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David, The PTO circuit has no fuse or circuit breaker (except for the fusable link) due to the wide variety of accessories that could be used. The mower PTO cord is #8 conductors (#10 used for shorting
to regenerate the mower motors). #14 wire then goes to each mower motor. The short or tall mower motors are rated .6HP @36VDC and 15 amps. Now comes the gray area. Overloads can cause commutation/brush issues and also long term heating which is really what shortens the motor life. I could never find the rating of the thermotector (or circuit breaker) that
was used but it’s job was to limit the motor heating over time. Sudden transients can probably loosen the epoxy holding the permanent magnets in the stator. Best advice is to not allow the power use gauge to be in the high range too long indicating high amps over time. Start/stop current inrush may approach 100+ amps instantaneously. When we made
the METI tractors and controls we used a 100amp CB which seemed to be okay even when we made mower decks with 1HP rated motors. Unfortunately based on the age of the original ET motors and how you mow grass (speed/height) it’s impossible to predict adequate
ratings for a fuse or CB…. I’m okay with 100amps as it has worked but I don’t mow with the ET except for trimming under pine trees. These motors are pretty robust but also approaching 45 years old….. …Walt From: Chad Bush via Elec-trak [mailto:elec-trak cosmos phy tufts edu]
David, Obviously there is a reason the fuses are blowing. $10 each for a mower motor fuse isn't bad, considering the cost of a new armature, which is probably PRICELESS.
I found newer circuit breakers for the GE motors at Waytek, but they aren't the slow-blow type. Might take a look at what they have, but I think you will run into issues of trying to compare amperage at the working voltage. There is a general amperage-squared
times T parameter that is used to compare most fuses. I believe GE used the slow-blow fuses to eliminate issues with the startup surge, hence the 30A slow-blow for the lift motors. The GE deck motors for 36" decks with
18" blades use a 30A breaker. I think you could reasonably size a breaker based on that, as long as the motor ratings are similar. Good Luck, Chad Bush -----Original Message----- On 19 Nov 2013 at 13:30, David Roden wrote:
The Avco R36 blew a mower motor fuse this past weekend. Mouser wants
> $9.77 each plus shipping!
And yet another this past weekend. Mouser is now charging $10.05 each.
I'd like to replace these 40a slow-blow fuses with circuit breakers. I see
quite a few DC breakers on Ebay for 50% to 100% more than the MDL 40 fuse.
That looks like a pretty good deal.
The question is, what size breaker should I use?
Given the size of the wire connecting to the mower motors, I'm guessing that
GE oversized the fuses - and used slow-blow type - so they'd open on a
serious long-term overload, but not on a normal starting surge. I need to
choose a breaker that will act the same way. That might not necessarily be
a 40 amp breaker.
Ideas? Suggestions? All appreciated. Thanks.
David Roden - Akron, Ohio, USA
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