Has anyone tried controlling their dust collector motor speed?

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Has anyone has looked into controlling the motor speed of their dust collector or vacuum to save on their electric bill and reduce noise? I currently use a shop vac (10A, maybe 250cfm) which works well enough for me, but I also have a 1.5hp dust collector (11A, 1000ish cfm). They both use a lot of power, but are also really quite noisy.

I’ve been running 3d carves that take all day (12ish hours, roughly $1.75/day by my estimates). This may be an apples to oranges comparison, but if it’s anything like pump curves it seems like I could greatly reduce the wattage without a proportionate reduction in cfm. It could also potentially reduce the noise, which will make me more sane and my neighbors more happy.

I’m guessing there’s probably a bunch of issues to consider by using some type of controller, but I’m motivated…

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I have a 3-phase, 2 hp Jet dust collector. I got a really good deal on it because the cannister filter had some dents from shipping damage. The price was low enough that I could add a VFD, and still be well under the price of a single-phase unit.

Your hunch about acoustic noise is spot on. If I reduce the motor speed by 20%, the perceived noise level drops by probably 80%. And I presumably still get at least 80% of the airflow, which is a great tradeoff in my book.

I heven’t measured the difference in power draw, but I wouldn’t expect the improvement there to be as dramatic as the noise reduction.

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Thank you for the response. In a search online, i noticed that people who use vfd’s generally seem to use them on multi phase motors. Not sure if ill be able to make this dream work without considerable investment.

But its good to know that at least the sound was better. Even with a negligible energy savings, it would still be worth it to me.

Thanks again!

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Hey Captain,

three-phase induction motors are very common on professional dust collection systems like the Felder AF 22. Unlike other motors, an induction motor’s speed is controlled by the frequency of the three-phase electricity. A VFD is a device that provides three-phase electricity with a variable frequency allowing for a wide speed range. One advantage of a VFD is also that it can produce three-phase electricity from single-phase electricity, e.g. from split-phase electricity that is common in the US. In the Felder AF series, Felder uses Motek motors (Made in Italy), three-phase induction motors as well as capacitor start induction motors for single-phase electricity.

One advantage of induction motors is that they are relatively quiet, because they have no carbon-brush commutators, they are brushless. Also they can have a large speed range, and, in combination with a VFD, a constant torque over the entire speed range. Induction motors are considered as the workhorse in the industry.

However motors that are designed to run on three-phase current without a VFD are designed to run at 50 Hz (or 60 Hz in the US and Japan), and before attaching them to a VFD and altering their speed, it is good to look at the datasheet of the motor or to ask the motor’s manufacturer about the allowed speed range. Usual VFDs can produce 0 – 400 Hz, my Omron MX2 can produce 0 – 580 Hz.

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Yes, VFD’s are really suitable only for three-phase motors. Please forgive the brief nerdy deep-dive here, but “single-phase” induction motors are actually two-phase motors, with a start circuit tacked on to generate the second phase until the motor gets up to speed. That start circuit is highly specific to the motor’s design voltage and frequency. Change either by very much, and you are highly likely to release the “magic smoke”, without which no electric motor can operate.

That said, there’s a good chance that you could find a three-phase motor that’s a mechanical drop-in replacement for your current motor. Check the “frame number” on your current motor, and you might be able to score a deal on a compatible used or surplus 3-phase motor.

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Hey Dennis,

that’s true since they produce three phases :slight_smile:

That’s perfectly true, it is explained if one clicks on the “capacitor start induction motor” link I provided above (the german version of the article provides even more detail). But I did not mention this since it does not matter here, I mentioned it just to explain how the single-phase versions of the Felder dust collectors work, and to emphasize that the Felder single-phase dust collectors models are driven by induction motors too (=capacitor start motors are induction motors too). I didn’t mention this type of motor for talking of altering the speed, the delay with which a capacitor produces the auxiliary phase, as you pointed out, is necessarily specific to a fixed speed.

You are right, I should have added “… (of which the latter are not suitable to alter the speed)” to avoid any misunderstanding.

By the way, to run an induction motor on single-phase electricity using capacitors is also possible with a three-phase motor, by using a Steinmetz circuit. Just like the two-phase capacitor motor, it produces less torque. However, since the advent of VFDs, which can generate three phases from single-phase current, that circuit has become little used.

Why not, nerdy deep-diving is fun (guess how I know) :slight_smile:

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Hey Dennis,

congratulations on the good deal, that is of course always delightful when you have the luck to get such an opportunity

What is the range to lower the frequency that is usable with your motor?

Did you rely on datasheet/manufacturer specifications when lowering the frequency?

