VFD derating 3

[itsmoked]

I have a VFD that I have to derate because of its sissy temp rating. It has to be put in an enclosure and if the air temp in the enclosure exceeds a mere 104F/40C I have to derate the drive to 80%. So it's a 25HP motor stating 29A I'm using a 30HP (47A)drive.

How do I have to rate the wiring and protection. Do I have to scale that all up when I'm only actually using the drive's larger semiconductors and heat sinking and none of the drive's larger HP capability?

Any chance I can rate it all against the motor's name plate FLA?

I can image the answer... Code answer always = sux.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

You're right, it's mostly the sucky answer you don't want to hear. 430.122 dictates that the conductors have to be rated for 125% of the VFD rated INPUT amps (which, by the way is usually LESS than the output amps).

But nothing is going to force you to de-rate the OCPD or anything else, you do size the OCPD for the motor amps (per the NEC HP charts by the way, not the nameplate). The VFD mfr will likely give you a MAXIMUM CB rating you can use with the drive, but other than to avoid nuisance tripping you are not bound to a minimum.

Although TECHNICALLY, CBs are rated at 30C... and most mfrs will tell you to de-rate them for higher temps. But as the OCPD for a VFD it will likely only trip AFTER the VFD blows up, so who cares...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Thanks Jeff! I can certainly live with larger wire. They already have me installing ultrafast semiconductor fuses. Funny they're 90A fuses going into a 47A circuit. I laugh that they think it will protect the drive..

Actually, I bet it does protect the drive, it protects it from completely disassembling on failure.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[che12345]

Dear Mr.itsmoked

I am thinking in another approach to circumvent the problems.
I am of the opinion that by installing a force cooling fan with the enclosure fabricated to the IP rating required. It would not be a big fan to lower the inside temperature to say 30C if the outside ambient temperature is say 20C. The different is 10K . A 30hp VFD is not going to generate too much heat for a small fan to handle.
The fan is to be controlled by a thermalstat, operates only when the inside temperature exceed say 30C. An alarm may be activated in case the fan fails and the inside temperature reached say 35C, which is lower than 40C.
Che Kuan Yau (Singapore)

 

[mparenteau]

I have used vortex coolers in the past to get around this issue. Yea...it's a compressed air hog, but it works and has been reliable.

Mike

 

[jraef]

My opinion on the high speed fuses is that it is heavily promoted by the fuse industry because when your drive dies catastrophically, they can cash in by making you replace expensive fuses too. The point is, about the only thing that will make those fuses blow is the destructive failure of the drive, in which case, what were you protecting?


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Hello Mr. Yau,
I will be 3D printing a duct to duct ALL the air exiting the top of the VFD out the top of the enclosure. And, as you suggest, I will be adding a thermostatically controlled fan to force air into the enclosure doubling the air flow thru the VFD. The issue is that this is in a location where the weather ambient has reached 107F. Once you put a wire in a box in a weather ambient of 104F you're going to get a rise above even 104.

I'm including a second thermostat to signal the enclosure temperature is exceeding 115F to the PLC running the show so it can unload the VFD and put up a red-flag on the Human Machine Interface. This will eventually happen when the filter clogs or the fan fails.

Jeff; You may have a point. The drive maker specifies that fuse for UL listing.

mparenteau; Thanks for the suggestion. Vortex coolers. kinda Satan's ugly stepchild. You end up using all that air when the air compressors, out in the sun drenched shed, are struggling to cope with the same excessive ambient.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[EdStainless]

Do you really want to blow outside air into this? Are you sure that it is clean?
Maybe invest in an air-air heat exchanger would be worth it.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[waross]

Is Peltier effect cooling an option, Keith?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mparenteau]

Keith,

In my application, my compressors were remote and distributed compressed air site-wide, so no real ambient concerns with regards to the compressors themselves. The only problem we had was oil getting downstream where it wasn't welcome. This only happened a couple of time and clean up was manageable. Satan's ugly stepchild? That made me chuckle...

