Why VFDs Blow Up

J

J. MacDonald

Joined
Jun 8, 2000
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3,140
Having recently posted about my VFD variable speed drive on my KMG grinder blowing up, I thought I'd try and salvage something from the experience and share it to help prevent someone else from feeling my gtief.

For those not familiar, a VFD, or Variable Frequency Drive is a solid state electronic device that let's you wun a 3 phase motor from a single phase line, usually 230V, and provides for variable speed operation of a three phase motor.

There seems to have been a rash of folks having VFDs blow up in the last few months and I think I've figured out why.

My Toshiba Tosvert VFS7S was working fine, right up until it blew up. In an electrical autopsy to try and figure out why it expired, I discovered that it was designed as a NEMA 1 type enclosure which is an enclosure designed to keep you safely away from the high voltage electronics, but isn't well insulated from the steel dust that flies around a knife grinder :( That's why my VFD blew up, it was a NEMA 1 design being used in an environment with a lot of conductive dust flying around that finally caused a short.

What we really need to power our variable speed knife grinders is a NEMA 4, NEMA 12 or IP-65 rated VFD. These devices do the same job as a NEMA 1 type VFD, but are rated for wet wash down duty, so are both water and dust proof. Of course a NEMA 4 rated VFD costs a bit more :(

I think the reason so many knifemakers have had VFDs blow up is using a NEMA 1 VFD in a dusty environment.

The KB VFD that Rob Frink of Beaumont Metal Works sells is a NEMA Type 4 drive and should last nearly forever. There are a few other NEMA type 4/IP-65 drives around, but the majority of VFDs like what Dealer's Electric sells are NEMA 1 drives and disasters waiting to happen.

I hope this helps someone and helps save them from buying the wrong variable sped drive.
 
too late, I just set up my VFD from dealers electric (harold said that it would be ok in a dusty knifeshop:mad: ) Anyway I mounted it 2 feet to the left and about 1 foot below the grinder.although it doesn't look like it, the VFD is recessed a few inches and it is out of the major dust there, i also wrapped some pantyhose around it as a filter. I hope that it lasts there, I may box it in with airconditioner filters just to be safe...thanks for the heads up..:cool:
 
John is correct in stating that the NEMA 1 rated enclosures on most VFD's are not really adequate to prevent the conductive metal dust out of the electronics.

Most circuit boards are designed to an international standard known as "Polution Degree 2" in both the UL and IEC (European) safety standards. This means that a coating of "nonconductive dust" on the circuit board will not cause it to fail. There are other, higher Polution Degrees where there may be condensation on the circuit board. For each Polution Degree, there are tables showing the minimum required spacing between conductors, dependent on the voltage between those conductors. The higher the voltage and the higher the Polution Degree, the wider the spacing required to prevent arcing.

The (electrically conductive) metalic dust entering a NEMA 1 enclosure will reduce the effective spacing between the conductors. When there is "enough" conductive material between the conductors, there can be an arc-over. This usually causes a catastrophic failure in the circuit. There is a big bang, the "magic smoke" gets out and the circuit stops working. This is just like an open-frame motor accumulating too much metalic dust inside and blowing up.

There are companies that manufacture metal enclosures with filter systems for protecting NEMA 1 rated equipment in dirty environments but they tend to be quite expensive. You can make your own using a metal box, some fans and some filters, as Rocketmann states.

Please use a metal box rather than using a flamable material like wood. Every electrical code that I have seen requires non-flamable or flame-resistant enclosures. If you have a fire and your insurance company found out that you had used a flamable enclosure, they will probably try to deny any claims that you make. I think I had told someone that a plywood box would be OK. I must appologize for that mistake.

I forgot to mention that the ventilation slots on the enclosure of your VFD were put there by the designers to let the heat escape from inside the enclosure. Blocking the air flow through those slots will cause the internal temperature to rise, shortening the working life of the VFD. Depending on the particular model, operating at higher than normal temperatures could cause some damage after a while.

Hope this helps.

Phil
 
simple solution for those that already have them with a nema 1 enclosure: buy a big nema 3r box metal or plastic and stick the whole vfd in it. problem solved.
 
sleepy said:
simple solution for those that already have them with a nema 1 enclosure: buy a big nema 3r box metal or plastic and stick the whole vfd in it. problem solved.
Click to expand...

How about some links???
 
When I purchased my KMG, I was planning to get one of the open controller packages from Dealers Electric. Rob made mention to locate the nema 1 controller away from the grinder to avoid the dust and stated dust was the biggest problem for the controllers.

Looks like he was right!:eek:

I bought a 2HP KBAC-27D controller (nema 4, closed) and a 2hp Leeson 56H motor (113026), from a company named State Electric in St. Louis, for only $70 approx., more than the DE package. This is the same package that Beaumont Metal Works is offering. Fully closed, sealed from dust and was still fairly economical.

Here's a link to State Electric. http://www.state-electric.com They don't show their prices for the KB nema 4 controllers but I got an online quote, which was the cheapest I could find even on the searching the internet.

Here's a picture of my KMG setup. 2 hp with VFD, 220v. http://pg.photos.yahoo.com/ph/h2ostx/album?.dir=/7e2e&.src=ph&.tok=ph9okeEBP3NIXm82

I think that the open controllers have potential to be dangerous in knifemakers shops. I sure don't want to see anyones shop get burned down because of an electrical short.

