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
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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
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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
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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
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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
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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/
