protecting inverters from dust

[jim mills]

How important is it? I have two hitachin inverters in my shop that are NEMA1 enclosure. I am in the process of purchasing another drive, and the salesman says i'm crazy not to run NEMA 4x enclosures. Says wood dust is conductive, and its just a matter of time before the units get fried. What say you?

[Erik Loza]

I don't have an answer for you except to say that it never ceases to amaze me how dust can get into sealed enclosures. I'll pop the cover off the j-box on one of our machines and there's dust in there. Maybe not a lot, but there is. And that's a rubber-gasketed box with compression fitting for the cord. I've never personally heard of a machine get shorted out due to dust penetration but maybe inverters are more sensitive than regular switches. Curious to hear what other say.

Erik

[Anthony Whitesell]

I don't know about "wood dust", but the representative from APC (UPS manuafcturer) said that "dust" (didn't specify what kind) is insulating. So if the dust builds up on the heat sink or cooling fins, the unit (or component) will begin to retain heat and warm up to the point where it overheats. FWIW, I was speaking with the APC rep as he was disassembling a three phase UPS to replace on the boards that had blown, according to him, due to thermal failure caused by reduced cooling capacity due to the layer of dust on it (so think you could not see any writing anywhere on the unit).

Before you place the inverters in sealed NEMA 4X enclosures, make sure you have the enclosure the correct size enclosure and the inverter mounted such that the enclosure will act as the heat sink or supply NEMA 4X rated cooling fans (usually a fan with a finger guard, filter, deflective shroud). Hoffman enclosures has a calculator on their website that will estimate the cooling capacity of the enclosure based on the size of the cabinet, heat load, ambient air temperature outside the cabinet and the maximum allowable temperature in the cabinet.

[Joe Jensen]

I'm an electrical engineer and a bit obsessive. I put a NEMA4 drive on my 4HP jointer (wow it's huge). But I put a NEMA1 drive on the drill press mostly because it's a lot smaller and cheaper, but also because tons of folks said not to worry. Having said that, the first drive was defective and when I powered it up a Capacitor exploded and the fan shot flames 2 feet out the top of the drive.

[Malcolm McLeod]

If wood dust is conductive, then we can all save money and 'wire' our shops with scraps from the woodpile. Just be VERY careful not to touch the oak to the pine!!;)

Sarcasm aside, all materials are conductive - - to some extent - - IF you apply enough voltage. The lousy conductors are what we typically call insulators.

All the VFD's I've commissioned or worked on have been in dedicated enclosures, or had their own NEMA-rated 'self-enclosure'. The purpose of the enclosure is 2-fold, primarily to protect the personnel from shock/arc hazards, and also to protect the drive from the local environment. NEMA 1 serves very well for personnel and for most applications is adequate to the environment. NEMA 4X is overkill unless you have a water splash hazard nearby.

Wood dust can coat the heat sinks on VFDs (and fans on larger units) and lead to overheating. Good dust collection is probably the best way to prevent this. If not, and you have dry compressed air handy, you can just blow off the dust regularly.

Third alternative is to put the VFD in an enclosure. All that's required is that the enclosure be reasonably dust-tight. I'd recommend using a metal box (i.e. Hoffman or Rittal), but this also leads to either conduit, cord-grips, or grommets on all box wiring penetrations. Follow the VFD manufacturer's recommendations for clearances on top/bottom/sides/front of VFD - - this ensures arc-flash clearances and limited heat build-up. (I wouldn't use a wood enclosure - both because of potential fire risk and its insulation properties causing heat build up.)

[jim mills]

Thanks for the comments. As far as dust, its a wood shop. Despite all my dust collection efforts, its dusty! I just went & blew the drives out with compressed air. A little dust, but not too bad...

[Steve Peterson]

I don't have an answer for you except to say that it never ceases to amaze me how dust can get into sealed enclosures. I'll pop the cover off the j-box on one of our machines and there's dust in there. Maybe not a lot, but there is. And that's a rubber-gasketed box with compression fitting for the cord. I've never personally heard of a machine get shorted out due to dust penetration but maybe inverters are more sensitive than regular switches. Curious to hear what other say.

Erik

The air inside the box will expand and contract with temperature changes. The inverter will likely heat up well above ambient temperature when it is operating and cool back down at night. A small pinhole can allow dust laden air to enter.

Hard disks often have an air filter to provide a preferred channel for the expanding air to go through. They are extremely sensitive to dust particles, even the non-conductive kind. :)

Steve

[Erik Loza]

The air inside the box will expand and contract with temperature changes. The inverter will likely heat up well above ambient temperature when it is operating and cool back down at night. A small pinhole can allow dust laden air to enter...

