Do CIM elevator motors overheat?

[AdamHeard]

Originally Posted by Ether

Everything Adam and Thad said, plus this minor item, which may be key to understanding:

If you're commanding the motor controller via PWM, you're not commanding power, you're commanding output PWM duty cycle.

If the motor controller is linear, then 10% command will create 10% output duty cycle which will effectively cause 10% of the battery's voltage¹ to appear at the motor's input, which -- if the motor is stalled -- will create 10% of the motor's stall current to flow through the motor which will create 10% of the motor's spec stall torque.

The electrical power being put into the motor is Pin = current*voltage, which will equal (10% of spec stall current)*(10% of battery voltage) which will equal 1/100th of the input power at motor stall at spec voltage.

That's why the motor doesn't overheat.



¹Let's assume, for sake of simplicity, that the battery is a constant 12 volt source (same as motor spec voltage).

2The output mechanical power at the motor's output shaft is Pout=shaftSpeed*loadTorque. When the motor is stalled, shaftSpeed is zero so Pout=0. Therefore all the input power Pin is dissipated inside the motor as heat.

Good catch, I overlooked that his question was more related to the motor controller's "10%" output than to the motor itself.

[Alan Anderson]

Originally Posted by Conor Ryan

But if your cim's are running hot, you can always add one of these: http://store.nextgenrobots.com/products (hint, use some thermal paste, its legal), or just a couple of fans.

Unless you can fill the CIM itself with some sort of thermally conductive (but electrically insulating) fluid, putting a heat sink on its case is less effective than you might think. The power is dissipated in parts that have very little ability to transfer the heat to the outside. To make its way to the outside, most of the heat needs to go through the motor shaft.

[Ether]

Originally Posted by Alan Anderson

Unless you can fill the CIM itself with some sort of thermally conductive (but electrically insulating) fluid, putting a heat sink on its case is less effective than you might think. The power is dissipated in parts that have very little ability to transfer the heat to the outside. To make its way to the outside, most of the heat needs to go through the motor shaft.

Richard Wallace has conducted and reported some excellent tests documenting this. I'll post a link later unless someone else beats me to it.

[mathking]

We tested the CIM coolers last year and found that while they had a small (but consistent and statistically significant) effect in keeping the motor from heating up too much (on the order of 5%), they aided quite a bit in cool down time. Particularly if a fan was included. I wish our design had been better and we had included them on the final drive train. I have a friend who design motors for a living who explained that while most of the heat is dissipated through the shaft for a motor like this, the case obviously heats up so it is dissipating non-trivial amounts heat through the case. And that if you are operating the motor in a way that causes it to heat up excessively, you should try to redesign. But if that isn't possible even small amounts of increases heat dissipation can have a positive impact on motor performance and longevity.

[FrankJ]

One concern I would have with a stalled motor is the effect on the commutator. With the shaft even rotating slowly you are spreading the current over all the segments rather than the 2 the brushes are on with a stalled motor. Other that that--what Adam said.

[Ether]

Originally Posted by Ether

Richard Wallace has conducted and reported some excellent tests documenting this. I'll post a link later unless someone else beats me to it.

Here's one report I found:

http://www.chiefdelphi.com/forums/sh...13#post1215913