Thermal tests on encapsulated Motobrain

February 3, 2014 in 8 Channel PDU by Joshua Oster-Morris

I ran a test of the encapsulated Motobrain to confirm the thermal characteristics in its new potted state. I started the test by turning it up to 50A for 30 minutes. I took a measurement and then turned it up 10A and set a timer for 10 minutes. After 10 minutes I took a measurement, reset the timer, waited, took another measurement and then turned it up 10A. So it ran for 30 minutes @ 50A, 20 minutes @ 60A, 20 minutes @ 70A, 20 minutes at 80A, 20 minutes @ 90A. The next step was to turn it back down to 50A and check it every 10 minutes for 40 minutes to see how long it takes to get back to a similar temp as before. The ambient temp in the lab was 19C when I began and was 26C when I finished the 90A test because the power supply and DC load are really hot creating and dissipating all that current (so I can add 7C to the first 50A reading and say I achieved a baseline reading which I failed to achieve in 40 minutes). I then let it run with no load for 10 minutes and called the test complete. Results are below.
I took a few internal temp sensor readings along the way. The internal temp sensor is on the far side of a second PCB from the power transistors and the potting compound is not a great conductor of heat so I expect a large temperature delta. When I pot one of these in thermally conductive epoxy I expect to see much greater correlation between the two numbers. When no current is flowing, I would expect the external and internal temps to be roughly the same.

One edge of Motobrain is bolted to a table with most of it hanging in still air. See the last post for a photo of its location in space.

50A 30 minutes (internal temp sensor was 25C)


60A 10 minutes


60A 10 minutes


70A 10 minutes


70A 10 minutes


80A 10 minutes (internal temp sensor was 41C)


80A 10 minutes


90A 10 minutes


90A 10 minutes (internal temp sensor was 51C)


50A 10 minutes


50A 10 minutes (internal temp sensor was 41C)


50A 10 minutes (internal temp sensor was 39C)


50A 10 minutes (internal temp sensor was 39C)


0A 10 minutes (internal temp sensor was 35C)


Test completed. The results are suspiciously like my previous unencapsulated tests where 80A was in the low 60s and 90A was in the low 70s. This tells us that the epoxy is not significantly worse than nothing. The final 0A measurement shows that the internal temp sensor and external temp sensor are well correlated to each other and that there is a pretty significant thermal gradient inside Motobrain when it is running. When I get the fancy (and expensive) thermally conductive epoxy we should be in good shape!