How Do You Know When Your Control Cabinet is Too Hot?

How hot do you estimate it got to evaporate the grease?

Gotta remember to clean the fins out...

In a class I teach to electricians on VFDs, I tell them that heat is the #1 killer of VFDs. Most VFDs are rated for 40C ambient, some are 50C, a few can be used at 60C with de-rating and one I know of can be used at 70C with de-rating and an aux. fan for the PCB, but that's not the norm. VFD life is cut in half for every 10 degrees C over rated ambient, so if you have an VFD rated for 40C and the internal enclosure gets to 60C, your VFD life is reduced to 1/4 of it's expected life.

It's not too difficult to figure out how hot an enclosure will get, most of the enclosure mfrs will provide a simple on-line too to calculate it for you. They provide the tool to sell air conditioners, but you can use it to see how to avoid them. The tricky part is that most people grossly under estimate the heat rejection by looking only at the efficiency of the obvious things like the VFDs. So if it says the VFD is 97% efficient, they assume 3% heat rejection into the enclosure, but that alone is actually not enough. Control transformers are 95-98% efficient, switch mode power supplies are rarely over 85% efficient, even thermal sensing devices like circuit breakers and OL relays reject about 3W per running load amp per phase. It all adds up quickly when an enclosure is sealed.

I've found this to be a good training tool to hand out.

Thermal calculations should always be done by a reputable panel builder, if they engineered the design. If not, and were building to your design, then what were your engineers thinking when they sent the design to the builder without doing the heat rise calculations??? Especially with 5 VFD's in one small cabinet!!! You might get away with that if they are for 1/2 HP motors. Larger and, well, I guess you saw the outcome.

I saw a lighting/power panel in a Mennonite nursing home in the early 1970's, where the maintenance man just ran what ever size wire he had from what ever breaker, even from different breakers to get the correct phase, to install new three phase appliances in this building. Now these were old order Mennonites and had no electricity in their homes, but had to have them in the nursing home, and of course, the maintenance man was a Mennonite. He "learned electricity" on the job. The insulation on some of the wires was so hot, if you touched one, the insulation stuck to your glove. Now that was not the panel heating, but overloaded wires heating, but still an interesting thermal effect.

Heat + moisture is an issue too. This one was growing a little crop of tiny mushrooms on it...

It is still working by the way, at least for now...

I have a lot of bottom feeder customers who will not pay for an industrial Ethernet switch for the panel.

They use a consumer grade switch until that hot summer day when the network connection to the SCADA software fails.

They call me in a panic and I say, "I didn't supply the Ethernet components on that panel; you or someone else did. Call whoever it was that supplied the Ethernet switch and have them fix it."

I know you are "right", but the customer "is always right". If you tell them you will be right there, you have the opportunity to win them over right away, make the sale, and get them on the correct path, all in one fell swoop.

I do get your point that it is pay me now, or really pay me later.

I think at least half of us could write a book on electrical. electronics overheating issues we have worked on due largely to bonehead engineering issues we have seen.

Oversold and under built or just assembled and not maintained by those who have no clue.

I know I could.

In past jobs I have actually gotten a stern talking to by my superiors for taking it upon myself to clean out electrical cabinets and fix locked up fans.

Apparently I had 'better things to do' (like chase bonehead ghost issues on things that never existed outside of management's mind or deal with real issues that were so far outside of my area and capabilities is was stupid to have asked me or anyone else in the company do them) than mess with that stuff being, obviously, 'it was still working just fine' as is therefore didn't need any of my attention.

Easy. When the panel builder puts a 3hp line reactor going to a 5hp motor in the VFD cabinet. The cabinet gets so hot you can't hold your hand on it, and the wire insulation inside starts to melt.

And you have to prove it to the contractor, sub-contractor, and design engineer. That, by the way, was just 2 weeks ago.

You know when it's (a 6ft H x 8ft W, 480VAC panel) too hot when you have to leave the doors wide open so you can aim a large floor fan at the contents inside to keep the VFDs from tripping, so you can keep production going.

I remember once someone wanted a 75HP VFD in a sealed NEMA 4 box, no ventilation, no exposed heat sinks, going outside but shaded from the sun. I used the Hoffman AC calculator software and kept increasing the enclosure dimensions until I no longer needed and air conditioner. The box size got to be 120" wide, 90" high, 30" deep for JUST that VFD... nothing else.

