Mini CIM drawing 4 times the current of reg CIM

We have WCP 2S gear boxes with a CIM and a mini-CIM attached. When we are running the robot, all the mini-CIMS are pulling about 4 times the amps of the regular CIMS.

We are using 12 - 11 tooth gears on CIM - mini. The motor controllers are SRX’s

This is unsustainable due to the total current draw.

Anybody know what we could be doing wrong? Appreciate it.

First off, I hope you aren’t talking about running your competition bot at this point, since it is supposed to be sealed in a bag.

Have you checked to make sure that your mini-CIMs are going in the same direction as your CIM motors? If the motors are fighting each other, you would essentially be running the mini-CIM at stall.

A good way to check that is to unplug all the mini-cims and see if the robot is faster. Then plug in all the mini-cims and unplug the cims and see if the robot runs backwards.

Another way is to plug in one motor at a time and check wheel direction, in all cases direction must be same.

This is a good way to check, but it’s unlikely that miniCIMs running backwards would be the cause. If you’re giving the same voltage to the CIMs and miniCIMs in different directions, the system would run slowly in the CIM direction and the CIMs would draw more current.

Honestly, I’m not sure what the cause could be. My first thought was bad wiring to the miniCIMs, but the increased resistance would actually mean less current going to them.

I’m assuming that both of the Minis are drawing excessive current, since if it was just one I’d say it is likely that you have a bad one. Of course it is possible to get two bad ones but we will set that aside for now and assume all motors are good. Are you using the same model motor controller for all drive motors?

1st step is to look at the blinky lights on the motor controllers. Are both blinking at the same rate and color? Are both motor controllers set to brake or coast? Are you using a Y PWM cable or sending separate signals to each controller? If you are not using a Y cable then adding one into the circuit is the next step. Pick one PWM cable and disconnect both ends temporarily and then use the Y cable to connect both controllers to the other PWM signal. What happens now? Then try it at the other PWM port that you were using. What happens now? Does everything respond the same no matter which PWM port you are using?

If using the PWM Y cable fixes things then it is a software issue.

Thanks for the replies.

This, BTW, is on a backup duplicate robot.

We have watched the SRX’s lights and they are both running same color green lights.

We have them running, up on blocks, with no load and still more draw on minis.

Still perplexed as to why the miniCIMs on all four gearboxes draw such a heavy amperage

I am not aware of this happening with other bots.

Tomorrow, we will try running the bot without the minis and see what different speeds we get. If there is not much difference we will just run without the miniCIMs

Just to make sure I understand this statement; you are running 12 tooth on the CIMs and 11 tooth on the Mini CIMs, correct?

What is the actual current on each motor? You say 4 times but that really doesn’t tell the whole story.

For example, if you run 11t pinioned on both motors and put the robot on blocks then there is a good chance the CIM motor will not be drawing any current. It will be contributing negative current (yes, acting as a generator).

The mini’s free speed is 900ish RPM faster than the CIM so it having higher current at certain loading conditions is normal.

They will almost never draw the same current all the way from free speed to stall.

I need to know the actual current being drawn to know if you’re situation is normal or not.

For what it’s worth, we designed the mini to be able to run with the same gears as a CIM. We selected the design point to be matching speeds at about a 15% load condition. There was nothing magical about it decision but matching free speeds is not a requirement and is my last choice.


You may be correct that the CIM would draw more current in that situation, but it’s not immediately obvious that that’s true.

When the MiniCIM is being driven in the opposite direction that it’s being powered, the back emf voltage will
be in a direction to cause additional current to flow through it. You’d have to do the math to determine whether the CIM or MiniCIM would be drawing more current.

We are definitely doing that. We have the 12 tooth on the CIM and 11 on the Mini.

When we were driving it in on floor the CIMS were running around 1.4 amps while at the same time the Mini’s were pulling around 4.7amps. That was the same pretty much on all four of the gear boxes. That does not seem right at all.

Those are comparatively tiny current draws, and so Paul Copioli’s explanation seems very likely correct.

Try pushing against a wall, and see what current numbers you get then.

That’s exactly what i would expect given the conditions you have laid out.

This is operating well within the operating range for both motors. I can tell you that the minis are over driving your CIMs still. I bet if you push against the robot the gap between the current draw will reduce and, eventually, the current draw on the CIMs will be much higher than the minis.

Can you let me know what size gear they are both driving and what the rest of your gearbox ratios are? I can lay out an entire current scenario knowing that information.

Let’s do a rough calculation at 12 volts, and 1500 rpm (because that’s where the torques balance)

R = 12/133 = 0.090 Ohms
V = 12 - 1500/5310*12 = 8.61 Volts
I = V/R = 95.4 Amps
Torque = I/133 * 343.4 = 246.4 ounce_inches

R = 12/86 = 0.140 Ohms
V = 12 + 1500/6200*12 = 14.9 volts
I = V/R = 106.8 Amps
Torque = I/86 * 198.26 = 246.2 ounce_inches

So the MiniCIM would be drawing more current.

Here’s a simpler way to look at it.

The CIM draws 133 amps to produce 343.4 ozin of torque.
That’s 0.387 amps per ozin.

The MiniCIM draws 86 amps to produce 198.26 ozin of torque.
That’s 0.4338 amps per ozin.

So the MiniCIM draws 12% more amps for a given torque.

If you hook them up in opposition and power them up, at equilibrium the torques must balance, so the MiniCIM will be drawing 12% more amps than the CIM.

Given what PC has said about motor free speeds, my guess is that you’d be better off putting the 11T spur gear on the CIM and the 12T spur gear on the MiniCIM. I don’t think that this would explain your current discrepancy, but would be a better application of the gears you have on-hand than what you’re currently doing.

FWIW we’ve treated CIMs and MiniCIMs as the same in drivetrain applications with great success, but never dove into the details of current draw per motor.

**Bolded **for emphasis. This is exactly the intended use case of miniCIMs. We designed them to be interchangeable with CIMS. As a matter of fact, my team uses miniCIMs instead of CIMs in most cases.