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I have a braking resistor attached to a system with a large rotational inertia. Everything works fine in terms of startup, rundown and normal operation. I would, however, like to investigate whether I could in reality install a smaller breaking resistor.

I do acknowledge the math behind the initial dimensioning of a given system, but would like to measure how much is "left in the tank" of my breaking resistor. Both during start up, rundown, and especially during normal operation (in my case, the breaking resistor breaks the system for each and every cycle that the flyweights rotate due to a very specific control requirement). Is this possible? Measure amps? Temperature? Other things?

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  • \$\begingroup\$ Measure peak electrical power. Measure how long that power has to be dissipated for, measure how hot it gets. Estimate the heatsinking provided by the chassis to the resistor and do some calculations to see what is possible. \$\endgroup\$
    – Andy aka
    Jan 20, 2020 at 14:23
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    \$\begingroup\$ Compare rotational kinetic energy [W] with Resistor Power [W] and its thermal resistance ['C/W] , compare with driver thermal resistance and Resistance ratio to look for opportunities to improve. Compare Brake/Driver pair to keep drivers cool and measure brake R margin in 'C margin to design limit. (e.g. 150'C).. Just don't ;) "break your brake" R and add forced air cooling 1m/s over surface. \$\endgroup\$
    – Tony Stewart EE75
    Jan 20, 2020 at 14:39

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Compute the thermal capacity of the resistor's mass. Pure silicon is 1.6 picoJoules per degree C per cubic Micron (yes--cubic micron).

A 1,000 watt resistor, mostly made of porcelain with resistive-element around the porcelain core, likely has about 0.1meter^cubed volume.

This is 100,000microns^Cubed, or 10^15 cubic microns.

Multiply 10^15 cubic microns by 1.6 picoJoules per degree C per cubic micron, and your heat capacity is a 1,600 joules/degree C.

If you dump 100 joules into that resistor/core, the resistor/core will heat up by only 1/16th of a degree. The thermal timeconstant, given avout 1cm thickness of the porcelain core produces 1.14 second TAU .... is 1.14 seconds.

Again, your resistor can store lots of energy, with a 1.14 second thermal time constant.

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