justin_le said:So what I want to do instead is generate a good ballpark set of thermal parameters for a hub based only on easily measured values. Ideally you could feed in the mass of the motor core, the mass of the hub shell, the diameter and width of the casing, and out comes a decent first order guess for the heat conduction coefficients from the core to the shell, and the shell to ambient, as well as the heat capacities both of the Stator core and the shell.
Our first step in this was the heat capacity values. In the experiments with the 9C motor, I never actually measured or even computed the expected heat capacities of the core and the shell. Rather I played around with the two heat capacity terms in in the model until it fit to the data. The steady state temperature is independent of the heat capacity, so we used that to get the thermal conduction terms, and then adjusted the core and shell heat capacities until the temperature slope rises matched.
I've been really curious to know how well this "model derived" heat capacity compares with a measured value, but it turned out to be trickier than expected to make a DIY calorimeter that was nicely repeatable. Our initial thinking was that we'd put the motor parts in boiling water to get them at exactly 100oC, and then dunk them in a styrofoam cooler with a known mass of room temperature water in it, shake things around, and then from the initial and final temperatures of the water bath we'd know the total heat transferred over.
However, we were only getting at best 5-10% reproduceability when testing the same motor. In part this was because as the motor was lifted out of the boiling water,it was still dripping wet with water, and in the process of shaking it off we'd also be encouraging the evaporation of the water from the surface which would quickly cool the core, so we never knew the exact average core temperature when it was dunked into the water bath. We also found that much to our surprise that all the styrofoam coolers that we had actually leaked between the foam beads. So after a few tests there was a puddle of water building under the cooler and we didn't know at any time how many water was trapped between the beads of foam and not contributing to the heat capacity tests.
So in the end, we lined the cooler with a plastic bag to deal with leakage, and then glued a thermistor to each of the motor cores and side plates. Rather than relying on boiling water to heat the cores up, we could then use a convection oven, and not have to worry about evaporative cooling during the transfer from the oven to the bath.
Our goal here was to measure the specific heat capacities of a number of existing motor cores and motor shells, in order to see how much spread there was between different hubs and come up with a good average value that we can use to derive effective heat capacity by just weighing the motor. The motor components are made from iron, aluminum, and copper, and these have theoretical specific heats as follows:
Iron: 0.44 J/gK
Aluminum: 0.89 J/gK
Copper: 0.39 J/gK
So the actual specific heat capacity would depend on the ratio of these metals in the motor parts, which shouldn't vary too much from motor to motor.
All told we had 7 direct drive motors of various vintages for testing