I have two identical PMDC motors. I ran each of them on a dyno at the same voltage and the same load. They both consumed the same amount of current, yet one of them runs 300 RPM slower.
The one that is running slower was actually under test for several hundred hours, while the faster one is brand new. Wondering if there's something going on with the brush/com, I put each motor in a motoring dyno to check their BEMF (both motors were driven with the same speed). Not sure what I was looking for, just hoping to see a difference. The graph is below. The blue trace is from the motor that runs slower. There are obvious voltage spikes that the other motor doesn't have. Is this the reason for the motor running slow? If so, why?