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120A Main Breaker Thermal Analysis by: Ryan_Todd

Detailed thermal analysis of the standard 120A main breaker, especially addressing the question of whether or not there is any practical benefit from using canned compressed air to pre-cool the breaker before the start of a match.

The following analysis was the result of a discussion that ran from March to April of 2016, while various members of the Chief Delphi community debated the effect of using canned compressed air to pre-cool various parts of a robot before the start of a match.

We ran around in circles on the topic of using this method on a CIM motor; it would of course cause thermal stress and warping, which could in turn result in power loss due to demagnetization, and even a risk of catastrophic failure if repeated thermal shock cycling causes the ceramic magnets to shatter. At the same time, however, specific examples of these failures were hard to find; additionally, many teams had used the method without noticing any significant detrimental effect. Perhaps Al Skierkiewicz summed it up best: many teams won’t notice any negative effects from this method, but the practical benefit is also minimal.

Once that discussion wrapped up, the question remained of whether or not the same method would have any benefit if used on the standard 120A main breaker before the start of a match. With only one moving part (especially since that part’s entire purpose is to deform under thermal stress), might this be a better application of the canned-air cooling technique?

My original contribution to this discussion included an incorrect assumption about the series electrical resistance of this breaker, however, and so the resulting numbers were off by a factor of 6; since I can no longer edit that original post, this white paper includes a couple of corrections based on the responses I received, and everything seems to pass the sniff test now.

First of all this paper was extremely helpful in building an accurate (I hope) breaker model (more on this in a sec). Thanks so much.

I think found a minor mistake in your paper, if you look at https://imgur.com/QnEqnscl.png You’ll see that you substituted 8 for C when it should be .08. Thus, your figure for specific heat is off by a factor of 100. 0.002716 degC / Joule is what you reported, .2716 is correct. This error cancels out because the only place you use it is in Cct. As C is 100x too high and c is 100x too low they cancel. So https://imgur.com/MDcYwuml.png is correct. This mattered for me because I was using purely the differential equation with numerical integration and I put in the correct value for C.

Again another minor flaw. I think your math around the benefits of precooling the breaker is slightly flawed. You don’t take into account the thermal mass of the surrounding housing which would also be substantially cooled effectively modifying the ambient temperature. Thus, if the housing was cooled and potentially the metal the breaker was attached too, it is possible that the benefits could be slightly higher than you state. Maybe, (based on some napkin math) instead of lasting .4 additional seconds longer it lasts 1.4 additional seconds.

That was left over from the original calculations, and it had an even bigger impact on the final result than you described. Because the factor-of-100 difference was in an exponent, it threw off everything else completely!

This prompted me to go back and triple-check all my sources and calculations, and I found another issue with my Fahrenheit-to-Kelvin conversion at the start of the paper as well. (d’oh!)

Long story short:
I posted an update (rev.1) with all the latest corrections.

Very true!

My analysis is limited by what information I could glean from the internet, and it would be invaluable to (for example) bring in a proper CAD model and run some finite element (or even CFD) simulations and get some more useful data about the different paths that the heat can take in this system.

If anyone wants to take this analysis further, I’d love to see the result! ::rtm::

This happens with larger breakers for AC applications and it is very likely to happen with this smaller DC breaker. To confirm proper operation, we test using the enclosures for the final product with the maximum ambient temperature. Sometimes, we have to go up one rating to prevent false tripping.

Have you contacted the manufacturer, Cooper Bussmann, to get their input on how to do such calculations? They have Applications Engineers who are paid to answer questions such as these. For many electrical components, there are different thermal models for continuous operation and for transient operation. They may be able to provide you with such models. Often the models provided by the manufacturers incorporate proprietary data (fudge factors) that they have determined to be important to the accuracy of the models. This data comes from doing a lot of testing and comparing with the models and are not readily available otherwise.

You would also want to ask how much variation in the trip point one can expect to see over different production batches and if there are any other effects on the trip point such as aging. This will determine how close to a calculated trip point you can push your application.