I've never seen this idea presented anywhere before and it seems so promising that I think it would be great to spread it around:
Electric lawnmower motors look like great DC generators!
I speak of the common 120 V / 12 A brush type motors with big permanent magnets in the case, about 4.5" diameter and 6" long or so, with a four terminal plastic diode bridge mounted on top, that drive the blade directly.
Everybody thinks first of using used car alternators for cheap generators, eg for windplants, but they're crappy because the field coils use considerable power continuously (eg 36 Watts) and because they're quite high RPM (eg 3000).
If you can find a used lawnmower, they're usually cheap. I don't know what the RPM is, but spinning that big blade it must be much lower.
I have a couple here that are 120V / 12 amps (1.44 KW!) rated. It's a permanent magnet stator (so no wasteful field coils like a car alternator has), and there's a diode bridge on the AC line to convert 120 VAC to DC, meaning it's actually a DC motor. They're "instant stop", which is done by the switch shorting the motor leads together so the motor becomes a generator and generates into a short circuit. (If you hold the switch half way, they wind down slowly.)
It might make quite a workable PM, DC windplant (or micro-hydro, or ?) generator. At relatively low RPMs it would put out some lower voltage and current. My completely uneducated guess for direct drive: charge 12V batteries at 5+ amps with maybe a 3-4 foot prop. For higher volts and more amps, I expect it would need a bigger prop and gearing the speed up, eg with pulleys and V-belt. (Belts... and of course it has brushes that would eventually wear out - not so nice to replace if it's fixed in place on the top of a tall tower.)
Typical model numbers (same 120 V, 12 Amps mower, same motor):
Sears model #: C935-35512-3
Black & Decker model #: MM500
(If you can find a source for the motors alone, new ones might not cost a lot without the lawn mowing accessory attached.)
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Quick Tests:
I UNPLUGGED it, opened the cover, tied the power switch ON (as it shorts the motor leads when OFF) and disconnected the leads from the diode bridge:
- Giving the blade a spin by hand, with a voltmeter connected, I read up to almost 9 volts DC open circuit, + or - depending direction. (It spun quite freely; no cogging that I could feel. I trust a prop would spin it faster than I can and so provide over 12-14 volts. I was trying withall not to cut my hand on the blade.)
- Cranking the blade around by hand, with an ammeter across the motor leads, I read up to almost 3 amps. (?2.85A? Short circuit - Much harder to turn of course!)
Then I put it all back together and TOOK OFF the thing I used to hold the power switch on!
That's as close as I plan to come to determining performance specs for this as I have no sure plans for using it myself - it was just an idea that came to me while I was fixing my mower. (Unless I put a crank handle on one and connect a car headlight or two by switches just to educate all those "perpetual motion" bufs how much work it is to make 50 or 100 watts!) All readings were varying rapidly and depended of course on the vigour of my turning. Almost 9 V O/C and almost 3 A S/C were my MAX readings, but it looks like it should work great! WOW! Why did I never think of lawnmower motors as generators before?!?
Cheers,
Craig