## VFD Enclosure Passive Cooling

## VFD Enclosure Passive Cooling

(OP)

Hi everyone. I am designing stainless steel, NEMA 4X electrical enclosures for VFDs and to reduce costs, the goal is to use passive cooling only and dissipate heat through the enclosure itself. I've attempted to calculate the required surface area to dissipate heat through three different methods and received three different results.

1. Rule of thumb from Rockwell Automation which states A = (Watts dissipation)*8 (https://literature.rockwellautomation.com/idc/grou...)

2. Heat transfer conduction through a uniform material: Q = -k*A(T2-T1)/L, using thermal conductivity of SS for k = 16.2 W/(m*K).

3. Simple heat transfer equation: (Watts dissipation) = (heat transfer coeff)*(A)*(ΔT), with a heat transfer coeff = 5.5 W/(m2*K) for stainless steel. (http://www.vfds.org/vfd-cooling-and-ventilation-13...)

For a small VFD generating 18 W of heat, method 1 says 92880 mm², method 2 says 55.6 mm², and method 3 says 163636 mm².

Is one of these methods best, or is there another equation to consider?

1. Rule of thumb from Rockwell Automation which states A = (Watts dissipation)*8 (https://literature.rockwellautomation.com/idc/grou...)

2. Heat transfer conduction through a uniform material: Q = -k*A(T2-T1)/L, using thermal conductivity of SS for k = 16.2 W/(m*K).

3. Simple heat transfer equation: (Watts dissipation) = (heat transfer coeff)*(A)*(ΔT), with a heat transfer coeff = 5.5 W/(m2*K) for stainless steel. (http://www.vfds.org/vfd-cooling-and-ventilation-13...)

For a small VFD generating 18 W of heat, method 1 says 92880 mm², method 2 says 55.6 mm², and method 3 says 163636 mm².

Is one of these methods best, or is there another equation to consider?

## RE: VFD Enclosure Passive Cooling

You will rarely regret having more area than required and will soon regret if it is far too little. In between you may get 1 year instead of 5 years due to cooking the capacitors or some other trade-off.

## RE: VFD Enclosure Passive Cooling

No 2 is simply the square area of steel assuming a constant temp. Steel transmits heat very well, air much less so.

Remember - More details = better answers

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## RE: VFD Enclosure Passive Cooling

Expjsed to really hit still air outside and its a different story.

But 18 W heat rejection?? My laptop puts out more than that....

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: VFD Enclosure Passive Cooling

The 18W heat rejection is the given value for the smallest drive, I just took it as an example. #2 makes the most sense to me and was my first instinct, but a required area of 55.6 mm² isn't realistic compared to the other answers, which are so much larger.

## RE: VFD Enclosure Passive Cooling

The source for equation (3) shows a lack of understanding of the fundamentals. "

The better heat conduction of the enclosure the more heat dissipated."It might be safe to assume that the VFD is designed with adequate cooling, and therefore the cooling features on your enclosure need to be "about the same".

You probably don't want to use the VFD's max allowable condition as your design condition.

## RE: VFD Enclosure Passive Cooling

## RE: VFD Enclosure Passive Cooling

Why no fan in the enclosure?

This is 18W. It is a very low number.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: VFD Enclosure Passive Cooling

## RE: VFD Enclosure Passive Cooling

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

## RE: VFD Enclosure Passive Cooling

At the very least we used a set of fins cut into the wall of the enclosure with fins inside and out.

And fans on both sides as well.

If we really cared the enclosure was air conditioned.

= = = = = = = = = = = = = = = = = = = =

P.E. Metallurgy, consulting work welcomed

## RE: VFD Enclosure Passive Cooling

Q = [-k*A(T2-T1)/L]+[h*A*(T2-T1)], and estimate h = 12 W/(m²*K), then solve for A.

## RE: VFD Enclosure Passive Cooling

It's one or the other: heat transferred by conduction = heat transferred by convection.

Pierre

## RE: VFD Enclosure Passive Cooling

^{2}K may be a little high.I work in american units so I have values of inside and outside convection heat transfer resitance R both at:

0.68 deg. F per Btu/hr per ft

^{2}And the resistance of the metal as negligible.

So total thermal resistance is 1.36 and U overall heat transfer coefficient is 1/R = 0.735 Btu/hr/ft

^{2}-^{o}FAnd heat transfer Q = U A dT

I would check by the method 1 which is a rule of thumb method by the manufacturer and then check by this heat transfer equation and use the most conservative value.

For typical electrical equipment the maximum temperature allowed is 104 F which will then be the maximum inside cabinet design temperature and the cabinet external design temperature would be the maximum possible space temperature.

Typically a space with electrical equipment installed is at least ventilated with outside air to remove heat or air conditioned with HVAC units.

## RE: VFD Enclosure Passive Cooling

## RE: VFD Enclosure Passive Cooling

1/(0.68 deg. F per Btu/hr per ft2) = 8.35 W/m^2-K

In any case, if your spec limit for the VFD is 45 degC, then a 40 degC enclosure temp is inappropriate, since you'll have at least a 20 degC delta between the VFD and the interior surface of the enclosure

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm