Hi Jeff etc,
Maybe my milli-ohm meter that measured .2 milli-ohms was off but all 4 rear
brushes are wearing down fast and the front one's no wear at all - so that's
the only explanation I can come up with is non-symetrical brush bars. The ADC
motor brush rigging are symetrical . I guess it's no biggee, I can change
brushes fairly quickly now, just let the rears wear down in 3k, then the fronts
will start conducting for another 3k miles. I'm just used to taking motors for
granted, never bothered with brush replacement till about 100k miles in the
past.
The brushes that seat first start shunting away the current from the others and
then it's a positive feedback loop, the shorter they get the more current they
pull.
best Regards,
mark
Hi Mark,
I think you're barking up the wrong tree.? My calculations for added resistance
of the extra buss bar length to the inside brush connection at 20 degrees C is
0.01 milliOhms.? That only carries one fourth of the armature current so
represents 1.25 millivolt drop at a 500A load on the motor.? The typical
voltage drop per brush at that current would be on the order of several volts.?
So the voltage divider method would indicate a possible current imbalance of a
tenth of a percent due to the extra copper bus bar length.?
Another thing to consider is the fact that the rear brushes sit on the comm
further away from the armature so the current flowing through them has a longer
path through the commutator segments to reach the armature circuit.? This kind
of counteracts the shorter path from the A terminals to the rear brush.
In both cases you're talking about a short distance though a relatively large
cross section of low resistivity conductor which results in negligible voltage
drop compare to the rest of the circuit.
You mention 100K miles on your Prestolite and it had a similar brush connector
bus bars (coming from the opposite way).? Did those show uneven wear?? I think
your current problem is not associated with that small distance difference in
the brush connectors.
Jeff M
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