[Behind the Lines] Ep. 3 with Ken Stafford on Motors and Gearboxes!

[Jared]

For the calculation:

First, we must find the motor's current peak performance.
This is 41% efficient (given by datasheet). Assuming all losses are due to resistance of the motor (this isn't true, but I'm just trying to see if I can get anywhere close to the measured 30% difference), we can calculate the resistance R = v/I = 12/68 = 0.17647.

Copper has a resistivity constant of around 0.0039 per *C. So, the new resistance at a temperature 100 *C higher would be 0.17647*(1+0.0039*100) = 0.245 ohms. Our loss of power is proportional to resistance, and the ratio of the two resistances is 1.38, meaning there is a 38% loss of power at the higher temperature. This is higher than the actual value, because my approximation of the original resistance is too high. It could be more accurately measured if I had a motor in front of me.

[Ether]

Originally Posted by Joe Johnson

My criticisms should have been more temperate.

Hi Joe,

Just getting back to this thread.

Just to be clear, I wasn't criticizing your criticisms. Like Richard, I viewed them as to-the-point and thought-provoking.

One interesting follow-up:

I had a private dialog with Ken a few days ago about the method he uses in the spreadsheet to adjust the motor curves for off-spec voltages. He assumes that the free speed of the motor is a linear function of applied voltage, with a y-intercept of zero.

I used the same approach in the motor calculator I posted to CD several years ago, but I've never been entirely happy with it.

I suspect it's probably a good enough model for FRC work (especially considering the wide manufacturing tolerances), but from a theoretical or educational point of view it seems that this model is too simple. I would think the free speed vs applied voltage is not linear and does not have a zero y-intercept. The problem is, the model is likely different for the different types of motors (e.g. fan vs no fan), so a motor calculator would need more than just the 4 classic specs (free speed, free current, stall torque, stall current) to model it.

I plan to run a few tests once I get my hands on a couple of motors. Ken said he was interested in running some tests too.

I will of course post any data as it becomes available :-)