Back EMF, unexpected voltage jumps, and compensating
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The data is attached to the first post. Each voltage and RPM has two values,
- with Back Electromagnetic Force
- and adjusted for BEMF.
Originally, I simply set the transformer to 12V and measured the RPM. I compared it to the rated RPM and it was way off. A motor rated at 10,000 RPM 12v was running 14,760? Then I noticed the voltage jumped from 12V to 16.6V. Oh. BEMF effects. I left the leads attached and reduced the transformer until the meter read approx. 12V. Et voila! 10,380 RPM, close enough to 10K.
I re-tested all the motors at 2 applied voltages
- Voltage before attaching to motor (12V or as close as I could make it)
- Voltage/RPM after attaching to motor, measured V jumped or dropped, according to motor BEMF
- Voltage/RPM after raising or lowering voltage to about 12v:
The BEMF effect was dramatic with the new can motors. All raised the effective voltage. The open-frame Pittman motors dropped the effective voltage by about 10% and I had to increase the output of the transformer.
The current was measured at free running and stall, without adjusting for back emf:
- 12v measured before applying leads to motor,
- do not compensate for bemf, allow motor to run faster than specification, measure amps
- do not compensate for bemf, clamp the motor shaft to zero rpm with pliers, measure amps
As expected, the can motors used much less current than the open frame. I was surprised at the difference.
- Free running : Stall
- Open frame 0.347 : 1.1A
- Highest current usage from can motor 0.049 : 0.66A
Those poor little things got rather warm at stall. I clamped them very briefly to prevent the magic smoke from escaping.
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About: Greetings ! I'm Tanay, a hobbyist interested in making robots and sharing stuff. I hope that my instructables help you in solving your problems Happy Tinkering ! Note : Sorry for the inconvenience but I won't&#;
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