About a week ago, whilst on IRC, I thought to myself "hey, instead of chatting, I'll quickly convert a small motor I have". Expected it would take me 1.5 hours at most. Today, 8 days later, it got finished.
The main purpose of this conversion was proof of concept of the JacquesM-method of offsetting magnets. See these links for more information:
http://www.fieldlines.com/story/2005/1/8/2291/67790
http://www.greenbits.com/images/windmill2/rotor2.png
http://www.fieldlines.com/story/2004/12/25/34425/916
http://www.fieldlines.com/story/2004/12/24/6105/5834
specs of the motor:
1 phase 230VAC
(2nd starting phase, with 2.5uF cap)
- 23 A
- RPM (4 pole)
Shimano-Tokki corp., made in Japan,
X7807-201V
The principle of the Jacques' method of offsetting (skewing would be the wrong word for it) is simple. If you have 16 slots (as in this case), each pole is offset by a fraction of the slot-width (in degrees). I.e., in a 16 slot motor, each slot is 360/16 = 22.5 degrees. The first pole is at 0 deg (i.e, 12 o'clock). The 2nd pole is at 3 o'clock, but a little less: 1/4 * 22.5 = 5.6 deg; i.e. at 84.4 deg. The 3rd pole is at 6 o'clock, but minus 2/4 *22.5 = 11.25 deg; i.e. 168.75 deg. The 4the pole is at 9 o'clok, minus 3/4 * 22.5 = 16.85 deg; i.e. 253.15 deg.
See the drawing below
The downside of this conversion method is that it introduces an imbalance in the rotor, which must be compensated for. I haven't done this yet, but intend to do it by drilling holes in the steel rotor until the assembly balances. BTW, the more stator slots there are, the less the imbalance becomes; in my case of 16 slots, the a-symmetry of the rotor is quite high, thus is imbalance.
The conversion itself consists of 4 round magnets (one per pole), of N42 15x8mm. This gives a total magnetic volume of .345 cubic inch.
A new steel rotor was turned, an aluminium ring crimped over it. The assembly was Loctited (Loctite 638) on the old axle. The bearings were 608-2Z, they were replaced by 608-2RS for better weatherproofness. The pockets for the magnets were milled at 15.2 mm, 10 mm deep. The .2mm play is needed for the epoxy. I used epoxy to glue the magnets, not Loctite 638, because the datasheet is unclear whether Loctite holds to 'inactive' metals. I'm not sure whether nickel is (in)active. Henkel (mfg. of Loctite) didn't bother to reply to the question).
Some more pictures:
The goal was to provide proof of concept of the Jacques M-method of offsetting. It's an utter succes in that respect:
Not even the slightest hint of cogging is present.
As I write this, I'm trying again to turn the axle, but nope: not even the faintest trace of it can be felt. So the offset method works as it should, just as I expected from theory. The advantage is that it's much easier to do on a 3-axial CNC mill, as opposed to Zubbly's 'helix' method. The downside is the imbalance it introduces.
With magnets, it doesn't turn as freely as it did as a motor; there's quite a bit of 'sogginess' or 'stickiness', I suspect this to be the stator losses. When I give it a good spin, it turns freely only for about a second. I will be making torque measurements to get more accurate data of this.
The results, powered by a 300/600 RPM cordless drill (calibrated with a RPM counter before measurement):
- RPM -- 20VAC; 70mA
- RPM -- 45VAC; 140mA
As you can see, not exactly a powerhouse.
The stator resistance of the original winding is 300 ohm. I don't plan on doing a rewind, but who knows, maybe I will one day.
At 1500 RPM, it should generate 112VAC; whilst it took 230VAC to drive as a motor. I suspect I could have used more magnetic volume to get more V/RPM. However, rotor size prevented me from adding another row of magnets.
Power output is about 26W/cubic inch. Zubbly's rule states 150W/cubic inch. Mine is much less, then again, Zubbly's values were gathered from 'real' motorconversions (several hp's), whilst my mini-motor is just a plaything in comparison.
The output will be at about max. 90VAC. I have several DC-DC converters that accept 38...90VDC in, and give 12V at a few amp output. Maybe I'll let this motor drive such a converter.
Ok, that's it. The motor was never intended to be married to a prop and put into service, but maybe I will. The most important conclusion being, 1) JacquesM's method of offsetting works (just as others have stated it should) and 2) completely coggless motor conversions are not an illusion.
Thanks to Zubbly, RonB, JacquesM and everyone else on this board. Without Fieldlines, I'd probably still be messing with car alternators and DC motors.
Any questions, don't hesitate to shoot.