Blown CIM?

[Ether]

Originally Posted by evand4567

Quick question: is it possible to blow a cim motor

Yes, and this is how:

We have a couple cims that were stalled a number of times for a non-trivial amount of time.

how would you test it to see if its performance has degraded?

Measure the free current. This is easy to do. All you need is a $4 Harbor Freight digital meter.

Measure the free speed. You could do this with a calibrated strobe light or a special point-and-shoot rpm measuring tool. Or an encoder or tachometer.

Measure the stall current. You could use that $4 Harbor Freight meter to measure the voltage drop across a length of wire of known resistance. Or use the PDB current monitor.

Measure the stall torque. Put a torque wrench on the CIM shaft?

[Ether]

If you have 2 known good CIMs and a 2-motor gearbox, you could try the following:

1)

If you have a suitable encoder, install it on the gearbox.

Install the two good CIMs into the gearbox (with no load on gearbox). Run CIM1 with full voltage, and measure the voltage output of CIM2 (unshorted leads). Record the CIM1 applied voltage voltage, the CIM2 output voltage, and the encoder RPM. Now repeat with CIM2 powered and measure CIM1. Record those measurements. This is your baseline.

2)

Replace CIM2 with CIM3 (suspected bad). Repeat #1 and post your measurements here. Replace CIM3 with CIM2, and CIM1 with CIM3. Repeat #1 and post those measurements here.

[Ether]

Originally Posted by Ether

If you have 2 known good CIMs and a 2-motor gearbox, you could try the following:

1)

If you have a suitable encoder, install it on the gearbox.

Install the two good CIMs into the gearbox (with no load on gearbox). Run CIM1 with full voltage, and measure the voltage output of CIM2 (unshorted leads). Record the CIM1 applied voltage voltage, the CIM2 output voltage, and the encoder RPM. Now repeat with CIM2 powered and measure CIM1. Record those measurements. This is your baseline.

2)

Replace CIM2 with CIM3 (suspected bad). Repeat #1 and post your measurements here. Replace CIM3 with CIM2, and CIM1 with CIM3. Repeat #1 and post those measurements here.

3) Install the 2 good CIMs (CIM1 and CIM2). Short the leads of CIM2. Apply full voltage to CIM1 and record the encoder RPM. Replace CIM1 with CIM3 (suspect motor) and repeat. Record the encoder RPM. Post your measurements here.

[GeeTwo]

Originally Posted by FrankJ

Another possibility would be to run it with a known load & compare its speed & current draw against a new motor.

Originally Posted by Ether

Easier said than done, if you don't have a dynamometer.

You can compare a questionable motor to a known good motor by using the same load for each test. You don't necessarily have to know what the numerical value of the load is.

For example, if you have a CIM-powered elevator lift you can measure the lift time for a good motor and a suspect motor (using the exact same throttle command each time), and compare the results.

If you have a CIM-powered lift lying around (and for some strange reason a lot of teams do right now), you can use it to "measure" the stall torque by finding the weight of a load that you can just barely lift. You can also use it to evaluate performance near the maximum power by lifting approximately half of the stall weight and clocking the lift time. I fully concur with Ether that the simpler tests would be to compare your suspect CIM(s) to a population of known good ones, rather than trying to work through all of the gear ratios and possible sources of friction.

Also measure free speed and curent, as Ether put in post #1. Unfortunately, I don't think you can get a good stall curent measurement with the PDB as a CIM has a typical stall current of 133A, and IIRC, the current meter saturates at about half of that. Even if the limit is high enough, be sure to use a motor controller, as powering a motor directly from the PDP can damage the current sensors. Especially when you do the stall measurements, try to use the same battery in the same state of charge and the same wires for all the CIMs you test.