Motor input power?

Discussion in 'General Electronics Chat' started by Leeb1987, Jun 23, 2013.

  1. Leeb1987

    Thread Starter New Member

    Jun 23, 2013
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    Hi all
    Wondering if anybody could give me a bit of help with working out what size motor I can run. Basically I have a 1.7kw generator and on it there's a label that says it will run a 800w (input) air compressor. So I figure I can run a 1hp motor? Any info or if I'm way off the mark then Any input would be great
    Thanks... Lee
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    One horse power is approx. 750 electrical watts.

    Yes, it can run a 1 hp. motor. The only question being can it START a motor under load?

    a 1 hp. motor may require much much more than its stated power rating when starting. Being only 1700 watts MAX. The generator may have trouble starting motors which draw more than twice their 1 hp rating, such as refrigerators, Window A/C units, etc. Only by trying it will you know if it will work for you.
     
  3. Leeb1987

    Thread Starter New Member

    Jun 23, 2013
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    Hi
    Thanks for the reply that's what I was thinking I know the power needed to start the motor will be a fair amount more than the running power. And I was hoping that the rating of the genny wasn't the absolute max it will put out. Is there any way of running anything bigger? IE using a capacitor? Maybe.
    Thanks..... Lee
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
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    Why is that? The gen puts out twice the power consumed by the motor. The reason it says 800W is because the 1.7KW is peak power, not continuous. By keeping the motor under 800W, you should be OK.

    Nope.
     
  5. Leeb1987

    Thread Starter New Member

    Jun 23, 2013
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    Hi
    Ok yeah I get what your saying. I think I was hoping for a quick fix without getting a bigger genny. Obviously a stupid idea so I will just either get a smaller motor or a bigger genny. Thanks for the help
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Not a stupid idea. If the generator is rated 1.7kW maximum (not surge) power and it states it will run an 800W motor (about 1HP electrical) than it should also start a 1HP motor. If you have the motor, give it a try. It should do no harm if you shut the motor off immediately, if it doesn't start properly.
     
  7. Leeb1987

    Thread Starter New Member

    Jun 23, 2013
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    Ok so thought I would test the genny an see what kind of power it could handle. So I ran a 1250w 25L little portable compressor and it ran perfect slight cough when the compressor started but fine. Then tried to run a normal 1100w compressor motor and it wouldn't hardly turn it. Anybody care to explain to me the reason why
    Thanks......lee
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Likely the normal compressor required more torque to start which required too much surge current from the generator. Is it a piston type compressor?
     
  9. Shagas

    Active Member

    May 13, 2013
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    Won't connecting huge electrolytics across the supply solve the initial surge current issue?
     
  10. Leeb1987

    Thread Starter New Member

    Jun 23, 2013
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    Yes it was a piston compressor. If there was a way to get over the initial start surge would the motor run?
     
  11. Brownout

    Well-Known Member

    Jan 10, 2012
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    No! A thousand times, no! Don't ever try that.
     
    Last edited: Jun 23, 2013
  12. Brownout

    Well-Known Member

    Jan 10, 2012
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    So you're saying the smaller portable compressor was rated at higher watts than the 'normal' one? And the higher wattage one started while the lower wattage one didn't? Something doesn't compute.
     
  13. Leeb1987

    Thread Starter New Member

    Jun 23, 2013
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    Exactly that's what I couldn't understand I assumed it was because it was a different type of motor? Or the makers of the motor exaggerated the power of it

    And when I say "normal" compressor I mean a standard workshop piston type belt driven compressor
     
  14. crutschow

    Expert

    Mar 14, 2008
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    It's likely due to the larger starting torque required by a belt driven piston compressor. If you started the motor without the load (loose belt) and then slowly tightened the belt to start the compressor, the generator would probably handle the load.
     
  15. Brownout

    Well-Known Member

    Jan 10, 2012
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    Do you happen to have a "clamp on" ammeter? It would be interesting to see what currents both motors are drawing on startup.
     
  16. BillB3857

    Senior Member

    Feb 28, 2009
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    If the unloader valve is bad on the one compressor, the motor is trying to start under a tremendous load. The unloader valve is designed to bleed off the compressed air that is trapped in the line between the compressor and the storage tank. By bleeding this air off, the compressor is allowed to start under a no-load condition.
     
    shortbus likes this.
  17. Leeb1987

    Thread Starter New Member

    Jun 23, 2013
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    I have got a clamp on amp meter I will check the current draw when I have some more time. And I tried running it with zero pressure in the tank an the bleed valve open to make sure there was minimal load on start and still nothing. I have another small direct drive comp rated a 2hp and the genny ran that fine aswell so I'm starting to wonder if the piston comp motor is drawing more power than it should do but it runs perfect off mains power so I think checking the current draw would be a good idea
     
  18. Brownout

    Well-Known Member

    Jan 10, 2012
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    I'm still stuck on the larger compressor being less wattage. It's gotta be at least 3hp, so that would be 2250W. I think the data plate is wrong or the numbers have been scratched. The ammeter will tell the whole story.
     
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