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Old 06-03-11, 07:15 PM   #1
Weed Dog
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Default The AC Mower Essay

I replaced the cranky, leaky, noisy ICE on this walk-behind mower with a motor from a washing machine. Now I can mow without earplugs, hear the birds chirping and the cicadas droning, and even listen to an on-board radio. I'm now in the safety-check, longevity-testing and quality-of-cut phase of the experiment.

The motor is a ½hp, 1750rpm, 120VAC open-frame design that draws 9.8A. Mounting studs on the face of the frame made it easy to bolt onto a steel plate. That plate was in turn bolted over the opening in the lawnmower shell where the ICE sat. The on-off switch is a 20A DPST toggle, mounted in angle stock and actuated by a kill switch/dead man switch. This switch is very recognizable to walk-behind mower users as the skinny, spring-loaded bar that the operator has to hold against the main handle in order to keep the ICE running. Adapting it to this mower (the bar and cable came from another scrapped mower) makes it operate in a very standard and familiar manner.

The two biggest challenges were finding a wiring diagram for the motor and making a blade adapter. I bought the motor brand new for $5.00 by placing a “wanted” ad in a local online resource, so there was no wiring diagram or manual to be found taped inside a washing machine shell. Several “sensei” at the Samurai School of Appliantology generously provided links to wiring data. Grasshopper is awed by their wisdom...

Several respondents to the want-ad offered me a free but non-operating washing machine if I would just haul it away. Not a bad way to get a motor and some wiring, and the motor is rarely the source of the problem when a washing machine breaks down.

The wiring diagram was vital because the motor has wires for two speeds, 1140/1725rpm. Lawn mower blades often spin between 1800-3600rpm, so I wanted to use the high speed wiring to get as close to the normal rpm range as I could for this direct-drive setup. Motors of this type also have a set of starter windings, to initiate shaft rotation from dead stop, and those windings have a set of wires also. If you merely connect the wiring for the main windings and apply house current, the motor will simply buzz and remain stalled unless you turn the shaft by hand, somewhat the way you might crank-start a Ford Model T. I suspect that a relay or centrifugal switch in the motor frame cuts out the starter winding circuit once the shaft rotation reaches some predetermined rpm, because I hear a familiar “ka-chunk” sound.

Direction of shaft rotation is important: on this motor it's a function of how you connect the wires for the starter windings. Viewed with the shaft facing you, it should rotate counterclockwise, so that you can mount a conventional blade to the blade adapter for the right-side discharge, the predominant arrangement. Had I discovered the shaft rotating clockwise, I could simply reverse the wires for the starter winding circuit.

In the Pantheon of AC motors I believe this one is classified as a split-phase. One question more knowledgeable readers may help me answer is whether the 20A toggle can handle the surge current. From my limited reading on the topic, I understand that starting currents could, for some fraction of a second or more, reach 700 to 1000% of the motor's rated amperage. This switch has been operating since last year in this setup without blatantly obvious signs of failure. On a related note, and I don't have the knowledge base to begin to quantify this, I suspect that the blade and adapter, and the grass they cut, represent a lesser load than a pulley-and-belt driven tub of water and clothes. I am only too-willing to take guidance on the wiring, and on methods I could use to measure the surge current. The criteria I used to select this type of motor began with the observation that I could often find them for free or for a song, and I had a mere intuitive sense that it might have the power/torque required. I'm aware that both capacitor start/induction run AC motors and permanent split capacitor motors have lower starting current, but are not as prevalent in my price point (!)

My neighbors comment that I may be cutting grass, but my mower still sounds like a washing machine.

Because the shaft on a small ICE is often 7/8” diameter, and the shaft on AC washing machine motors is often only 1/2” or so, I made a blade adapter from a scrap piece of flat stock steel that is welded to a steel collar I found at F&F. The collar fit snugly on the motor shaft, already had a key-slot and a tapped set-screw hole on the side, since the collar was designed for a purpose similar to what I was using it for, and saved me the challenge of locating a small machine shop that might have to create the collar by turning some bar stock on a lathe.

