# how hot is TOO hot for a controller ?



## dtbaker (Jan 5, 2008)

I have my controller mounted on about a foot square finned AL heatsink... no fans. Most of the time it is barely warm to the touch; but when I do an errand on the west side of town I have a steady grade on the way home for a good 5 miles.... and by the time I get home the controller is pretty warm.

I am guessing its about 110-115 deg F since I can leave my hand on it, but not comfortably for long.

At what point does a controller's efficiency, or life, go down as it gets hot?

I am contemplating ducting some outside air over the heat sink... figuring that when in motion under load it would pull a little more heat if ambient air was flowing. Right now I do not have a belly pan or grill block installed, also the grill is pretty well blocks from the wall of batteries... I am concerned that if I seal up the belly, it'll get even hotter without ducted airflow.


----------



## etischer (Jun 16, 2008)

If it's too hot to touch, that is generally too hot. I would just stick a fan on the heat sink. Ducting may help, but you will also be blowing dirt and rain on your heat sink. 



dtbaker said:


> I have my controller mounted on about a foot square finned AL heatsink... no fans. Most of the time it is barely warm to the touch; but when I do an errand on the west side of town I have a steady grade on the way home for a good 5 miles.... and by the time I get home the controller is pretty warm.
> 
> I am guessing its about 110-115 deg F since I can leave my hand on it, but not comfortably for long.
> 
> ...


----------



## Tesseract (Sep 27, 2008)

dtbaker said:


> ...
> I am guessing its about 110-115 deg F since I can leave my hand on it, but not comfortably for long.
> 
> At what point does a controller's efficiency, or life, go down as it gets hot?


Without knowing the total thermal resistance from the semiconductor junctions to the heatsink/case there's no way to predict whether a case temp of 120F is too hot or not.

That said, my feeling is that 120F is too hot for controllers like the Curtis, Kelly or Logisystems because they mount their semiconductors onto bars that "tee" into the case, so there is relatively little contact area to conduct the heat produced. Automatic thermal cutback on the maximum amps ensues relatively soon in these controllers because of this (extremely soon with the Kelly's - pretty much within a few seconds of operation).

Putting a big heatsink on the case - as you have done - will help slightly, but the real problem is inside. The thermal resistances here will likely be on the order of 1.5C/W for junction to case, 0.5C/W for case to heat spreader/bus bar, and then some unknown value between the heat spreader and the the case, but 0.1 to 0.5C/W is reasonable. Thus, if each semiconductor is dissipating 30W (a reasonable maximum for parts in the TO-220 case) then the relative temperatures at each point will be thus:

Controller Case = 120F = 49C
Bus Bar = (30W * 0.5C) + 49C = 64C
TO-220 Case = (30W * 0.5C) + 64C = 79C
Junction = (30W * 1.5C) + 79C = 124C

124C is too hot a junction temperature for plastic packaged devices such as the TO-220, btw; 100C is the recommended maximum. Is this what is necessarily happening inside your controller? Who knows, but a fan certainly couldn't hurt. You might notice some more pep from it, too.

Oh, and the rule of thumb for semiconductor and electrolytic capacitor life is that every 10-11C rise cuts lifespan in half. This is a much more serious problem for the capacitors inside motor controllers, rather than the semiconductors, because the capacitors have much shorter lifespans to begin with: 2000-5000 hours at 85 or 105C is typical.


----------



## dtbaker (Jan 5, 2008)

Tesseract said:


> ...the rule of thumb for semiconductor and electrolytic capacitor life is that every 10-11C rise cuts lifespan in half.


very interesting... and a good reason to pursue additional cooling... I will put this on the list of things to do sooner rather than later...

thx


----------



## grayballs (Aug 27, 2008)

I've spent a good portion of the last 20 years, building large chiller systems for computer rooms. 
The guys on the hardware end of those things, tell me that they can measure, in $, (I suppose that is time and replacement parts) the effect of 10 DegF. 
In these cases, 80 DegF is the alarm point, and they prefer 60 DegF or lower.


----------

