I asked this in the electronic section and been recommended to take it here instead. I've come to this site to ask some advice. I recently bought a 2 HP (1500 watt) workshop cyclone dust extractor, and I became suspicious about its rating.

So I ran it with minimal loading, so just the filter but no additional piping and measured the AC power input (or output from the mains socket) this measures as 1000 watts. (240 volts). Then when I covered the inlet (to increase the air flow restriction) the current draw from the outlet reduced to ~800 watts.

I've been told that the motor rating may not be reflected in the current draw as it simply may not be set up to be fully loaded, and by throttling the inlet I infact actually just reduce the load, (not sure how this works).

I've initially been thinking the motor ISN'T a 2hp motor, but maybe this is irrelevant?

On another point, I bought an anemometer to check the airflow. It gives me readings suggesting that it's only sucking about 80% of the stated. so about 500cfm vs 600cfm. Does anyone have any good ideas on how I can check the calibration of my anemometer?

  • By covering the inlet, you are just spinning air, not moving it, so it's no surprise the current draw goes down. Next: motors seldom draw their rated current. Once you get a duct system hooked up and it's doing some real work, measure again. Check the nameplate on the dust collector motor too. Bottom line: If it performs well, no worries. Apr 12 at 4:08
  • A 1500W input 100% efficient (doesn't exist) motor could not possibly be more than 2.0107 HP. The 99.47% efficient motor to make 2 HP is not very likely. Typical cheap 80% efficient motor more like 1.6 HP. Pretty sure my "not import crap" dust collector only claims 1.5 HP for a motor that needs a 20A 120V circuit and prefers to be run off a 240V circuit at half the amps. So, you've been lied to by the marketing department....
    – Ecnerwal
    Apr 12 at 11:10

1 Answer 1


There's two things going on here.

First: most anything cheap-ish with a motor lies about the ratings. It's entirely possible that things won't melt at 1500 watts but it actually draws less than that almost all the time. It's entirely possible that 600 CFM will only happen in perfectly ideal situations. Or it may never truly happen.

Second: blocking off the inlet of a vacuum always results in the motor using less power, always results in it increasing in speed, and almost always results in it getting hotter.

  • Presumably the downvoter is yet another one of those people who don't understand how centrifugal fans respond to restrictions. Or just the downvote fairy... This is entirely accurate. I made some measurements recently for a different question: diy.stackexchange.com/a/270485/18078
    – Ecnerwal
    Apr 12 at 11:16
  • @Ecnerwal Thinking the same thing.
    – JACK
    Apr 12 at 12:23
  • I've read that consumer electrical goods manufacturers who use HP as a marketing tool routinely overload their products to destruction and record "peak horsepower" at the moment that sparks and flames first appear. Small engine manufacturers used similar tactics until all such HP ratings were banned in the U.S. Now they can only quote torque, not HP. For consumer electrical goods, best to believe amps and ignore HP. More insight at ecmweb.com/content/article/20892800/…
    – MTA
    Apr 12 at 12:57
  • Basically, I misread the ad and I thought I was buying a 1050cfm unit with 2hp but the units were in m^3 /hr which is about 600cfm. So thats my own fault, Im looking for any reason to claim it's not as advertised and get my money back. So far, it seems as though the motor rating is a little embelished but the airflow is harder to show without a properly calibrated anemometer. I appreciate all the responses! Apr 12 at 21:12

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