Never thought of that. You learn something new every day.

Erik

[Anthony Whitesell]

I had not looked at VFDs in enclosures in that regard (gasoline in plastic cans yes, not VFDs-electronics-etc.). Since we are not as concerned about water infiltration, a foam gasket that is breathable but would provide filtration could be appropriate.

In looking at the OP and in light of the above. I am now reminded of the NEMA panel classifications.

NEMA 1 enclosures are typically used for protecting controls and terminations from objects and personnel. This style of enclosure, while offering a latching door, does not have a gasketed sealing surface. NEMA 1 enclosures are used in applications where sealing out dust, oil, and water is not required. Motor start/stop stations are often housed in NEMA 1 enclosures.

NEMA 4X enclosures are made of stainless steel or plastic. NEMA 4X enclosures are used in harsher environments than standard NEMA 4 units. Applications where corrosive materials and caustic cleaners areused necessitate the use of a NEMA 4X enclosure. Applications include food, such as meat/poultry processing facilities, where total washdown with disinfectants occur repeatedly and petro-chemical facilities, including offshore petroleum sites. NEMA 4X is used when protection from the worst environments is required. NEMA 4X enclosures are available in sizes from small wall mounts to two-door floor mount models. Wiegmann NEMA 4X enclosures are made of 304 or 316L stainless steel.

Interesting to our applications would be NEMA 4. NEMA 4 enclosures are used in many applications where an occasional washdown occurs or where machine tool cutter coolant is used. They also serve in applications where a pressurized stream of water will be used. NEMA 4 enclosures are gasketed and the door is clamped for maximum sealing. They have continuous hinges, mounting feet, and padlock hasps. NEMA 4 enclosures are available in sizes from small wall mounts to two-door floor mount models.

I now have the feeling the seller of the VFD was thinking metal working tools, not dry wood working tools.

For a full summary https://www.automationdirect.com/static/specs/encnemaratings.pdf and http://www.siemon.com/us/standards/nema_comparison.asp

It actually appears that a IP52/NEMA 5 enclosure is what should be used in woodworking if you are interested in keeping the dust out.

[Joe Jensen]

Keep in mind that Inverters (VFDs) generate quite a bit of heat. The cost of the upcharge from a NEMA1 to a NEMA 4X version was a lot and it's because the heat sinks alone on the NEMA4X are bigger than the NEMA1 VFD. The links Anthony posted look to be for enclosures no considering the heat generated by what's inside the box. You can't buy a NEMA1 VFD and put it into a NEMA 4 enclosure. Just like you can't put a PC or Laptop on a sealed box and expect it to survive. If you want a dust resistant VFD you need to buy the version from the VFD manufacturer that's built that way.

[Anthony Whitesell]

Keep in mind that Inverters (VFDs) generate quite a bit of heat. The cost of the upcharge from a NEMA1 to a NEMA 4X version was a lot and it's because the heat sinks alone on the NEMA4X are bigger than the NEMA1 VFD. The links Anthony posted look to be for enclosures no considering the heat generated by what's inside the box. You can't buy a NEMA1 VFD and put it into a NEMA 4 enclosure. Just like you can't put a PC or Laptop on a sealed box and expect it to survive. If you want a dust resistant VFD you need to buy the version from the VFD manufacturer that's built that way.

As I mentioned previously, with proper mounting the enclosure can be used as a heat sink. This is standard practice. Hoffman Enclosures has a calculator on their website. You need to know the size and material of the enclosure, maximum internal and external temperatures. I have upsized enclosures to make the required heat dissipation. I have also included internal fans to circulate the air within the enclosure to provide a more uniform and increase heat dissipation to keep magic smoke in all the parts.

[Malcolm McLeod]

... You can't buy a NEMA1 VFD and put it into a NEMA 4 enclosure.

Joe, my apologies if this seems like piling on, but Anthony is correct. I have worked on or with dozens of control panels with a NEMA1-rated (or even unrated) VFD installed into the appropriately area-rated control enclosures - up to and including air-purged and explosion-proof enclosures. The VFD can be installed alongside PLCs, control power transformers, braking resistors, and numerous other devices - all producing heat. If the panel is properly sized, it has enough surface area to shed the internal heat load to the outside utilizing just the large surface area (heat sink) of the panel. Of course, as the heat load increases the panel can get very large. Quickly. Really quickly if the panel is located in an unconditioned space, but its done all the time.

