Variable Frequency Drive for Dust Extractor Fans

Smart design with variable-speed fans and operable blast gates can save a lot of energy while providing good extraction where it's needed. December 31, 2012

I have heard different stories about variable frequency drive (VFD) for extractor fans. How do they work?

Sellers Ive researched about the drive claim it will reduce electricity costs. I suppose it will reduce noise as well. I have spoken to two suppliers of ducting in the UK. They both say that it causes an increased risk of fire as shavings tend to build up in the ducting. They also say that extraction fans are designed to run at one frequency and varying the frequency makes the fan very inefficient. Is this true?

Forum Responses
(Dust Collection and Safety Equipment Forum)
From contributor K:
They are correct. If you slow the fan down, you risk fallout in the duct. You can measure the duct velocity to determine how much play you have. If the velocity is high enough, you can reduce the fan speed and still maintain necessary carrying velocity for the wood dust. Slowing the fan down may help with sound, but it's unpredictable unless you have access to original fan data, and even then it's a guess.

From the original questioner:
Would anyone ever install VFD or recommend it? How would I measure the air velocity in the duct? What sort of data would I need from the fan manufacturer to set up a VFD or is it a case of tweaking the frequency to vary the fan speed until the velocity in the duct reached a sufficient level to remove the waste?

From contributor K:
Yes - vfd's are a great energy cost saver when you have varying flow requirements based on multiple processes using the same duct. If you need all lines operational, then you would run vfd 100%/whatever needed to maintain duct velocity. If you wanted to shut a line off, then you find the new rpm that maintains minimum duct velocity and mark it, and so on and so forth for each process. Since fans are variable torque machines, most motors are adequate for running VFD's up to 4:1 turndown - which is way plenty for most applications. The motor nameplate should have all necessary details to get the VFD with.

From contributor M:
VFDs are great for a lot of things. I will not take you through all that. Just look at the kind of analog inputs your VDF will take and you can see what it will do. One hindsight concern: I spent about $10K on engineering. When I was done, it all works great. The vendor of my particular bagger would have done all but the most complex custom mods (some of the analog data on static at different points in the system used by the VFD to set its performance) for free. Talk to your vendor. They may save you big money. Avoid HVAC companies with their engineers in tow unless they agree by contract to work directly under the vendor's direction.

From contributor J:
I talked to Dantherm about their VFD and auto gate system. Evidently they use the Ecogate system. Is that what you have?

From the original questioner:
The VFD would have to keep up the minimum velocity in the biggest diameter duct near the fan to stop fall which ever machine is being extracted from. So, if you have a machine like a mortiser that has a 63mm ducting port, the waste from it would end up in, for example, a 600mm ducting pipe before it goes into the fan. How could the vdf possibly maintain a minimum velocity in the 600mm ducting while drawing air through a 63mm ducting inlet? Surely other blast gates would have to be opened. Would they?