Author Topic: Electric Lawnmower Motors: Great Generators?  (Read 13881 times)

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CraigCarmichael

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Electric Lawnmower Motors: Great Generators?
« on: May 06, 2009, 08:10:56 PM »
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.)


-----


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

« Last Edit: May 06, 2009, 08:10:56 PM by (unknown) »

RandomJoe

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Re: Electric Lawnmower Motors: Great Generators?
« Reply #1 on: May 06, 2009, 05:26:26 PM »
If I remember correctly, my Black & Decker "Lawn Hog" runs at 1800 RPM.


My dad had one too for a while, then it "burned up".  My experience with this one leads me to believe his just had a bad switch.  Mine went bad too, they are very poorly made.  Mine went open, but I could easily see how it might fail to a short of the line in!  (Which is what my dad's did - just popped the breaker when he pulled the switch handle.)  Now I wish I had gotten his "dead" one when he offered it...

« Last Edit: May 06, 2009, 05:26:26 PM by RandomJoe »

DanB

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Re: Electric Lawnmower Motors: Great Generators?
« Reply #2 on: May 06, 2009, 06:22:26 PM »
yes I think you're probably not far off thinking it might make an OK little wind turbine for 12V with a 4' or so blade on it.  I'm not sure how they'd hold up to the abuse... a lawn mower gets used a few hours/week, a wind turbine rarely gets a break.


So rated at 10 amps, they might work for 100 or 200 Watt wind turbines @ 12 Volts, with direct drive.  The thought has crossed my mind before but I've never messed with one.


I wonder if it cogs much...

« Last Edit: May 06, 2009, 06:22:26 PM by DanB »
If I ever figure out what's in the box then maybe I can think outside of it.

CraigCarmichael

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Re: Electric Lawnmower Motors: Great Generators?
« Reply #3 on: May 06, 2009, 08:19:01 PM »
I couldn't feel any cogging and I was checking for it. There's just a bit of even drag from the brushes and bearings.


I agree the quality isn't the best. There's the switches, and more than one other person besides me has reported the magnets coming unglued and breaking up. My repairs were that I glued most of the magnet pieces back in, twice - once for each magnet in two successive years. (This time, a couple of magnet bits had come off and were grinding in there.) BTW it ran fine with one magnet, but presumably at half power.


(I wonder what glue the manufacturer used? I used "epoxy steel".)


On the one hand, the windplant as noted runs continuously, but on the other hand, it doesn't get dragged over rough ground and hit rocks and branches, and on a small direct drive windplant it will be running way under its power capacity.


And you don't need the switch. or the diode bridge.


--Craig

« Last Edit: May 06, 2009, 08:19:01 PM by CraigCarmichael »

Bruce S

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Re: Electric Lawnmower Motors: Great Generators?
« Reply #4 on: May 07, 2009, 07:06:03 AM »
Craig;

I've been working around with an older one as well. The only problem I have, when I get them home and replace bridge rectifier and the momentary switch ( which together might cost $10.00) someone wants to buy it from me, due to the quietness of them.

The bridge is usually a 10Amp one and easy enough to find even at RShack.

The brushes seem to also be easy enough to find, but haven't had a need for them yet either.


The newer 24V battery powered ones don't short the unit like the AC powered one does, just a disconnect from the batteries.

I'm impressed, I have a small 100'x50' backyard and it'll cut the entire area even with the grass and weeds up beyond the 6" mark and still have enough battery left to cut the pretty front yard.

I tested the charger and merely built, (clugged together) a small cable to charge it dirrectly from my little solar bank.


O bother, another field "test" :-))


Keep Smiling;

Bruce S


 

« Last Edit: May 07, 2009, 07:06:03 AM by Bruce S »
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wooferhound

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Re: Electric Lawnmower Motors: Great Generators?
« Reply #5 on: May 07, 2009, 05:57:34 PM »
I asked the same question about 3 years ago

http://www.fieldlines.com/story/2004/10/25/93721/579

« Last Edit: May 07, 2009, 05:57:34 PM by wooferhound »

CraigCarmichael

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Re: Electric Lawnmower Motors: Great Generators?
« Reply #6 on: May 10, 2009, 04:55:52 PM »
Woof, It's too bad you asked instead of trying it out for yourself, since they look very good.


("Field testing" a lawnmower?, what can I say but AWRG!)


Craig

« Last Edit: May 10, 2009, 04:55:52 PM by CraigCarmichael »

cedarworkshop

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Re: Electric Lawnmower Motors: Great Generators?
« Reply #7 on: August 28, 2014, 02:25:41 PM »
Thought about these models for the next VAWT project.
Then a few months later, I found a 24 volt Yardworks cordless lawn mower at the dump. Brought it home, removed the blade and hooked it up to a cord drill, ran at 1150 rpm and got 7.45 volts. So, it wouldn't be idea for 12 volt charging. Took it apart to check out the motor. The stator was inside a housing with two large black magnets on opposing sides. It had massive brushes too! I'll keep it on the back burner for a future project.
I'll try to post a video soon.

electrondady1

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Re: Electric Lawnmower Motors: Great Generators?
« Reply #8 on: August 29, 2014, 07:52:16 AM »
to make use of those mags you would need to rewire the armature with more turns  and finer wire.