That said, the above is describing an industrial environment. The OP is looking at a shop. You can install a VFD virtually any way you want, as long as you follow good design standards and codes.

As an aside, something I see mentioned only rarely, is to remote mount the VFD. You can put the VFD in another room or clean space, power it from your CB panel, run VFD output to the tool (or a wall plug), and place the control module on or next to your tool. For control, you have 2 options: 1. Set it up for discrete control (i.e. FWD/OFF/REV) plus a potentiometer for a speed command; or, 2. Remote mount the control module (many VFDs have a detachable control faceplate and mounting kit). Interface wiring from VFD to faceplate can be as simple as a CAT5 cable. Costs and complexity can grow quickly ...but to each their own.

Hope this is some help to those looking for options.

[Mike Heidrick]

On my cncs i put the hitachi drives in 4x boxes as they are pretty cheap on ebay and i like doing nice neat wiring on the gland plates in them. I follow the spacing reomendations. My vfds also have fans. That being sad i have a teco under my jointer and a fuji undet my shaper from laguna that way not in boxes per say just the under tool enclosure. They have been that way since 2006 and 2009 respectively with zero issue.

[Rollie Kelly]

The only way you will keep dust out of any enclosure is to put a slight positive pressure on the inside. It goes without saying, use clean air.
Rollie

[Allan Speers]

This is probably much ado about nothing - unless one routinely slows a motor down a lot, or uses the dynamic braking feature.
Then again, most folks probably DO use dynamic braking, so...

I'm thinking that if one were using an add-on braking resistor & heat sink, (available as an option for most VFD's) the that alone would protect the VFD's electronics from heat damage, and one could probably get away with a sealed metal enclosure. This certainly worked with my 3 HP Unisaw, for almost 10 years, though it probably would have been fine even without the extra braking module.

------

Malcome, you mention remote-mounting the VFD. The only caveat might be (MIGHT be) that longer runs from the VFD to the motor can exacerbate certain negative electrical problems (spikes, or reflections, or something) that can theoretically damage your motor if you take the speed down to say 50% or less, especially with a non-inverter rated motor. I don't feel like getting into another useless debate, but this is what I was told by techs from 2 VFD manufacturers, plus Baldor & Leeson. Users can choose to believe what they want.

[Tom M King]

I cut some filter material out of a furnace filter, wired it over the top of the heat sink, and have a small fan blowing air down through the filter. I can feel the air coming out of the bottom of the heat sink fins, and it's never very warm. I blow the filter off once in a while with compressed air.

[Malcolm McLeod]

.. that longer runs from the VFD to the motor can exacerbate certain negative electrical problems ....

^True that, but distances when I start to worry are typically >200ft and >10hp. Seems a stretch for most here, so I omitted. (I can wire a lap around my whole shop in 100ft:()

Edit: I believe this ties back to your earlier thread about line reactors, inductance, harmonics, and reflected wave issues. But since harmonics and dust accumulation are COMPLETELY different, I'll shut-up now.... Or, we could discuss zombie apocalypse?!;)

[Anthony Whitesell]

FYI. The 3% voltage drop occurs around 50ft at the maximum amperage per wire gauge (ie., 15A on 14g wire, 20A on 12g wire, etc.).

[Allan Speers]

Edit: I believe this ties back to your earlier thread about line reactors, inductance, harmonics, and reflected wave issues. But since harmonics and dust accumulation are COMPLETELY different, I'll shut-up now.... Or, we could discuss zombie apocalypse?!;)

Nah. It goes back further to my thread about the value of using a true inverter-rated motor for vari-speed operations.

Thanks for the clarification. That's very important info.

[Joe Jensen]

As I mentioned previously, with proper mounting the enclosure can be used as a heat sink. This is standard practice. Hoffman Enclosures has a calculator on their website. You need to know the size and material of the enclosure, maximum internal and external temperatures. I have upsized enclosures to make the required heat dissipation. I have also included internal fans to circulate the air within the enclosure to provide a more uniform and increase heat dissipation to keep magic smoke in all the parts.

Anthony and Malcom, I agree with you. One can certainly put a VFD in a NEMA enclosure, IF the enclosure is rated to dissipate the heat the VFD will generate. You can't just drop a VFD into a NEMA4 box that will physically hold it.

Also, I don't think woodworking will require a completely dust free positive pressure environment. Dust proof is like water proof. Means a lot more than resistant.

[jim mills]

Thanks for all the feed back. Sounds like blowing the inverter out with compressed air on a regular basis is effective & cost effective way to maintain it in a wood shop enviroment.