paper: 4 CIM versus 6 CIM theoretical calculations

[asid61]

Originally Posted by cbale2000

I'm aware that mechanically it's not critical, I was just curious as to how it affects the efficiency of the overall system. Presumably the MiniCIMs would be working harder since they would be trying to pull the other motors to a higher RPM, but I'm not sure if this would make enough of a difference in motor current to really matter.

I think a mix of MiniCIM + CIM at a 1:1 ratio could work fine. The MiniCIM having a free speed about 10% higher and having a lower stall current (and therefore a higher resistance) than the CIM means that when they're running at the same RPM (say, 1,500 rpm) the CIM draws 95 amps and the miniCIM draws only 66 amps. That approximately matches up with their weight proportions, which is analogous to heat capacity, so both of the motors will heat up at the same rate. That means no dead motors.

The closer you are to stall, the more the CIM:MiniCIM heat ratio goes down (which is bad), but not by a large amount. The ratio gets closer to 1:1 as you go nearer to free speed as well, but by the time you are running that fast the heat generated isn't too much anyway.
It's possible to gear the MiniCIM such that it's always running at a higher RPM than the CIM, perhaps by using 11t and 12t pinions, but it probably isn't necessary. Maybe a team that has done CIM + MiniCIM combos can chime in here?


I wonder if using higher-resistance cables to increase your resistance would be a valid strategy to help prevent brownout of 6+ CIM drivetrains.

EDIT: The numbers for a 2 CIM + 2 MiniCIM drivetrain are as follows:
Approximated as 3.33 CIMs
2,931 theoretical rpm
333 W/CIM
1,108w total, compared to 1,280w for a 4-CIM drive, or about 86% as efficient. Not too great of a drop if you're running a significantly lighter robot and don't need the power.