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Amps, HP, & true HP for motors

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  • Amps, HP, & true HP for motors

    Ok, two years ago I purchased a Ridgid shop vac and can't use it for fear of burning down the house from it's 5.5hp rating! I got 3 questions:

    1-how are the electric motors rated for this HP?
    2-can the true HP be found when the volts & amps are known?

    3-I'm in the process of replacing my 15 yr old sears compressor which is rated at 4hp, 15amps@230 volts. When I went to home depot the one I like is rated at 7hp, 17amps@230 volts. What's the true hp for the compressor? I presume the old sears may be a true rated HP motor?

    Thanks all

  • #2
    Hi Woodchuck, I got 3 answers.

    1.inaccurately and deceptively
    2.Yes. amps x volts = watts. 746 watts = 1 hp.
    so (amps x volts) divided by 746 = actual hp.
    3.(17 amps x 230 volts) divided by 746 = 5 1/4 hp.

    This is the basic formula and doesn't account for motor efficiency, temperature, the space-time continuum, etc. [img]smile.gif[/img]


    Great article on the different types of motors and how they're rated.

    hope this helps.
    "Did you put the yellow key in the switch?" TOD 01/09/06


    • #3
      Universal motors tend to be rated notoriously, outrageously, untruthfully, deceptively, erroneously, optimistically, incorrectly, and sinfully high. It's an absolute crock that should be outlawed. Liars everyone of one them! (well, maybe not all...)

      My Ridgid shop vac is 6.25hp but plugs into a 110v a$$ it's 6+hp! My DC is 2hp and draws 12amps @ 220V....the motor probably weighs 45lbs. I'm not picking specifically on Freud router boasts 3-1/4hp, Sears makes mega hp claims that aren't so. So does DeWalt, Bosch, Milkwaukee, PC, and most others. Those motors are not capable of sustaining anywhere near that output for more than fractions of a second before they go up in smoke. Induction motors tend to be rated much closer to their actual hp, but there's still a bit of overstating going on. They will at least sustain somewhere near their rating.

      The efficiency factor that Lorax mentioned falls roughly in the range of 0.68 (low) to 0.80 (high).
      My router draws 15amps @ 110v. 15x110x(an avg eff rating of 0.74 divided by 746, gives the Freud a rating of closer to 1.5hp.

      We as consumers need to collectively make a big stink over this. Write, call, email, or shout, but somehow let mfgers know that we don't appreciate being misled.