How do I fix a Black & Decker electric lawn mower motor?

A neighbor asked me to have a look at her B&D electric mulching mower (MM1800 Type 1) after it stopped working. When I pulled the control switch, the blade didn't turn, but there was a buzzing sound and kind of a burned plastic smell. I also found that it was very hard to turn the blade by hand. At home, after a lot of trouble I was able to remove the 6 screws holding down the big plastic cover. (You should see how much grass was under there.) I can now see the outside of the motor, but how do I figure out which parts need to be replaced? The owner's manual has a parts list. I assume that some parts (like the magnet ring and armature) are OK, and that the problem is with something like the brush spring/terminal/brush insert or rectifier. How do I find out? (I'm hoping it won't be that hard to replace the parts of the motor.) Thx.

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bkbkbkbk (author) 1 year ago

I took the top off the motor and found that the magnet ring was cracked and parts had broken off, then I trashed the thing. The neighbor has ordered a new mower. Thanks to all.

Yonatan241 year ago

Do you have any pictures?

Grass probably got into the motor and clogged it up. Time to get a new mower! ;)

iceng1 year ago

"Cherchez la Femme"

Using Google(r) images, I was looking for an image to explain, symbolically, the parts inside a typical mains-powered electric lawn mower.

Because, it is a story that has been told before. I mean, the explanation of how typical mains-powered electric lawn mower works, has been explained before.

This page,
http://nickviera.com/electrical/lawn_mower/bandd/

contains the circuit diagram I was looking for. It also has a pretty good explanation of how these mowers work and discussion of the usual ways they fail.

What I suspect, regarding what you have told us about your neighbor's mower, is this mower was mostly suffering from mechanical friction, from having been fed too much grass all at once.

Also there could be rust bunnies in the bearings. They move in sometimes when they sense a machine is wounded. The usual solution for rust bunnies is oily spray lubricant, e.g. WD-40(r), or whatever brand you like, or whatever similar thing is sold where you live.

Moreover, I suspect simply removing the friction, this ?might? be enough to fix the mower. I mean, it will be enough, if all the electrical parts are still good.

If the electrical parts are NOT good, well, that makes it more challenging, but I am hopeful Nick Viera's page linked above, can give you some clues about that.

+1

Great page Jack, definitely a PM DC brush motor with compromised cooling as the grass debris insulates the motor and trivial bridge heatsink. Which will cause accelerated component failure.

The armature powered burned plastic smell is indicative of wire shorting damage.

The page I saw has no ground so a short even on a double insulated PM tool increases an electrocution risk...

bkbkbkbk (author) 1 year ago

I'll try to look at some of the easier things on my neighbor's mower. I'm not sure if she can afford a new machine, so I might have to mow for her. I have a similar mower (Craftsman-branded B&D) that's older than hers, but still works fine. I use a 14-gauge cord, but she only used 16 gauge with a cord the same length and a motor the same number of amps, and she often mowed wet grass. I also found that her blade and insulator washer were pretty beat up, and two fins were broken off the plastic fan. (I won't let her use my mower.) I'd like to help her out and I like to tinker, but there's only so much I can do and a part like the armature seems very expensive. Thanks for all the advice.

-max-1 year ago

I reckon it will either need a new motor or to rewind the old one. If you stop it from rotating while it is energied it will overheat pretty quickly.

Sounds like the cord and switch work.

The smell indicates the motor is burnt.

The blade should be a little hard to turn by hand, the drive is gear reduced.

Open the gear box and see if the armature is stiff without the reduction gears.

If it is stiff you may need to replace the brushes, armature, and bushings.

To check the rectifier on the DC output check with a multi meter to the power cord, one way you should get continuity the other you shouldn't.

If you get continuity both ways you may need to replace the rectifier, brushes, and armature.

Ether way you are in for work if you can get the parts, it might be cheaper and easier to buy a new lawnmower. Black & Decker is cheap and made disposable.

There's a gear box ?

Yea almost exactly the same as a skill saw.

So an AC universal series motor or PM DC motor exchanges armature RPM for torque and still enough speed to cut grass and mulch grass...

This seems as an overly expensive design with necessary service versus a simple AC induction lawn mower... What think you ??

That style of motor get up to 8 to 10,000 rpm, most of them are low torque high rpm to get some modicum of power they need gearing. I have a half horse not much bigger than my fist but it is 8,000 rpm

iceng1 year ago

Electric motor repair is not simple once the unit won't run... As you pointed out the smell, that may have compromised the armature wire insulation to fail even if you replace the brushes and the commutator is not gouged...

If you like the lady, be honest, the machine is beyond easy fix and you don't have the equipment to make sure it will be safe if by some rare event you get it to spin again...

BTW always first see if the motor shaft is free to turn as a prelude to applying power.

First rule of doing repairs: Do not offer them if you have no clue.
Sorry but I fail to see the point here as it is a bin case anyway.