Two very quick thoughts, probably both bogus: 1) You're striking an arc inside the bearing, and the controlling thing is the voltage; below the critical voltage needed to strike the (very short!) arc the resistance is much higher
Or.... 2) Somewhere in the bearing, at the junction between two materials (like between the balls and the race, or maybe between the graphite lube and something), you've got an accidental diode, with a junction drop of a couple volts Well anyway those are the things which came to mind first. Horace Heffner wrote: > This is a quick check of BB motor resistance at 5 different rpms at low > current using 4 runs. Fig. 1 is the circuit. > > > (+)---P1---R1---P2---A---P3---Motor---P4---G > > Fig. 1 - BB motor resistance check circuit > > > A is a DVM ammeter. R1 is 48.4 ohms. The current in the 5 runs was 0.19 > A +- 0.01 A. The voltage drop from P1 to P4 is shown in all runs in CH2, > and is about 13 V. The voltage drop in all 5 runs is shown in CH1 and is > about 2.4 V. > > Surprisingly, it was possible in the slower runs to see the rpms in the > slow runs because the resistance apparently varied depending on angle of > the shaft. The rpms for runs 1-3 are approximately 222, 540, and 167 > respectively. The last run was very fast so I could not pick up the rpms > from the trace. > > The resistance of the motor in this configuration is thus : > > Rmotor = (2.4 V)/(0.19 amps) = 12.6 ohms > > This is amazing. The resistance must drop a lot with higher current. If > this were the resistance during the high power runs the current would > have been about an amp. The voltage drop must reduce with increased > current in this low current range. > > Here are the first 5 runs. > > http://www.mtaonline.net/~hheffner/HullR1_222rpm.jpg > > http://www.mtaonline.net/~hheffner/HullR2_540rpm.jpg > > http://www.mtaonline.net/~hheffner/HullR3_167rpm.jpg > > http://www.mtaonline.net/~hheffner/HullR4Fast.jpg > > http://www.mtaonline.net/~hheffner/HullR5_0rpm.jpg > > - - - - - - - - - - - - - - - - - - - - - - - - - - - - > > R1 was then reduced to 23.2 ohms. The current in the next 2 runs was > 0.38 A. The voltage drop was about 3 V. This gives > > Rmotor = (3 V)/(0.38 A) = 7.9 ohms > > The resistance drops with increased current. R6 was at 675 rpm, R7 was > at 462 rpm. Here are the traces from Runs 6 and 7: > > http://www.mtaonline.net/~hheffner/HullR6_675rpm0.38A.jpg > > http://www.mtaonline.net/~hheffner/HullR7_462rpm0.38A.jpg > > > - - - - - - - - - - - - - - - - - - - - - - - - - - - - > > For Run 8 R1 was dropped to 9.4 ohms. R8 was run at about 1140 rpms. > Current was 0.96 A. The voltage drop looks to be about 3.2 V, with lots > of spikes. This gives: > > Rmotor = (3.2 V)/(0.96 A) = 3.3 ohms > > http://www.mtaonline.net/~hheffner/HullR8_9.4ohm_0.96A.jpg > > > - - - - - - - - - - - - - - - - - - - - - - - - - - - - > > For Run 9 R1 was dropped to 4.2 ohms. This was run at 0 rpms. Current *I > think* was 2.4 A. The voltage drop looks to be about 2 V. This gives: > > Rmotor = (2 V)/(2.4 A) = 0.83 ohms > > http://www.mtaonline.net/~hheffner/HullR9_4.2ohm_2.4A.jpg > > - - - - - - - - - - - - - - - - - - - - - - - - - - - - > > For Run 10 R1 was still at 4.2 ohms. Current was 2.13 A. R10 was run > at 1430 rpms. The voltage drop looks to be about 3 V, with nominal > spikes. This gives: > > Rmotor = (3 V)/(2.13 A) = 1.4 ohms > > http://www.mtaonline.net/~hheffner/HullR10_4.2ohm_2.13A.jpg > > - - - - - - - - - - - - - - - - - - - - - - - - - - - - > > This is pretty weird. The voltage drop remains about constant as > current increases, so the apparent resistance drops. The effect at this > low current does not appear to be rpms dependent, except for resistance > when stopped, which is erratic. > > > Best regards, > > Horace Heffner > http://www.mtaonline.net/~hheffner/ > > > >