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Thanks, Aiph5u. If Jet publishes detailed motor specs, I wasn’t able to find them. But as a practical matter, the operating torque drops rapidly with RPM, so power requirements are minimal at very low speeds. I’ve tested the speed down to 10% with no problem. Of course such low speeds aren’t actually useful for dust collection, as sawdust wouldn’t stay in suspension in the ductwork.

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Thank you both, this has been an insightful conversation. I’m not sure I want to swap out the motor on my dust collector just yet. I’m a habitual upgrader of machinery and I’ve found that custom modified machines tend to shrink the buyer pool.

Cutting these long projects is currently a hobby, but it’s getting to a point where it may become a part of life. It probably is worth exploring a more expensive option… or at least one with more effort. I think I’ll start by keeping an eye out on Craigslist and see if I can build one from parts. Maybe I could make some sort of hose valve to use the same filtration/collection as the dust collector.

Anyway, this has given me a lot to think about, thanks!

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Hey Dennis,

do you use the Jet 2 hp dust collector just for the Onefinity, or for other machines too at the same time?

I ask because what I’m thinking of is to use a dust collector (3 kW/4 hp) for both the CNC and a thickness planer as well as a jointer, the former to run constantly while the latter will only run from time to time. Besides closing the blast gates I would like to slow down the dust collector when only the CNC is running. I am also not sure yet if I will use a 100 mm or a 70 mm hose for the Onefinity.

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I would start by measuring the current draw. Just because the nameplate on the motor says 11amps that’s not normal use. Neither of my dust collectors are very noisy (compared to a shop vac). I even bought a 1hp Harbor Freight special that’s very quiet specifically for my 1F. A simple box built around the dust collector with some insulation would be worth trying. To reduce power switch to a larger filter for less restriction. If possible remove the filter and just send the dust out of the back of the shop.

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I’m still getting my Journeyman set up, but that’s basically my plan as well. The Jet is rated for 1200 cfm at 4" (of H2O) of static pressure, so it should be able to handle two or three machines at the same time.

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There are obviously very technical considerations beyond the scope of this forum but yes theoretically power consumption is related to the cube of fan speed - divide your RPM by 2 and you divide your kW by 2^3=8. Actual achievable energy savings would be a bit less but if you can get away with lower fan speeds by all means do so. However, retrofitting your dust collector at ~$1/operator-day savings might be an unacceptably long payback. And “get away with” is up for interpretation. Personally, I want the maximum flow rate possible in my dust collector for as much help moving the <0.5 micron dangerous dust as possible.

Lastly you may be disappointed with the sound savings. Here’s a chart showing the relationship between flow, static pressure, and sound:

Screen Shot 2022-09-20 at 12.49.30 PM
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It’s in metric so I’ll convert. 4 inches of static is about 1000 pascals. 0.5 m3/s is about 1000 cfm. So on this generic graph that’s about 98 db. Cut your flow rate in half to 500 cfm (~0.25 m3/s) and your sound pressure is still 95 db. Your mileage will vary but don’t expect amazing reductions in sound.

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Dadodad, your interpretation of the Engineering Toolbox graph left me scratching my head, trying to understand the major discrepancy with my own experimental observations. I think I may have figured it out.

Here’s the thing: if you reduce the air flow rate through a given ducting system, the pressure drop will also be reduced. I’m certainly no fluid dynamics expert, but my recollection was that the pressure drop is more-or-less proportional to the square of the flow rate. Which is consistent with the power being related to the cube of fan speed, given that the mechanical output work of the fan is equal to the airflow rate times the pressure differential. I found a good reference just now that appears to confirm my recollection.

So let’s revisit your example, except using 800 Pascals as the baseline pressure, at 0.6 m3/s flow rate, just to make it easier to read the graph. At those baseline numbers, the graph shows a Sound Power Level of 96 dB.

Now let’s cut the flow rate in half, to a volume flow of 0.3 m3/s. The static pressure goes down as the square of that change, i.e. by a factor of four, which brings it all the way down to 200 Pascals. In that scenario, the graph shows a Sound Power Level of 81 dB.

I’m sure you’ll agree that reducing the fan noise from 96 dB to 81 dB is indeed a major reduction.

However, my “perceived” reduction in noise was even greater. I suspect this is because the quality of the sound also changes. My ears find high-frequency noise to be much more objectionable than lower-frequency noise. I have absolutely no objective data to back this up, mind you, but it stands to reason that running the fan at lower RPM will shift the noise-frequency spectrum downward. The Engineering Toolbox graph doesn’t address this difference at all, but presumably just includes the sound power contributions from all portions of the audio frequency spectrum.

In any case, I stand by my previous report that a 20% reduction in fan speed resulted in major noise reduction, at least for my particular dust collector and my particular pair of ears. When I have a bit more spare time, I’ll download a sound-meter app to my smartphone, and report back here with some more objective test data.