My old faithful electric lawnmower died today.  I took it all apart and the
problem does seem to be the motor.  I'm going to (try to) buy new brushes
tomorrow since it seemed like one of the brushes was about gone.  But what
else should I look for?  It's the kind with two brushes to an armature, and
in series with a surrounding coil.  Everything else tests OK, but it won't
start.  It started running weak, and in about 5-10 minutes it was nearly dead.

9 responses total.

Sounds like a series wound AC motor...  The bearings aren't grabbing,
are they? That is, can you still spin the motor easily just after it
dies? 

It spins fine (probably better, now that I've regreased the gears).  I'm
worried that the armature has a short or break, though.  The first bad sign
was that it started tripping the GFI outlet I normally plug into.  It's
possible that it was the result of one of the brushes wandering off the
armature, or perhaps the amazing amount of grass clippings which had
accumulated inside the casing.

Could be shorted or open armature.  Those are not worth trying to fix as it
means rewinding the motor.  try replacing the brushes.  The brush material
is soft enough so that a slightly oversized one can be reworked if you can't
find an exact replacement.  The armature can be tested with a device called
a "groaner".  They were common in automotive shops in the days of generators.
You can also go at it with an ohm meter and check for shorts to the shaft and
resistance between commutator segments.

If it was tripping a GFCI, you probably have some kind of current leak 
to ground.  It could be as simple as wet grass clippings stuffed in a 
bad place, but it could also be a breakdown between the insulation of 
the windings and armature.  If it keeps tripping GFCIs you should 
probably replace the mower.  That indicates a potential safety hazard.

No shorts measured.

I found a pretty close brush at Stadium hardware, put it in, and (after a few
minutes of jockying things around until the case would close) it worked!  But
after a few passes down the lawn I turned it off to fix something else, and
it wouldn't turn back on.  :(

Maybe it's time to replace the beast after all...

May Jim come over and play with your mower some time first?  He had some ideas
but I could not follow them well enough to summarize.  
Any idea on why my front-loading semi-antique washer clacks a lot but won't
spin unless I first remove the clothing, wring it out and return it?  My guess
is the belt needs replacing - any other possibilities?  It starts to try to
spin, goes slowly for a while, then cuts out and smells sort of burnt -
slipping belt burning up?  Jim says it is supposed to slip some, but not too
much.

Sounds like the belt. I mean, smells like the belt...

Well, I wimped out and bought a new electric mower.  Display model (last one
at Meijer!) for $40 off.  Probably the old one still has brush trouble; I
think the factory brushes are all one unit with the casing and so my
replacement can't keep solid electrical connection.

Jim is welcome look at it, and assuming the new one works OK the old one will
be given away anyway.  I really got my money's worth on that puppy; 4 years
ago I found it for $15 at the ReUse center, and I've only spent about $30 on
repairs in all that time.

Some interesting motor- and generator-related info I ran into by
accident today:
http://members.1stconnect.com/anozira/SiteTops/energy/Alternator/alternator
.htm

It describes how automotive alternators work, and how to modify them for
higher voltages.  It also describes how to use an induction motor to
generate 60 Hz AC.

Response not possible - You must register and login before posting.

- Backtalk version 1.3.30 - Copyright 1996-2006, Jan Wolter and Steve Weiss