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
R1 is 48.4 ohms. The current in the 4 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 4 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 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. 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
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