Kelly
 
When I was evaluating VFD's a few years ago...These are the issues that I was faced with. The majority of them are open NEMA 1 enclosures with a soft key pad to programming. I weighed all of the costs associtaed with setting one of these up....which included an enclosure, switches and a pot mounted on the outside of the enclosure and the additional wiring. I must have evaluated a dozen different drives. You need a big enclosure for cooling the drive...or you need to circulate fresh air through a small enclosure since the heat sinks are internal.

enclosures are avaliable through most industrial and electrical suppliers: Grainger, MSC, McMaster Carr, Loeb electric, Hughes Peters,...etc.
 
every time I fire up my air compressor, I give the VFD a good blast - I still lost one a couple years ago though. I've never been able to find an enclosed one, guess I'll need to look harder next time.
 
I am not surprised by what Ron found when he chose the VFD that he sells. My previous employer was in the niche of manufacturing IP54 dust-proof and splash-proof VFD's for "dirty" applications. I was on the R&D side and never saw a price list but had heard that they sold for at least double what equivalent NEMA 1 rated VFD's sold for. There are many industrial applications for VFD's that are at least as hostile as a knifemaker's shop and they had lots of customers who got frustrated trying to implement their own protection scheme for their NEMA 1 equipment. It took PDL quite a bit of R&D effort to develop their sealing system and most customers can not spare the time and resources to do that. They just want to be able to install a system that works reliably in their environment and get on with their main business. This is why there is a market for sealed drives (such as the KBAC-27D, the PDL drives and others) even though they cost much more than a NEMA 1 VFD.

Be very careful about just putting the VFD in a sealed box or blocking off the ventilation slots. The VFD market is very competitive and the components in most of the designs are pushed right to the limits to keep the costs down. This leaves very little margin in terms of thermal capacity, etc. Your VFD may run for a short time then trip on an overheat alarm. If you are not so lucky, some component will fail after a short time and the VFD will stop working or blow up.

Oh yes, please turn off the power before blowing the dust out your VFD. One of the other forum members blew his up when vacuuming while the power was on. I suspect that the air flow from the vacuum moved the metal dust that had accumulated to where it was able to do some damage.

Phil
 
Buying a vfd in a in anything other than a nema 1 enclosure automatically doubles the price(or close enough to anyway). This is why iI suggested the nema 3R box. You can get them at any electrical supply house and stick your vfd in it for less. As long as you leave it in the enclosure it comes in you could stick it in any kind box to isolate it from the metal dust: plywood box, plastic storage tub, big metal or plastic tool box, etc. Just make sure whatever you use has enough space left in it after you mount the vfd to dissipate the heat it will produce or give it a clean air supply with a small cooling fan from anywhere like radio shack and some cheap flexible hose.
 
If you have no choice, or can't afford the NEMA4 enclosures, you can simply seal the entrance ports.
Only one of mine is NEMA4, the other three NEMA1.

I just ran a bead of silicone seal around the tops and the fronts to seal the bat switches. It's been working for 13-15 years. I also keep them clean by vacuuming as soon as I finish grinding, something I do to the shop anyway, as pets use it and the garage at night.
 
I just got my VFD and is a OMRON 3G3MX - only ip20 -
calling OMRON and asking for advice . there told me to Extent the VR nobe out - like a volume control for the speed . using a 2k B Style 5 watt VR . - it cost me less then $1 to get it set up as the inverter offer the option and also it had a removalable control panel which i can extent it onto a Box . using a Computer LAN line. -

After taking to OMRON , the advice i got is that IS more Importance to keep it away from water then DUST . but of cos if we can kepe it safe at the same time allow it to cool off it would be great . one advice is to case it and put it High . away from Water and dust . basically the Inverter 3G3MX AE022 come with it own HEAT SINK and mini FAn to cool the inverter .

i will post some photo soon once i figure what would be the best setup .

Great read and advice . after all Inverter is not cheap and is alway worth the extra read up to protect what you have .


Thanks
 
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Keep in mind that (within reason) you can mount the VFD quite a distance from the motor...and many/most come with terminals for a remote speed control.

We (at my primary job) put them in explosion proof enclosures to speed control pumps in flammable atmospheres. Ya think an arc in a shop is loud...try one in a refinery or pipeline pumphouse. :eek:

Yes the cost goes up greatly when you put one in a dustproof box because the engineering calcs to insure proper heat dissipation are time consuming and over-engineering is rampant. Our typical designs also mount the main inverter to the metal box so the box itself becomes part of the heat sink. I haven't tried any of the ones usually recommended here but we mostly use Allen Bradley and ABB...both get reassembled in custom boxes this way. It isn't for the timid, but can overcome a lot of the heat issues previously mentioned.
 
heat dissipation is very importance for the Inverter - So if you are trying to DIY your own BOX . try to keep in mind . Metal conduct heat better then Plastic .

and using metal will give you a better heat dissipation factor . i was also advice by OMRON if i were to DIY my Box a small fan on the metal box would work great also .
 
This is a good post, and good advice. I have worked as a equipment technician and manufacturing engineer and seen these drives fail due to conductive dust. I gave this advice to one prominent knife maker when he first started and he poo pood it ~ wonder if his has blown up yet?
 
CDH has it. Mount the VFD anywhere but in the dusty shop area. Then put the remote switch and speed control pot at the grinder. Stick it in a closet, in a storage cabinet, or build a big Plexiglas box.

Adding a computer fan to provide air flow will also greatly reduce VFD failures.
 
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