I'm learning about PM alternators, waiting for an aircraft 3-phase 400 Hz unit, so I ordered a single-phase John Deere rider lawn mower replacement unit, $50 from Amazon, to play with.
There are 12 stator coils, all in series but opposite winding direction on alternate coils. I haven't figured out the rotor magnets, but they are probably simple N-S-N-S radial fields.
The detent is really hard.
Maybe the magnets are magnetized in their long directions, circumferential field, making a closed magnetic ring with small gaps. That would better correspond to the very strong detents.
A pair of coils bridges a magnetic gap. One detent later, they bridge the gap again, but the induced voltage direction is reversed.
It looks like there's three coils and pole pieces per magnet section. Is it 3-phase into the diodes? Tell us how to understand it.
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Thanks,
- Win
It's single phase, AC output, just a pair of wires connected to all 12 coils in series.
I assue that the action happens at the four gaps in the circular PM loop, so only four sites are active at a time.
Thanks, John for telling us about it. One more thing to understand.
--
Thanks,
- Win
These things usually drive a bridge rectifier and a cap, or a battery, and are regulated by shorting the alternator. It's mostly inductive so doesn't much mind being shorted. Open-circuit voltage could be very high, which is why a series regulator isn't a good idea.
So it's really more like a magneto, then. Interesting!
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Most magnetos had a set of points to keep the coil shorted as the mag field built up. At the critical time, about at peak current, a cam pushed the points open and there was a voltage kick. I think the points may have shorted only a part of the winding, to make more voltage into the spark plug than points could break. Or there was a separate step-up coil.
PM things like this are, when the speed over-runs the L/R time constant, approximately AC constant-current sources. Ideal for charging a bettery.
We're being asked to provide a 3-phase AC power source programmable for frequency, voltage, inductance, and resistance per phase, to simulate a PM alternator that runs an engine control computer. The customer is tied in the currently popular security knots as regards giving us any real numbers or waveforms to work with, so I'm reduced to buying lawn mower repair parts.
My Yamaha 250 had a magneto ignition. It was enormous fun. I must have crashed a few hundred times, dirt riding (illegally) in the Bonnet Carre' Spillway and some old gravel pits and such.
I was thinking of a bicycle magneto rather than an IC engine.
Cheers
Phil Hobbs
OK, the constant-current effect is good there too.
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I'm pretty sure Deere doesn't make engines. The small stuff is either Brigg s & Stratton or Kohler, custom manu'd to Deere specs. The alternators shoul d be the same. Looks like a bunch of stator coils in series and a PM armatu re. Deere and Kubota are the most expensive parts you can buy. You can get B&S and Kohler for significantly less, like maybe half price.
I got a Chinese copy replacement Deere unit, which looks very nice. But I really need a genuine jet engine FADEC power alternator, which I haven't been able to find for retail sale. There must be an old one out in the rain out back somewhere.
Something like this:
very different from the Deere.
I asked them what the coil inductance was, and they replied "that's proprietary."
Looks like redundant 3-phase windings. I get to sell more simulators!
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