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  #1  
Old Mon 08 October 2007, 18:35
Tom Caudle
Just call me: Tom
 
Texas
United States of America
Using a metal enclosure for the electrics vs "plastic" enclosures

Cut from another thread:

I hate to pee on the fire here but you need to understand how heat gets transferred and why airflow is important. When an object heats up it radiates heat or passes it through convection to an object cooler than itself.

The source of the heat in a Gecko is the small die inside the FET's and the voltage regulators. It is concentrated at the points those devices attach to the back plate. It has a thermal transfer rate between it's tab and the die (not 100%) and then between the tab and the back of the Drive module (even lower). From there it has to couple with the cooler surrounding air via some form of heatsink. If the back of the plate is at 130 deg F the die itself will be a lot hotter. It's all about making the heat move away from the Die as quickly as possible.

What happens next is important. As the heatsink transfers its energy to the surrounding air, a layer of hot air forms just off the surface of the heatsink that acts as a thermal blanket. If the heatsink is oriented vertically natural heat convection will cause the hot air to migrate up faster and be replaced by cooler air up to the point where the air up top is so hot it too starts to act as a barrier. See, dead air is a really poor heat conductor. The hotter the layers of air the worse it stratifies and less convection there is.

Just increasing the size of a heatsink (total surface area) does not give you a proportional decrease of the semiconductor junction temp. The further the junction is away from the transfer area the less efficient. The most efficient part of the heatsink is the spot closest to the source. The thermal transfer of the heat laterally through the heatsink structure works a lot slower.

Giving the design lots of room to prevent localized heat build up and stratification is one way but space comes at a cost. Bigger heatsinks comes at a cost.

It's amazing what a small amount of forced air cooling will do for the equation. It breaks up the insulating layers of hot air and improves the heat transfer ratio by a factor of many times. Just a small amount does wonders. We aren't talking 3%; we are talking 300%. Being able to bring in external air and vent the heated air out makes everything run cooler

Yes, fans fail but good quality commercial fans don't usually just roll over and stop. They start making lots of noise first.

Also don't ignore the other heat producers in the cabinet. The transformer will self heat even with small loads. The bridge will heatup as will things like the drain resistor on the caps. Bottom line is that over 50% of the energy overall will go up in heat and a lot of that is concentrated in the controller.

If your intent is to build a bullet proof power controller, then perhaps an internal heat monitor to detect the failure of a fan or the clog of a filter would be a good approach versus trying to make a closed box big enough to work under all conditions. We will be introducing a "smart" power controller very shortly that has that feature (measures the temp of the heatsink) as well as other features that are just as important.

I know I have an "agenda" since I sell CNC stuff but the experience comes from years of design work in power systems to 20KW. If it runs in my shop in Texas in August it will run anywhere (:-)

Now, I'll shut up and go home...............
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  #2  
Old Tue 09 October 2007, 09:23
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Tom, let's keep this short . . . . . . .will your control box work maintenance-free for a year in a dusty shop? Totally hands-off?

Ours does.
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  #3  
Old Wed 10 October 2007, 08:55
Tom Caudle
Just call me: Tom
 
Texas
United States of America
Quote:
Originally Posted by Gerald D View Post
Tom, let's keep this short . . . . . . .will your control box work maintenance-free for a year in a dusty shop? Totally hands-off?

Ours does.

Short answer: NO

Neither does my Ford Truck or even my shop PC (controller). I have to stop and clean my linear slides and lube the leadscrews every 16 to 24 hrs of use. On my plasma table the maintenance intervals are shorter given that the smoke from plasma is vaporized metal and conductive. Even with extraction there is enough to deposit a fine film on everything. Lost one PC last year and it's in an enclosure.
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  #4  
Old Wed 10 October 2007, 09:03
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Thanks. And that explains why we don't use leadscrews and linear slides either.
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