Mini Cim Drive Motors?

[Richard Wallace]

Originally Posted by Ether

Did you record the CIM RPM under those test conditions?

Yes, but that data doesn't matter for the question you posed: compare rates of temperature rise. Waste heat dissipated by the CIM and mini-CIM depends almost entirely on torque loading.

My test was intended to check Paul's design goal for the mini-CIM, which he has stated was to share load with a CIM (at 1:1 gearing) while taking about 2/3 of the CIM's torque; i.e., the mini-CIM takes 2/3 N-m for each 1 N-m taken by the CIM, so the motors share the torque loading about 60:40.

My results indicate that Paul actually exceeded his design goal () since the mini-CIM's internal temperature is rising more slowly than the CIM's while it delivers 40% of the total load torque.

Conclusion: if your drivetrain needs more power than one CIM can deliver, but not as much as two CIMs, then one CIM and one mini-CIM is an excellent combination. You can run them at the same speed, and mini-CIM internal temperature rise will NOT be a limiting factor.

[Ether]

For the sake of the discussion below, let's agree that the mini-CIM meets the stated design goal for FRC competition use, but pursue the heating question just a bit more.

Given these two motors, how would one gear them if the goal were instead to share the load in such a way as to balance the motor temperatures as much as possible?

There is no single answer. It depends on the location of the temperature you are trying to balance, and it depends on the duty cycle.

Using the CIM motor curves, at 27A at 12V (Richard's test), the CIM would be spinning at 4320 RPM and generating 119 watts of waste heat. Only 66 of those watts, or 55%, is due torque-producing current (I²R heating). The other 45% (54 watts) is due to eddy currents, hysteresis, internal friction and windage, etc, which also cause motor heating.

If the mini-CIM is constrained to the same speed as the CIM (1:1), then at 12V and 4320 RPM it would be generating 133 watts of waste heat.

Under these conditions, the mini would be producing just slightly less torque than the CIM.

So it appears at first blush that the mini-CIM would run hotter than the CIM under these conditions.

However, these numbers obviously change as the motor heats up due to changing coil resistance and magnetic properties. And the temperature has a distribution within the motor due to internal structure and the different sources of heat.

It's possible (but quite a bit of work) to model all these parameters accurately. With the right equipment and setup, a test would yield the answer.

For sake of discussion it would be interesting to include non-competition duty cycles. Are there any teams out there who have a CIM+mini 1:1 on a practice/demo bot? Is either motor hotter than the other after a good hard workout?

[Richard Wallace]

Originally Posted by Ether

It's possible (but quite a bit of work) to model all these parameters accurately. With the right equipment and setup, a test would yield the answer.

One possible test set-up could be a brake dynamometer, a two-motor gearbox (e.g., AM CIMple Box included in recent FRC kits), thermocoupled CIM and mini-CIM motors, and a laboratory dc supply capable of feeding the motors at 12 Volts, with combined current draw up to 84 Ampere. I have all that on hand. The CIMple Box ratio is 4.67:1. Test loads of interest might be 4, 5, and 6 Newton-meter at the CIMple box output shaft -- loads beyond that would require more current than my supply can deliver. At each load, the test procedure would be to record temperature rise until the brush guide reaches 125 degrees Celsius and compare the plots, CIM vs. mini-CIM.

Please check my figures above and suggest changes.

[Ether]

Originally Posted by Richard Wallace

Please check my figures above and suggest changes.

Sounds like a good test. Thank you for doing this.


I would suggest only 2 changes:

1) record the outside case temperature of each motor (in addition to the brush guides), and

2) add a lower torque (higher speed) test point of 2.5 Nm (at the gearbox output shaft).


Rationale:

1) Since motor case temperature is the metric most familiar to FRC students, it would be enlightening to record that and show how it does (or doesn't) correlate with brush guide temperature.

2) Higher speeds are where the mini might be expected to fare worse than the CIM. Useful to have a data point there.

[Richard Wallace]

Originally Posted by Ether

I would suggest only 2 changes:

1) record the outside case temperature of each motor (in addition to the brush guides), and

2) add a lower torque (higher speed) test point of 2.5 Nm (at the gearbox output shaft).

Good suggestions. Our previous tests (see earlier post on CIM temperature rise at peak mechanical power) included thermocouples at four locations: brush guide, internal air, motor case, and lab ambient. And I agree a lighter load point will illustrate the effect of differing free speeds when the CIM and mini-CIM share load at 1:1 gearing.

PM me if you want to come by the lab when this test set up is ready.