|
Insulation
resistance and High Potential Testing are means of troubleshooting or
evaluating a winding insulation.
Insulation is
typically made from organic compounds and ages due to time and
temperature....theoretically, motor designers establish rules based on
temperature rise to see how much they can "push" motor ratings.....remember the
old motors that were huge and low horsepower and today they are small, run much
warmer with more horsepower? The reason is early insulation systems used
to be able to handle 75C temperature rise and survive a "normal" lifespan and
today the newer insulating materials may be able to handle 135C rise with the
same lifespan. Now, for every 10C above rated temperature you run a motor
you cut the insulation life in half.....likewise, if a motor runs cooler, it
will last longer.
Megger Testing
evaluates leakage of current through the insulation to
ground:
DC voltage
(500V) is applied between the winding and ground and a resistance is
read. New motors would typically have infinite ohms to ground where a
real dirty wet motor may actually be grounded. Thorough cleaning and
baking/drying will improve readings. Minimum recommended reading is 1.5
meg ohms.
Hi-Pot testing evaluates a voltage gradient
of the insulation without puncture or rupture:
DC
Hi-Pot:
This is
intended to be a non-destructive test used to determine if a winding has aged or
is weakened. Current is limited by the tester. It's a leakage test
to ground through the insulation. New windings: 2 x rated volts + 1000V so
the ET motor gets 1,072V.....approx 50% on a used motor. Pass or fail
based on leakage current.
AC
Hi-Pot:
This is a
destructive test. AC voltage stresses the insulation to try to puncture
holes through it.....2 x rated volts + 1000V. Pass or
fail.
Service Shops use
this test to show a customer their motor won't last much longer and it won't
with this test unless the winding is in really good shape.....now, on new motors
or if the shop rewinds a motor, you want to make sure they AC Hi-Pot it to prove
their winding job was good.
I'd
recommend not hi-potting any used motors....especially for ET use. If you
want to clean the motor, carefully disassemble it, take it outside and blow it
out with clean dry 50psi air and inspect it for any insulation damage at
connections, pole pieces, or end of the commutator as these areas can likely be
the source of leakage to ground. You can't use a ohm meter to check for
megger as it won't drive enough voltage/current for a good reading. Look
for arcing to ground and repair those areas with epoxy, insulating paint, or
varnish. Brush dust is conductive and gets on everything....mix it with
grease/oil and it's a mess. Do not use solvents to clean a winding as you
can wash contaminants down into places you can't get them out
of.
Clean the
commutator with a toothbrush and between the segments (where the mica is) with a
razor blade or small wire brush. Hand sand it's surface if it shows any
signs of arcing or copper bar damage (you can shoe-shine
it).
When you
assemble the motor, replace the bearings....they are cheap and available from
local sources (Grainger, etc..). Heat in a toaster oven to no more than
300F and pop them on. Lettering on races always faces
out.
Sorry, I got
carried away.....hope this helps.
...Walt
From: elec-trak-bounces cosmos phy tufts edu [mailto:elec-trak-bounces cosmos phy tufts edu] On Behalf Of Michael Macdonald Sent: Monday, January 26, 2009 11:18 PM To: elec-trak cosmos phy tufts edu Subject: (ET) re; motor trouble Walter
was wondering if you could explain more
fully
"....don't let them hi-pot the
windings"
i recently took my snowblower motor in to give it a
once over.
result: $120.00 to find out "it is
fine"
i need to ask more questions in
future.
Mac
|