(ET) Klixon breaker failure mode on mower motors

["Geoff Rich" <grich FIRSTSOLAR COM>]

Hello All,

Perhaps this has been discussed before, but since it's generally 
acknowledged that mower motor components (specifically armatures and 
shell-magnet assemblies) are some of the things in the "hard to replace" 
category (i.e. no cheap, readily available substitutes), I'd like to 
mention a failure mode that could lead to armature problems, and possibly 
to an expensive rewind job, if you can't locate replacement armatures.  In 
extreme cases, it may even lead to partial demagnetization of the magnets.

I have an early front-mount deck (nose roller & "screen door" handle era) 
which has the smaller diameter motors, and recently I found that two of 
the armatures had winding problems.  These motors would draw unusually 
high current, run hot, vibrate, and not attain the usual operating RPM.  
When operated on the bench at 5-10VDC, they would stall at certain points 
in the rotation, then spin 180°, then stall... the classic behavior of 
armature coil problems.  Also, I felt some dragging/cogging when spinning 
the shafts by hand with the motor disconnected... I had replaced bearings 
last year, so this was electromechanical drag... not good!  Growler tests 
at the local motor shop confirmed my suspicions: a couple of shorted coils.

More investigation turned up the fact that the little 25A Klixon breakers 
on top of the motor caps were WELDED CLOSED!  So, under heavy loads, the 
motors would draw more and more current, get hotter and hotter, and 
perhaps this eventually burned out some of the varnish on the coils, 
leading to shorts in the windings.  A little searching on the web turned 
up some (incomplete) specs on the Klixon CA-series breakers (Texas 
Instruments now owns this operation... see 
http://www.ti.com/mc/docs/precprod/docs/tcb.htm). They mention a 30VDC 
rating for these breakers... no wonder they couldn't clear the arcs.  In 
one case, the disc was actually burned away near one contact, so at least 
that unit did finally stop the current flow.  These are old units, so it's 
also possible that they welded from resistive heating during heavy 
operation, since the contacts may have become dirty and pitted over the 
years.

So, I suggest the next time you are working on a mower motor, remove the 
Klixon breaker on top, hold a lighter underneath it for a second or two to 
heat the bimetallic disc, and check to make sure you get an open circuit 
across the terminals.  If not, it could be welded closed.

As for me, I'm planning on replacing these Klixon guys with some 
48VDC-rated molded-case breakers (Square-D QOU125 or similar).  The QOU's 
mount on DIN rail, so I'll probably just mount them to the deck surface 
somewhere.  Hope they can tolerate the dust...  I'd rather not have to put 
them in an enclosure.

BTW, I've never had any troubles with the main thermal disc breaker CB-1 
(the one mounted on the traction motor).  I think it's an SDLA-series 
part, 80A rated, but I have to check for sure.  Same goes for CB-2 (40A?) 
on the electrical panel behind the dashboard.  Browsing the TI link above 
will yield contact info for tech support and for distributors of the 
Klixon line, if anyone needs to buy some new breakers.

Geoffrey Rich
E-16