The AC Mower Essay

[Weed Dog]

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...