Monday, September 11, 2017

An Electric Lawn Mower Motor Failure

I actually know better than to pick up discards like this from the roadside, but I've got nothing else pressing to do, and I thought I might learn something about electric mowers.

It's an elderly Sears Canada model C935-35516-1. The odds of it being repairable are slim to nil. But I have no experience with or knowledge of electric lawn mowers, so I may as well open this one up and at least discover what its problem is. At worst, I'll have one more hunk of scrap metal for the next run to the scrapyard.

The blade is not in too bad a condition, but when I try to turn it it won't turn but a bit, and there are crunching noises -- not a good sign.

A 3/4" A/F hex nut holds the blade on. Removing that with an impact wrench reveals the blade's 1/2"-20 spindle stub.

There's a plastic centrifugal fan blade that's seized in place. That fan blade has to come off in order for the motor to be removed.

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To make a long story short, after much puzzling out and dismantling, I got the motor out of the deck and opened it up. Here's what happened to the motor.

One of the permanent magnets came loose from the frame, met the spinning rotor and shattered, jamming the rotor. Here's another view after I'd removed some bits of the shattered magnet.

That's what's known as catastrophic failure. That motor is unrecoverable.

So, that's a roadside find that didn't pan out. All I'm left with is scrap value, which is precious little since the mower's deck is all plastic.

Anyway, I did learn a bit about electric lawnmowers -- they use permanent magnet DC motors. That's how they're able to have adequate power with a motor of minimal size and weight. A solid state bridge rectifier is what makes it possible for the motor to operate from AC household current. Here's a view of the rectifier mounted on top of the motor.

There's quite a good article on electric lawn mower wiring and servicing here.

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