I did make some rudimentary efforts to balance the blade adapter/blade combo by clamping a 7/16” diameter bar horizontally in a vice, making sure it was level, then spinning the blade/blade adapter combo on it, minus the set screw, while watching it for any tendency to settle with one blade lower. A magic marker line on one blade helped me tell them apart. With RPMs at what seem to be minimal for grass cutting, I regularly check the blade for sharpness.

The biggest expense in this project was the extension cord (!) I got the mower shell free from a neighbor (he'd already recycled the ICE), the AC motor for $5.00, added a few nuts and bolts and the 20A toggle switch. The extension cord is 12 gauge, and the size of the conductors makes these cords pricey. Smaller gauge cords would overheat, so I read, which helps me understand why most plug-in electric mowers actually use a DC motor and rectify the AC power, and require only 14 or 16 gauge extension cords. I do like the fact though that, unlike a DC motor, the AC motor does not have brushes that require eventual replacement.

The photos do not show the plastic box that clamps in place over the toggle switch to cover the contacts. I'm also looking to dress the wiring, perhaps in a loom or harness, and to cover the very- exposed motor. The latter solution may be as crude as an inverted plastic bucket with ventilation holes drilled in it. I recognize the need to child-proof the mower, and curious dogs invariably have wet noses...

The AC-motored mower works well in my small front and side yards. From the start I accepted that I would need to learn some strategies to keep the cord away from the blade and my feet, and to work around trees. Now it's as automatic and unconscious as clutching and shifting in a manual transmission car. And of course, there is no gas or oil to buy, no plug or filters to clean or replace, no decaying fuel lines to clog the carb jets, no gum or varnish clinging to the carb's interior surfaces, no exhaust stink, no oil-leak grime. When I finish mowing I hang the cord on the mower and roll it into the garage.

In the off-season I may convert the mower to pulley-and-belt drive with an idler wheel, the pulleys sized to increase RPMs on the separate blade shaft. I'm toying with the idea of having a shop make that blade shaft, or seeing if I could rent time on a metal lathe. I see a shaft with stops on it an inch or so in from each end to facilitate press-fit bearings, and perhaps even turning a brake disc surface on the shaft to allow the installation of a simple shaft brake.

Next project: cobble together a DC-motored walk-behind mower, maybe AC power-sourced, maybe even a battery-powered unit. Then put solar panels on the garage roof to charge those batteries...

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Old 06-04-11, 09:17 AM   #2
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Quote:
Originally Posted by Weed Dog View Post
I replaced the cranky, leaky, noisy ICE on this walk-behind mower with a motor from a washing machine.
Great write-up.

From my experience:
The 1/2" shaft on your mower is pretty frail and if you hit a big object (rock, brick, etc) you'll bend the shaft, so take care.

If you can rig up a GFI to your mower, it will provide a much needed margin of safety. I made my entire exterior plug circuit GFI, so it was already done.

Keep your eye open for a 3600 rpm elec motor, it really makes a difference.
Best Regards,

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Old 06-04-11, 06:19 PM   #3
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I find the biggest challenge to be finding a pattern to mow that didn't tangle the cord. I have a lot of trees and bushes in my yard which makes it a puzzle as to how to cut and not have to untangle cords.

My mower was bought at the Mart of Wal in November for $100, priced down from $180.
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Old 06-05-11, 03:04 PM   #4
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Default Gfci

AC is right: a GFCI circuit adds greatly to user safety.

The steel of the motor shaft is notably soft: I nicked it while removing a pulley.

I'm slowly building a network of sources for the odd components my tinkering requires. A 3600rpm motor, reasonably priced, would be worth its weight in gold. It would probably have heftier wiring in the starter windings, too, making it more durable and having a lower surge current.

