High Armature Resistance of Kostov 9 DC motor

[727jjmc]

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

 

[ScottyUK]

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.

 

[727jjmc]

Thanks for the post Scotty.

 

[elinBG]

I am suprsied that these bulgarian motors are well known abroad

http://www.eldvigateli.com

 

[waross]

What Scotty said. You can't get an accurate reading through the brushes with a multi-meter.

Bill
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