Mike

 

[che12345]

Dear Mr.itsmoked

Q. " ... I will be adding a thermostatically controlled fan to force air into the enclosure doubling the air flow thru the VFD. The issue is that this is in a location where the weather ambient has reached 107F."
A 1. If the outside ambient air is 40C, it would be difficult to bring down the internal temperature to 30C. An alternative way is to install a small refrigeration unit, see e.g. Ritta or other manufacturers' brochures.
A 2. To cut down the power consumption, it is recommended to run the refrigeration unit only when the inside temperature exceeds say 30C.
Caution: if you are in a location with [high ambient temperature and high humility], be very careful of possible condensation. On the other hand, with refrigeration unit; no cooling fan is required and the enclosure IP can be as high as IP65, if at all needed.
Che Kuan Yau (Singapore)

 

[waross]

If the enclosure is refrigerated, there is no need to exchange air.
With captive air in the enclosure, condensation will quickly dry the air.
Then condensation will cease.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

EdStainless; Cheap er.. very cost sensitive company. The drive and motor starters and PLC in this box are about $5K, trying to get them to swallow another 2K for the a low end A/C is more than I want to tackle. The old units were simply bolted to the walls. New, they want in enclosures. Their argument is so it's easier to move everything, when in-fact, I know they will never move anything.

Bill;

Is Peltier effect cooling an option

LOL. Peltier is NEVER an option. Satan's Ugly Stepchild is pinnacle-like quality compared to Pestilencer. The very finest Peltier is 5% efficient. The VFD alone dumps 634W. That would require 12kW of Peltier cooling. It would fail in less years than you can count on 3 fingers, because, that is what Peltier coolers do.

Mr. Yau; The problem with A/C is that since the enclosure is only 48" x 37" x 12" (1.2m x 0.9m x 0.3m) the VFD dumping 634W into that space would over heat it on a cold day in.. 2 minutes. This would require the A/C to always run and the very real possibility of short cycling it.

I have a fan that moves 191cubic feet per minute that will turn over the air in the enclosure 15 times a minute. It should be as good as out-on-the-wall.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[che12345]

Dear Mr. itsmoked

1. My apology for my unintentional misleading opinion by suggesting (force cooling fan) or (refrigeration unit); which is far far over-kill, for this small box.
2. " I have a fan that moves 191cubic feet per minute that will turn over the air in the enclosure 15 times a minute. It should be as good as out-on-the-wall."
I agreed that it would be a simple, far better and cost effective solution, instead of any other over-kill proposals.
3. BTW: A 191 cfm fan would consumes around approx 32W. It may not be economical running it 7/24 or 5/7. In addition, if the fan fails say within two years? (for running 7/24), the VFD has to be stopped or down-loaded. Perhaps installing a [higher rated VFD which takes care of the de-rating] should be taken into consideration?
Good luck.
Che Kuan Yau (Singapore)

 

[itsmoked]

Mr. Yau; No apologies needed!
Ever. Well, maybe ever.

I welcome all ideas, any one of which can trigger; thought, action, better action, and changed directions. I don't think of alternate solutions at all sometimes that people here like yourself do.

As for the fan control, you're correct that that fan is too much for all day use and that's why I've included a thermostat to control it. My concern though is that these things really need 'correct control'. The process places a large demand on the VFD for about 10 seconds, then perhaps 1/5th load for 5 seconds, than 1/20th for 4 seconds, then no load for 5 seconds, the repeat. I can easily see this cycling causing the temperature to spike, the fan start, the heat load taper off the fan drops the temperature below the thermostat hysteresis, fan goes silent. Then a few seconds later back on. Possibly highly annoying. Depending on the jobs and the weather I can see this happening some days and not other days making it very hard to quantify, characterize, and prevent.

