# Thread: Equipment / stirring Fans in a VFD panel

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## Equipment / stirring Fans in a VFD panel

I was talking to one a certain individual and he said he uses a certain simple formula for identifying once a VFD panel will not be cooled enough by stirring fans (fans simply mounted inside closed enclosure with no air access to exterior) based on the size of the enclosure and the power rating of the VFD and maybe a couple other auxilary components I'm guessing.
Is there a certain formula methods used for this type of calculation?
At what point should forced ventilation be a concern?

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Originally Posted by Ramos617
I was talking to one a certain individual and he said he uses a certain simple formula for identifying once a VFD panel will not be cooled enough by stirring fans (fans simply mounted inside closed enclosure with no air access to exterior) based on the size of the enclosure and the power rating of the VFD and maybe a couple other auxilary components I'm guessing.
Is there a certain formula methods used for this type of calculation?
At what point should forced ventilation be a concern?

There is no simple formula that could possibly work that does not take into consideration what the exterior ambient temperature is and what the internal operating temperature limit of the VFD is. Not all VFDs of a given power rating will create the same amount of waste heat a full power, and even then it might depend on the details of the load it is driving. At best a formula can let you calculate how much heat must be rejected from the enclosure when the inside surface is at the maximum allowable temperature. Then you can calculate what ambient air temperature will allow that. If the ambient goes higher, you need ventilation. (Although a fan blowing on the outside of the enclosure might still work.

What is possible is a "rule of thumb" calculation based on experience and requiring a safety factor of two or so. That can provide a good starting point, and the finished installation can be tested to see whether it was right.

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Originally Posted by Ramos617
I was talking to one a certain individual and he said he uses a certain simple formula for identifying once a VFD panel will not be cooled enough by stirring fans (fans simply mounted inside closed enclosure with no air access to exterior) based on the size of the enclosure and the power rating of the VFD and maybe a couple other auxilary components I'm guessing.
Is there a certain formula methods used for this type of calculation?
At what point should forced ventilation be a concern?

You can calculate it based on the surface areas available for cooling. For example, if your enclosure is wall mounted, the back panel can't be included. Most panels are painted mild steel and companies like Rittal or Eldon have information on this.

I did my own spreadsheet with a simple yes or no on which surfaces to include.

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Hoffman has a formula in their catalogue for this. IDR if I’ve put it into one of my spreadsheets but as B indicates, it certainly would not be difficult

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A stirring fan doesnt do anything. If properly installed according to the manual a vFD will circulate the air around it. Everything else is natural convection...hot air rises, cold air sinks, just like an oil filled transformer. The big limit, and Hoffmann as mentioned has formulas for this, is it's hard to radiate much off a panel. VFD heat load is easy. Multiply the maximum input in watts or if not given maximum continuous input rating x volts x 1.732 by the losses which is 100% minus the efficiency which will be in the specs too. Don't forget to add in all the other heat sources like any and all transformers and reactors/filters which you figure the same way.

This is conservative...worst case. If you won't be running the VFD wide open and you know the efficiency at points other than peak power (you won't) you can design for other conditions but in those cases you're basically guessing or taking measurements and hoping the process never changes.

Usually the calculation tells you that you will need external fans or thermal electric or air conditioning coolers.

Another approach is using flange mount VFDs. The heat sink and fans on the VFD are mounted on a plate on the back of the VFD. You cut a big hole in the enclosure and mount the VFD so that the heat sink is on the back or side. The only thing left in the enclosure is the control board and wiring.

Along the same concept and this is strictly in space limited situations large (1000+ HP) VFDs can be water or oil cooled so the radiators go outside. The cost is much higher than air cooled so this is only done when space is a premium. I used this on a set of 5 VFDs running 2000+ HP motors on a 4 axis servo system, 5 motors per axis in parallel, with roughly 12-15 MW peak power. It was an excavator so space is a premium. Running air ducts that big would not be practical compared to a couple large fan coooed radiators.

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Originally Posted by paulengr
Another approach is using flange mount VFDs. The heat sink and fans on the VFD are mounted on a plate on the back of the VFD. You cut a big hole in the enclosure and mount the VFD so that the heat sink is on the back or side. The only thing left in the enclosure is the control board and wiring.
Interesting approach..............

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Originally Posted by Besoeker3
Interesting approach..............
Danfoss is big on this. Only certain models are compatible.

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Originally Posted by drktmplr12
Danfoss is big on this. Only certain models are compatible.
We built some VSDs for a naval application. They were outdoor mounted right on the dockside. They had to be weatherproof but we still had to dissipate the heat. We mounted the power semiconductors inside the IP54 panel and copper heat sinks on the outside of the back panel

Just one of many non-standard applications we got involved in.

9. Fins-out is the best first step for sure. Move the majority of the heat out of the box.

Most large drives have internal stirring fans in their packaging already. Adding more is kind of pointless. But if you are talking about really small drives that are convection cooled, it will help a little bit. I have seen some that say you can get an additional 10deg C ambient by adding a fan that blows onto the control board (PowerFlex 520 goes from 60C to 70C by adding a stirring fan after derating). But that is a specific tested value, not a formula. YMMV.

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