhorbitant delivery charges. In Canada, all I've been able to find is disc magnets, a few common sizes of cubes, etc, and jewellery and craft magnets. I would prefer to shop locally, but maybe I'll have to order direct from Chnia. I'm NOT going to pay UPS charges-

Ok, there will be one other problem- putting several strip magnets together side by side inside a steel ring with the polarity the same. This could be a mess- magnets jumping around, epoxy all over the place-

[flylo]

I buy things on ebay & shipping from china is cheaper than I can ship across the street. It's amazing.

[customcutter]

I'm with Deja Vu, I'd like to see video's on your experiments.

CC