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High Armature Resistance of Kostov 9 DC motor

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727jjmc

Electrical
Joined
Jan 23, 2014
Messages
2
Location
US
Hello. I have a quick question. I have a Kostov 9 series wound DC motor (21 kW @ 144V ) for an electric vehicle application. The motor is brand new, and I have yet to run it.

The other day, I tested the armature circuit resistance with a DC ohmeter, and the resistance was very high (1.5 Ohms). According to my calculations, total resistance (armature plus field) should be about 124 milli-ohms or so. What gives? It was cold outside (27 degrees F), but I had a 200W motor heater strip on it at the time. The motor was warm, not hot, but then again, some parts were cold.

I was thinking that the problem would be in the brush/commutator interface. Maybe I need to adjust brushes (ie the contact pressure)? Maybe thermal expansion/contraction is to blame? Any thoughts? I would think the commutator would expand during operation and improve brush contact, which would lower armature circuit resistance. Thanks
 
Brush resistance is highly non-linear at low voltages. Try the test again with a higher current source, or very carefully make connection to the armature itself avoiding the surface where the brushes bear on the commutator.
 
Thanks for the post Scotty.
 
What Scotty said. You can't get an accurate reading through the brushes with a multi-meter.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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