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 ________________________________ From: Mark Hanson <[email protected]> To: "[email protected]" <[email protected]> Sent: Monday, May 13, 2013 3:03 PM Subject: [EVDL] Brush uneven wearing, premature failure - fix Hi, I measured with a milli-ohm meter my Netgain-warfield-Impulse 9" brushes at .5 milli-ohms and the cross bar that connects two in parallel at .2 milli-ohms. I calculated that the rear brush (since it didn't go through the cross bar) was seeing about 85 more amps at 500A motor current going up a hill (when i smell brush smoke). I get about 3k miles out of the rear brushes but the front 4 brushes are at the same length as new (with calipers). I could just let the rear burn down and then it would be forced to the front brushes - but the fix is symetrical brush rigging like ADC has - same bar lengths from the connection armature bolt to each brush. When the first brush seats, then it shunts and hogs all the current away from the other and the front brush doesn't seat (until its' forced to by current). I saw only 1/2 contact area when I removed the front brushes after 3k miles when the rears were worn down. I could just look at it like changing oil every 3k miles or let the others wear and go to 6k miles but my ADC's and prestolite's over the years went to 100k miles before changing brushes. Cheers, mark in Roanoke www.REEVA.info RE & EV's -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130513/4e5cc90f/attachment.htm> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130513/4f2446d6/attachment.htm> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