I would almost bet that was a request from an old boss of mine.

My first real job was as a maintenance technician for a pasta plant and they had a new processing line go online a month or so after I was hired on.

The main control panel was good sized ~( 20' x 8' x 3') but not for what power it was handling in the environment it was put in.

It has something like 6 racks PLC units plus ~ 200 HP in VFD's along with multiple variable power control units for ~ 50 KW of additional heater and other process line operation loads on top of the motor drives.

Beyond that it was mounted above the main pasta ovens in the main production area that ran at an ambient temperature of 100 - 105F at 90 - 100% humidity.

My bosses solution to the constant overheating issues was to add a 5000 BTU pneumatic vortex cooler at one end which did nothing of any measurable good other than take the plant's main air compressor to it's limit and beyond causing even more problems elsewhere due to other systems in the plant occasionally shutting down from low pneumatic system supply pressure issues.

Blow that.

I looked into the vortex coolers on a project once where I was putting VFDs on bridge cranes that were over the pot lines at an aluminum smelter. The ambient was upwards of 180F (82C) in the summer. The SMALLEST vortex cooler unit I could get required 100CFM at 40PSI of "instrument grade" air, meaning no oil or water vapor. When I ran that by a compressor supplier, that came out to a 10HP air compressor! A refrigerant based air conditioner was going to take a LOT less energy to keep that box cool...

Yeah, vortex coolers seem to be best for small applications, where you only need to drop the temperature to around -20F relative to ambient.

I personally don't even think of the problem as 'cabinet cooling,' but rather as 'heat venting.' You don't need to 'chill' the cabinet as get the heat OUT. If there is no external heat source (like a smelter underneath the control box) then typically, properly placed louvers and exhaust fans will do the job. If the enclosure is going someplace where external head will be applied, then a water-cooled system looks much more feasible, possibly even replacing existing 'finned' heat sinks on components with a 'heat transfer plate' so the heat-producing components are connected directly to the water-cool system⁽¹⁾.

Notes:

1. Yes, I'm basically copying the same system that high-end water-cooled gaming PCs use. However, it is an Engineering Maxim: Don't Reinvent the Wheel. The Computer water-cooling system works, it is scalable to larger systems, as long as there's no patent issue involved, why NOT use the concept and safe the time that would have been spend designing something 'similar, but different'?

When the OSHA guy shows up with marshmallows?

Your control cabinet is too hot when anything plastic related or connected to it, or included within it begins to resemble Salvador Dali art.

You know the cabinet is too hot also when you would not drink it if it was tea.

Not VFD but heat related.

I had a client call me because their CPU which I installed in their office had crashed and would not reboot without crashing again.

When I got there the CPU was not where I had placed it in the office (beside a desk in the open office area)..."It was making it too hot in here and it was noisy so we put it in the closet over there and shut the door".

We had a nice talk about air circulation and the need to keep the CPU where it would not overheat...I fixed it, put it back where it was, and closed out my contract with them.

Great responses from everyone. I especially liked the reference to Salvador Dali. I've heard some great storied from machine shops as well. I was thinking to myself when I wrote the title that this sounds like the opening line of a joke. Sadly, it is not funny.

They say that experience is the best teacher. I think that is only true if it is able to bitch slap the person who made the stupid mistake. Unfortunately, that does not happen enough. My rule of thumb is this. If the cabinet is warmer than ambient (assuming it is not next to an oven) or perhaps any warmer than 85°F (30°C) it is just a matter if time before the next breakdown occurs. Of course, there are many environments where it never seems to cool down to that temperature. In those cases duplicate replacement parts really need to be available. Blessed are those who keep the plants running despite the morons. You all deserve a hearty show of appreciation but if you are like me, all you can do is dream about that. Ah, well. Pleasant dreams!

It rather depends upon whether there is a need to fry eggs on it because of hunger and the absence of a stove.

IEC61439-2 could help you - the design should be compared with a tested unit to ensure compliance and temperature rise tests are a large part of that i.e. you will know before it is even installed.

This applies to switchgear, MCCs, control cabinets, call them what you like.

We have seen many folks rely on past experience alone (and a good reputation) but lets face it sooner or later you will get caught out. Follow the standard, do the tests, simples.