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Old 11-28-2009, 11:18 AM   #1
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AC Electric lawnmower

I took the 1/3HP AC motor from a washing machine, bolted it into the shell of a walk behind pushmower, welded up a blade adapter, and made a blade. It works. Now I want to make a mower that works better, using 120VAC as the power source.

I made the blade because the motor shaft rotates in the opposite direction from the engine shaft. I had to make the blade adapter because the engine has a 7/8" or 1" shaft, while the motor has a 1/2" shaft.

This mower features direct drive, but I could reposition the motor and use pulleys or gears to change the shaft rotation, so that I could use a conventional blade and increase the blade rpm. This is appealing, since the motor is 1750rpm, while the engine rpm was in the 1800-3600 range.

This mower still features a "dead-man" switch to cut power to the motor, though ideally I'd have an electric clutch/brake to stop the blade faster. Any ideas?

The appeal of a plug-in lawnmower? Very quiet operation, very low maintenance. Working around the cord isn't the nuisance I expected it to be. I'm even exploring some type of cord retractor that would sit on the mower deck and dole out and retrieve cord as needed.

Should the next mower use a DC motor, so that I can install it as a direct drive? (Shaft rotation is user-controlled.) Can I buy an off-the-shelf converter to change 120VAC to, say, 90V DC? Speed control isn't vital, just safe on-off operation.

The 1/3HP AC motor is just adequate for a level, well tended lawn, but it does bog down in my more "wild" and lumpy back yard. Do I save myself time and trouble by sticking with AC motors, and just try using one with a higher horsepower rating? I'm even wondering about the motor size required for a self-propelled walk behind mower...


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Old 11-28-2009, 03:35 PM   #2
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The electric mowers that I have worked on had universal motors. There was a 25A bridge rectifier to convert AC to DC and thus run the motor on DC with increased torque. You may be able to take a DC motor from a Treadmil and use it in the same fashion. If you stick with an AC motor, you maybe should try and get a 3/4 hp or 1 but remember that induction motors do not like long thin extension cords! Voltage drop becomes an issue. You would need to use like a 12 awg which would get old fast carrying that around. That is why I think manufacturers use universal brush type motors. They are light, very cheap, powerful for their size, and voltage drop does not bother them as much.


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Old 11-29-2009, 11:06 AM   #3
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Motor Choices

I use a standard 100ft, outdoor-use extension cord, 14AWG. For grins I'm going to measure the voltage drop, though it doesn't seem to be great enough to seriously impair mowing.

I took some pains to make sure the blade had good static and dynamic balance, so the mower vibration is minimal. Anyone who does a lot of mowing with a walk-behind mower has probably experienced that tingling in your hands from a mower that vibrates too much.

I see that DC motors in the 3/4 to 1HP range are not cheap, and that I'd be changing out brushes as they wear down. I don't mind that, but the AC induction motors are less expensive, don't have brushes that need replacement, and some offer the choice of clockwise or counterclockwise shaft rotation.

Funny thing, the mower still sounds a bit like a washing machine...
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Old 11-29-2009, 11:11 AM   #4
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Treadmill Motors

Hmmm...just looked up treadmill motors. Promising. Need to see more specs on them. I wonder if they have the requisite durability...
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Old 11-29-2009, 11:34 AM   #5
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DC motors are indeed expensive.

I'd use an AC one. Generally speaking, the HP rating of a gas engine is about 3 times that of an electric motor. A 1 HP electric motor will produce about the same power as a 3 HP gas engine.

If you can get an electric motor with a frame size of 143TC or 145TC, it'll be 1 - 2 HP and have a 7/8" shaft. It'll also be face mount. Meaning that it has a machined surface on the shaft end that has tapped holes to bolt it to the lawnmower deck. You'll likely need to make some sort of adapter plate though.

1 HP, maybe 1-1/2 is about all you can get from a 20 amp 120 volt circuit. A 2 HP motor running on 120 will trip the breaker if it's loaded very much.

Electric motors come in a variety of speeds, the speed is not easily (or cheaply) varied. They are 3500 - 3600, 1700 - 1800, 1100 - 1200, and so on. The actual speed will vary a bit with load, but it'll be within these ranges.

The blade to shaft adapter of a typical lawnmower has set screws, and a hole in the middle to hold it on. A gas engine has a tapped hole in the shaft, an electric motor does not. I'd recommend drilling and tapping the shaft, so that the set screws are not the only thing holding the blade on.


P.S. All AC motors are reversible, some are an easy re-connection in the terminal box, others are a bit more difficult.

Last edited by micromind; 11-29-2009 at 11:36 AM. Reason: Added P. S.
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