Skyl4rk's yard is a de facto mindgame, a chessboard of moves and resulting consequences. Keeps him sharp...
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Old 06-06-11, 01:59 PM   #5
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Weed Dog,

Sounds like you have a good project there. Two thoughts: If you are willing to go to dc, you might look at Surplus Center Surplus Center - Hydraulics, Engines, Electrical and More . They have a lot of dc motors, including a 1 horse 3400 rpm 120v dc motor for $40. You might could use a 120v dc motor and a big full wave bridge to make a corded 3400 rpm mower. I'm no electrical guy, so there might be other parts needed, but I have used such a simple circuit at lower voltages. I saw bridges rated for 35 amps at 600 volts selling over at American Science and Surplus in Milwaukee for $3.95 each (not a typo) last week.
One last word. I am about fifteen minutes south of daox in Menomonee Falls, with a basement full of hobby machine tools. If you're in need of a jackshaft for your current setup and can get over here, I can probably turn up something for you. I've got stock in my shop to about 2" diameter. I am currently among the economically independent (translation: got laid off Friday) so I have time to do some tinkering. My electric rider project blew up last year along with the gearbox on my old Toro, so maybe I can pick up some ideas by helping another tinkerer.
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Old 06-07-11, 09:01 AM   #6
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Default That 120V DC Motor

toolingjim:

I found the motor you mentioned at the Surplus Center website. It even includes the rectifier and has an 8' power cord(!) The motor draws 8.15A at speed and has a 5/16" diameter shaft, though the latter only protrudes 1&3/8" from the face.

The challenge here would be to find or turn a collar that extends the shaft say, 2", and which would then have the flat steel scrap welded to the bottom to facilitate attaching a blade. Very accessible/doable...Hmmm....

BTW, do you know Daox and The Milwaukee Electric Car Club crowd? I've been reading many of their posts and viewing their videos. I'd love to stop by on a build day and pitch in, but I often work on Saturdays...
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Old 06-07-11, 09:33 AM   #7
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Good thing the electric car club meets on Sundays. Actually, I believe this Sunday is our next build day.
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Old 06-07-11, 02:52 PM   #8
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Weed Dog,

Never even noticed the rectifier in the description. That means that $40 plus shipping gets you a 1 horse power unit.
My offer of machining help stands if you need an adaptor. With that small a shaft, using a belt drive would be a necessity to avoid trashing the motor the first time you hit a rock.
Just brainstorming here, but best bet might be a welded sheet metal tunnel mounted on the top of the deck. It could double as a motor mount and belt guard. Set your pulley centers to fit some common v-belt. You will also need a blade shaft housing and shaft. Couple of flange bearings would do, or make a weldment and machine for bearings. Six months ago you could have had the center out of my old rider's deck, but it's probably a car fender by now.
You might want to do a quick check on blade speed. IIRC, mower motors run about 3600 rpm, so you should be ok, but it never hurts to check. I think the rule of thumb is that the tip speed should not go supersonic.
No, I don't know daox. I did read his thread on the electric rider, and realized where he lived when he showed us his yard on the thread.
BTW, can anybody tell me where this Milwaukee Electric Car Club meets? I'm not a big electric car freak, but mechanics of all sorts interests me. Might as well take a look...
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Old 06-07-11, 03:11 PM   #9
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We meet at the Milwaukee Makerspace. I'll be there on Sunday working on the plug in kit for my Prius, and testing the AC motor controller I picked up a few weeks ago. Lots of guys come just to chat away and discuss different things they're doing with cars, house, whatever. So, feel free to stop by.
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Old 06-08-11, 08:13 AM   #10
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Quote:
Originally Posted by toolingjim View Post
Weed Dog,

Never even noticed the rectifier in the description. That means that $40 plus shipping gets you a 1 horse power unit.
My offer of machining help stands if you need an adaptor. With that small a shaft, using a belt drive would be a necessity to avoid trashing the motor the first time you hit a rock.
Just brainstorming here, but best bet might be a welded sheet metal tunnel mounted on the top of the deck. It could double as a motor mount and belt guard. Set your pulley centers to fit some common v-belt. You will also need a blade shaft housing and shaft. Couple of flange bearings would do, or make a weldment and machine for bearings.
Exactly where my thinking was going...felt I should produce a few scale drawings and views to help determine the most efficient use of materials and a means to construct it that would lend itself to maintenance.

And the belt drive/pulleys approach does prevent damage to the rather delicate motor shaft...cool.

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