CIM motor starting current

[Richard Wallace]

Responding to a request from Ether in another thread, my lab measured the starting current of a CIM motor supplied directly from a fully charged FRC battery, with the shaft free. The resulting current surge reached about 120 Ampere peak within ~5 milliseconds, and settled to about 3 Ampere average after about 70 milliseconds. See the attachment for test details.

We will also measure CIM motor starting current with the shaft loaded by a dynamometer set for rated torque, and report those results.

[Al Skierkiewicz]

Thanks Richard. That was about my calculation (117 amps) for 2' of #12. Using .011 ohms for the internal impedance of the battery brings it to just over 120 amps.

[billbo911]

Not being an expert in motor behavior, I have a couple of assumptions/questions.

If this motor went through the same test, but this time with a constant load applied to the motor, would the differences in the waveform be:

1) The peak start-up current be approximately the same and with a similar rise time?
2) The settle out current would be higher and dependant on the load applied to the motor, but a longer period taken to reach a settled state?

[Richard Wallace]

Originally Posted by billbo911

If this motor went through the same test, but this time with a constant load applied to the motor, would the differences in the waveform be:

1) The peak start-up current be approximately the same and with a similar rise time?
2) The settle out current would be higher and dependant on the load applied to the motor, but a longer period taken to reach a settled state?

(1) About the same peak current, but slightly longer rise time due to mechanical loading.

(2) Yes. See attached data.