TS DC Motor on Electric Lawnmower

Welll, the 20 year old Ryobi BMM-2401 beast stopped in its tracks today. Any ideas on where the short might be? Here's my story....
I didn't hit anything unusual - just a thick patch of grass and it just stopped. I could see that the thermal fuse had tripped. I reset it, started up again and it tripped again. This time I saw smoke coming from the relay board, and the red wire connecting the hot power from the power relay to the motor hot terminal was hot.
So I tore into it, took the relay board out and sketched the wiring diagram.
First thing I saw was a large burned spot on the relay board at the connection of a 12awg wire that connects to the motor positive terminal. Sorry, camera is not working right now....
I disconnected all the wires connected to the board and checked continuity. I found that the load contacts remain closed whether the coil is energized or off. So that pretty much tells me I need a new relay. Luckily they are still making an exact replacement and not that much $$...
http://www.digikey.com/product-detail/en/VF7-11F12/PB710-ND/813777
That wouldn't explain why the board was burned, so I'm guessing that there's another short somewhere. Probably on that same board.
But just in case it's not, I started checking the motor for short. I disconnected the pos & neg motor terminals and measured the resistance at 2 ohms. That sounds pretty low to me, but motor TS info online says that's in the proper range. I=E/R means 24/2 = 12A wouldn't it? That's pretty high. Maybe that's not important -- the battery spec is 5A max.
Motor part number - 971051-003. Obsolete and no specs I could find online. Measurements: Shaft length end to end 8" Motor diameter 3 7/8" Shaft diameter .675" (just under 11/16th) Iron stator housing length 4.5"
Thanks in advance!
Bob
Misc Info for future reference...
Similar to BMM2401:

https://www.youtube.com/watch?v=mFpP-4tv0tw

Review in Popular Mechanics, March 1994, page 76:
http://books.google.com/books?id=heMDAAAAMBAJ&pg=PA76&lpg=PA76&dqryobi+bmm2400+motor&source=bl&ots=0RD3A9nuqu&sig=G7C5_6615AgRiDZTyC zaRlizQoQ&hl=en&sa=X&ei=vmAFVN7uHMOtigKuloGgDw&ved FoQ6AEwCA#vonepage&q=ryobi%20bmm2400%20motor&flse
Ryobi Outdoor Products 550 N. 54th Street Chandler, AZ 85226
Original costs: Mower $379 (Mine was $20 at garage sale with bad onboard charger. Paid $25 for a charger at the auto parts store)
New battery in 1994: $125 (Actuall it's two 12VDC in series) $20-35 each now for new ones on ebay
319
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Guv Bob" <But just in case it's not, I started checking the motor for short. I disconnected the pos & neg motor terminals and measured the resistance at 2 ohms. That sounds pretty low to me, but motor TS info online says that's in the proper range. I=E/R means 24/2 = 12A wouldn't it? That's pretty high. Maybe that's not important -- the >battery spec is 5A max.
YOu can not use the normal i=e/r on motors. That is true for only a fraction of a second. Once the motor starts up, it also acts like a generator and creats a back EMF. The current will be a lot less as the motor reaches its running speed. This is true for any common motor , AC or DC. It might also be know as the LRA or locked rotator amps. It will often be 3 to 5 or more times the running current.
For a motor you have to know what it is rated at at full load and measure the current when the motor is running.
--
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks a lot, Ralph. That's what I was hoping to hear.
I was expecting to see some kind of voltage regulation or filter circuit, but don't see anything. Is it normal for this type of motor to run connected straight to the battery like this? With this units, there's a chance that someone else has fiddled with it before I got it. Before the relay went back, never had any trouble in 15+ years.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 9/2/14, 3:58 AM, Guv Bob wrote:

I'd check those two terminals on the relay. I'd check the board without the relay.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks, JB. That's exactly what it was. I ended up bypassing the board and installing a relay & base elsewhere and rewiring the other connections around the board.
This was an interesting project for me. I was surprised at how little there is to an electric mower. It's basically a motor and battery. Add a thermal circuit breaker or fuse, relay, switch and charger and that's it. Mower chassis is 2 pieces of molded plastic. Only and some brackets are metal/ New ones made in China sell for $300 and up. Hmmmm......
I took way too many photos and will upload a few soon.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.