VFDs and Temperature/Environmental Limitations

[jmbelectrical]

I'm working on a project in which the mechanical engineer is locating VFDs, each one serving a roughly 10 to 15 HP pump, in an outdoor equipment yard adjacent to each associated pump. Knowing that VFDs don't fare well in high temperature, high humidity environments (The project is located in the southeast U.S.), I asked the mechanical engineer to specify NEMA 4X enclosures with air conditioning. Before proceeding, he asked that I confirm the necessity of a NEMA 4X, air conditioned enclosure, as it seemed unusual to him.

Much to my surprise, I found literature that claimed that, due to advances in power electronics, modern VFDs can typically operate reliably in environments whose temperatures don't exceed 120°F. I also found that an electrical enclosure manufacturer is offering NEMA 3R enclosures specifically for VFDs. Again, to my surprise, they're only equipped with ventilation fans.

This leads me to the following questions:
1. Was my initial suggestion of a NEMA 4X, air conditioned enclosure excessive?
2. Are newer VFDs really more tolerant of higher temperature environments than they used to be?
3. Will VFDs hold up in an environment whose outdoor ambient temperatures often exceed 90°F, in addition to whatever temperature increases result from direct sunlight exposure?
4. Are there any warranty issues with respect to installing a VFD outdoors?
5. Is there anything else I need to consider in this scenario?

 

[ScottyUK]

Most VFDs use computer-type fans for forced cooling. These fans don't like untreated external air in my experience. We installed a number of Rockwell Powerflex 70 drives of about 15kW rating in an enclosure where the drive itself was within the enclosure but the heatsink was mounted through the wall, thus allowing a high IP rating to be achieved while ensuring that heat was rejected to the external atmosphere. It avoided the need for an actively cooled enclosure. The heatsink fans failed within a month or two and we didn't get a lot of support from Rockwell, certainly not to the point where they admitted there was a problem, although they did provide some replacement fans. When they also failed the directive from high up was to get rid of the fans completely so we fabricated our own venturi-based airmover in place of the fans and that ran successfully if somewhat inefficiently from then on.

1) In your application I would stick with the requirement for the actively cooled enclosure.
2) Temperature is the enemy of all things electronic, but especially electrolytic capacitors. They haven't changed much in 30 years.
3) See above. Cool is good.
4) See my tale of woe above, and that was for an IP66 rated drive.
5) Give the mechs the alternative of a fully air-conditioned drives room instead, then they'll like the NEMA enclosure. ;-)

 

[jraef]

Some Gen 4 IPMs (the integrated power modules used inside of VFDs now) are capable of operating at higher temperatures, and combined with what are called "glued fin" heat sink designs, it is now possible to get to operating temperatures of up to 70C (158F) ambient with de-rating in some designs up to around 30HP. Larger drives will limit out to 60C (140F) ambient with de-rating, 50C (122F) without. But there are still a lot of drive designs that have not been updated yet, so be careful, because many are still limited to 40C ((104F), and for those you need AC units if outdoors. That said, a drive mounted outdoors in direct sunlight with a 100F+ ambient will get MUCH hotter than 122F inside, often even with ventilation. I ALWAYS recommend using a sun shade, especially for afternoon exposure.

I'm not a fan (no pun intended) for ventilated outdoor designs on VFDs because they need filters to keep out dust, and people don't change /clean filters; it's just human nature to forget that sort of detail eventually. So it only saves on initial cost at the expense of long term reliability. In addition, no filter is going to keep out airborne moisture, which has a tendancy to collect and condense on metal surfaces inside of the VFD. Conformal coating of the PC boards helps, but that can't protect everything.

I'm working on a very large project right now with almost 50 75HP VFDs going outdoors at a tank farm and after much consideration for reliability, we are going with AC cooling in NEMA 4X boxes, even though at the HPs involved, we could have done the less expensive "fins out" option here as well. The biggest objection to the fins out package by the way was because wasps tend to make nests in the heat sinks, which then causes added build up of dirt and loss of cooling, so you trade filter changing with heat sink cleaning, which again, eventually stops happening.

ScottyUK,
Did you use the fans that came on the PF70 drives, or did you use the optional NEMA 4 fan retrofit kit? A lot of people miss that.




"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

jraef,

This was about 2005 / 2006 I think when the IP-rated Powerflex 70 was fairly new on the market and I don't recall a NEMA 4 fan kit being available. Rockwell UK certainly didn't offer it, either at initial purchase or once they knew we had problems.

I wonder if the NEMA 4 fan kit was developed as a result of our rather vociferous criticism of what, on paper, should have been a damned good solution. I'm sure we weren't the only ones who had this problem, so if Rockwell did respond then credit to them. I wish we had known about it - the constant bleed on our instrument air system from our in-house solution wasn't exactly welcome although the reliability certainly was. From a quick look on their website I didn't find the NEMA 4 kit. Do you have a link by any chance?

 

[jraef]

ScottyUK,
Most likely true. I've had others complain to me about the fan failures on PF70s of that vintage. To be honest though, I just looked and the NEMA 4X fan kit is for when you buy a flange adapter kit to convert a NEMA 1 wall mount drive drive to a flange mount, so you have to change the fan that comes on the NEMA 1 version. On the PF70s, if you order the flange mount version, you automatically get the NEMA 4X fan, or at least now you do. Sorry for the false alarm on that.

I've always had my suspicions as to whether or not any muffin fan can really be made to stand up to that kind of environment for very long. Clean wash down production areas, sure, but outdoors also means a lot of other environmental factors such as dust, vermin and icing. That said, we use a lot of flange mount versions on irrigation pump systems in Central California, haven't been getting complaints in the 3+ years I've been here.




"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

The fans at that time were little (60mm?) fans of the type normally found on computer processors and the like. The bearings seized solid on most of ours, and we had a couple where the electronics failed before the bearings. I guess the guys responsible for initial development of that product line don't get out of the lab very often to see the operating environment their products go in to.