It also seems I purchased a real freak of a fan. This dang thing BLOWs out thru the grill thru the filter element! It blew the element into my face. Why would you want an enclosure fan to blow out thru a filter? So I need another blow IN fan. I think I'll maybe down size to a box turnover more like twice a minute. Thoughts?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[che12345]

Dear Mr.itsmoked

1. Perhaps it is better to install a [higher rated VFD that is required by the load]. A unit which is de-rated to operated at 40C ambient. The manufacturer would guarantees that it would take the load at 40C ambient [with their build-in fan] that is (usually located at the bottom) of the VFD housing. Let the VFD [internal fan] cycles on-off based on their (internal temperature detection). [The manufacturer knows better]. Therefore, no external force cooling fan or thermostat is required. Now you handover the responsibility to the VFD manufacturer and washed your hand clean.
2. The enclosure as insisted by the owner is superfluous. It may be fabricated to IP00 or IP01; but always wise to maintain say at least 100 to 150mm clearance* on all sides including top any bottom. Caution: see the VFD (manufacturer's installation manual) [carefully of the minimum clearance* requirement!]
3. You are a well respected and experienced engineer. My proposal is for your reference only.
Che Kuan Yau (Singapore)

 

[waross]

Time to start another thread Keith;
"How can I reverse this fan motor?" Grin.
Yes, I know, reversing the motor may be easier than reversing the fan blades.
or
Mount the fan on the outside of the enclosure, blowing in.
Joking aside, consider an outside the enclosure thermostat and let the fan run continuously when the ambient air rises above a certain point. (possibly above an ambient of 30C)
Not replacing the inside T-stat but working in parallel with it.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Mr. Yau; I embrace your, "let the VFD fan do it". I 3D print a three piece vent structure. Here's one. This external structure exactly matches an internal duct that reaches to within 1/16" (1.5mm) of the VFD's heat sinking exit. All the air going thru the VFD exits out these roof-top exits.

Of course this means any extra fans like the one discussed above has to add air into the enclosure it can't blow air out or it could and would stall air leaving out the above vent stack since it would be a counter flow.



Bill! Yes! Reversing this silly fan I have.. Do you see what I see?

Current side blowing out:

And here's the current sucking side:

Would you agree it looks like I should be able to simply reverse the motor and fan? Keeping in mind they make a blow and a suck version of this fan - the reason I got tripped up buying a blow.





Keith Cress
kcress -

http://www.flaminsystems.com

 

[Bill West]

Sometimes this board seems to have a very neat auto writing system. Last night I thought of the VFD heat figures and an enclosure fan. Bang, today Keith has written that out for me. Then I thought of an external thermostat to control fan running hours. Bang, a few posts later Bill R wrote it out. What's left? Well I spent many tries a while ago trying to fan cool my furnace room, it was overheating a pantry cupboard half a floor above it. It took a month to get it to barely work. What I can pass on at this point is that despite the generous size of your fan Keith, the temperature rise is still going to be around 10F. I think you outlined a 104F VFD limit and a 107F worst ambient. I got the ~10F rise by putting 2000BTU/hr (~634W) into 200CFM (~191cfm) on the "Example - Heating Air, Sensible Heat/Imperial Units Chart" at

https://www.engineeringtoolbox.com/cooling-heating-equations-d_747.html

Also fan CFM's are always optimistic, your exit grille alone will impact it. Then there is the question of where will the heated air go and where will cooler makeup air come from? I fear your 104-107F limit may just be too tight to solve.

Okay, I read farther. Your duty cycle is ~20%, the VFD heat sinks may dampen the temperature swings and you may only have to deal with 127 watts loss.

Good Luck, Bill

 

[waross]

Keith.
Cut a mounting hole in your enclosure for the fan. Then mount the fan backwards in the hole.
Hold the filter down with lots of coat hanger wire.

Bill
--------------------
"Why not the best?"
Jimmy Carter