B&D Electric mm575 motor internally shorted?

[karosis]

Hi all, I got a used B&D electric mower, started getting weak and losing power, then shutting off. I researched and checked the switch and rectifier config. Tried bypassing the switch, it tripped the breaker. Tried replacing the rectifier (which was bad, I tested it) and it still tripped the breaker. I read that the switch is an alternate-action double pole switch, which interrupts the AC input when the handle is released but also shorts the motor leads together to stop the blade sooner. Okay, whatever, but I found that the two leads to the motor read as a short even with everything disconnected. That can't be right. I can only conclude that the motor has an internal short. Any suggestions, comments?

 

[1 Lucky Texan]

I'd expect the motor windings to be very low resistance. Check each lead from the motor to ground/case. If one seems 'closer to zero' you might look at that side for a pinched or otherwise compromised wire.

sometimes, a quality analog ohmmeter is easier to use for troubleshooting shorts. You may be looking for .25 ohm differences.

maybe another owner or someone with experience will reply with better info.

 

[Mark Widmer]

It is normal to read a short across the motor. My B&D CMM1000 would read a short when measuring across any two contacts, even ones that were not "paired up" on opposite sides of the rotor. It is just how the windings are connected inside the motor.

If it's getting weak and losing power easily, it's a good bet that the batteries need replacing. Do you know how old the batteries are? If they were treated properly, they should last 4 years or so. And be aware that many people do not treat mower batteries properly, so 1 or 2 years battery life is fairly common too.

A bad breaker should still provide full strength while the mower is running.

And at the risk of stating the obvious ... has your grass gotten very tall? These mowers are not as powerful as gas mowers, and will bog down more easily in thick (or wet) grass.

Hope that helps.

 

[karosis]

Hey thanks all, I appreciate the tips. I never worked on an electric mower before. Yeah, I figured on the windings kinda reading as a short, but not just how much (sorry to say my analog meter is kaput.) I have no doubt at this point that it's the motor that's burnt out. And I also have read up a little and I have had the revelation that, yes, using a puny electric mower (AC powered btw, no batteries) especially an older one on tall grass where it bogs down and overheats is a bad idea! No doubt that's what happened here. The owner later revealed that she was cutting tall grass and "smelled something burning". The lesson here I guess is that these are not the type of mowers that will take abuse, and are for light duty only. Thanks again.

 

[Mark Widmer]

Aaaaah ... you have an AC-powered motor, and I was thinking DC/battery. You even said "AC" in your first post! Hate to admit this, but pretty much everything I said in my earlier post can be disregarded.

I would expect an AC mower to have more power than a battery-powered ones (seems to be true of my power drills anyway), but not having any experience with AC mowers I won't swear to it one way or the other.