Electric lawnmower repair

I have a Black and Decker electric lawnmower (model MM1800 type 1), which I have used for a few years, ever since my previous electric lawnmower failed.  A few weeks ago, the lawnmower failed while I was mowing the waist-high grass in the backyard (a heavier load than the mower is designed for).  The failure mode was an interesting one, though—it blew the circuit breaker for the outdoor outlet.  I figured that some of the wet grass had gotten into motor compartment and shorting out the circuit, so I left the mower in the sun for a couple of hours to dry out and tried again.  It still blew the circuit breaker, so I put the mower away until I had time to work on it.

Last weekend, I took the cover off the mower to see if I could see an obvious short circuit.  This looked like it would be an easy job, as the cover is just held down by 6 screws, but the screws are recessed, and there was so much impacted grass built up over them that it took half an hour or more to dig out the grass enough to turn the screws.  One I got the cover off, the amount of grass inside the motor compartment was scary—after dumping out about 200 cubic inches of grass, I spent half an hour with a vacuum cleaner and a screwdriver cleaning off the stuck-on grass clippings.  Here is what it looked like after cleaning:

The mower is very simple, and looks pretty good after all the grass clippings have been removed. All those spaces between the reinforcing ribs were packed solid with grass clippings.

I thought that perhaps the huge buildup of grass clippings had been causing the short circuit, but after removing all the grass, I saw no signs of blackened wires or other evidence of arcing. I also disassembled the switch handle to look for evidence of shorts there. Again, there was nothing visible. I tried powering up the mower while it was open—still the circuit breaker blew instantly.

The lawnmower design is a simple one:

The motor driving the blade is a large brushed DC motor. The switch either provides power to the motor (on) or shorts it out (off) to brake the motor. (Schematic fixed 2018 June 6)

There is easy access to the wiring.

Here is what the rectifier looks like in situ. The four wires are on slide-on connectors, and the rectifier is screwed to a small metal plate that acts as a heat sink.

I got out my cheap ohmmeter and started checking continuity and measuring resistances. I quickly determined that the switch was working correctly, and the the motor itself did not seem to be shorted, but the bridge rectifier was acting as a short across the AC inputs, even with no load connected. I then looked online and found a site that said that the bridge rectifier was one of the most commonly failing parts for the Black and Decker mowers, and that Black and Decker charged ridiculous amounts for replacements. They also said that the part was a 15A, 400V bridge rectifier in a standard package. The one in my mower was labeled GBC2504, which is a 25A, 400V bridge rectifier (so, for that matter, is the own shown on the web page that claimed it was a 15A one). I ordered a 50A, 1000V bridge rectifier from Digikey, which cost $1 more than the closest equivalent to the one Black and Decker used, but even with shipping was half the price of the list price of the poorer rectifier from B&D. Based on the derating curves on the data sheet, the 50A bridge rectifier should be able to run about 30°–40°C hotter before it fails. My only concern is that this might be too good—that B&D was using the rectifier as a cheap point of failure at high temperatures. With the better rectifier the motor might overheat and burn out before the rectifier fails, and a replacement motor is not a $3.50 part.

This morning I replaced the bridge rectifier with the new one, being careful to clean the metal plate and put on new thermal grease to get good thermal contact to the heatsink. The mower started up right away and gave me no problems with mowing the front lawn. I’ll try tackling the waist-high grass tomorrow, but today I need to do some grading. (I have a homework set and something like 22 redone lab reports to grade during this long weekend—I think I’ll be able to clear that this weekend before getting more redone reports on Wednesday.)