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  1. #1
    Join Date
    May 2009
    Location
    Kent UK
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    3,152

    Default power ratings of power tools, amps or watts

    just trying to understand a bit more about the rating of power tools

    i'd probably never have asked the question if i hadnt bought an american skilsaw

    my 8 1/4 wormdrive is rated at 13amps (my guess is it is the power draw) 110v 60Hz

    all my other tools are rated in watts (input) and happen to be 240v 50Hz

    is there any real difference in using amps or wattage as a measurement ?

    why the difference ?
    Limey Carpenter

  2. #2
    Join Date
    Oct 2005
    Location
    Grand Rapids, MI
    Posts
    62

    Default Re: power ratings of power tools, amps or watts

    Ohm's Law -

    Amps times Volts equals Watts
    Watts divided by Volts equals Amps

    Using algebra, if you have 2 of the 3 you can find the third. Your 13 amp saw @ 120 volts would be 13 x 120 = 1560 watts. If they gave you watts and volts, it would be 1560 divided by 120 = 13 amps.

  3. #3
    Join Date
    May 2009
    Location
    Kent UK
    Posts
    3,152

    Default Re: power ratings of power tools, amps or watts

    i remember ohms law from school, it was taught as V=IR

    i was under the impression that V = volts, I = Amps and R = Ohms

    like you say format in which ohms law it is learnt is immaterial because the unknown item can be calculated from the other two using algebra

    but where i am comming unstuck is that your calculation says that

    Watts (1560) = Amps (13) x Volts (120)


    the jump in understanding i am missing (all these years since school) is that your calculation, says that

    V stands for watts

    R stands for voltage
    Last edited by Tom Bainbridge; 12-20-2009 at 05:37 AM.
    Limey Carpenter

  4. #4
    Join Date
    Jan 2005
    Posts
    2,196

    Default Re: power ratings of power tools, amps or watts

    You guys got it all wrong


    This will clarify everything you need to know about watts and volts

    http://www.abbottandcostello.net/clips/watts.wav

  5. #5
    Join Date
    Nov 2009
    Location
    Broussard, LA
    Posts
    16

    Default Re: power ratings of power tools, amps or watts

    The Japanese often use wattage to specify the output of electric motors. It's easier to use if you're engineering an application involving it. On power tools, American manufacturers generally prefer specifying amperage and voltage.
    For power tools using AC/DC motors (DC machines with brushes and commutator), the wattage can be closely approximated by multiplying the amperage by the voltage. The amperage can be calculated by dividing the wattage by the voltage. It's not really important whether the tool's power is specified by amperage-voltage or by wattage. It's just a matter of manufacturer preference and industry standard.
    Keep in mind that these formulas don't apply to AC induction motors such as capacitor start or shade pole. Many table saws use capacitor start motors.

    Sherlock

  6. #6
    Join Date
    Jun 2004
    Location
    Des Moines, Iowa
    Posts
    3,866

    Default Re: power ratings of power tools, amps or watts

    Tom

    The 13 Amp rating on the motor is not what it will draw under normal load but rather when it is being pushed a little hard typically through a rip. You can exceed this amperage draw if you really drag down the motor. this will heat up the breaker and trip it, or if you have too big of rated breaker will start to smoke the motor.
    Remember all things electric or electronic have smoke inside them and when you let it out you have ruined them:)

    I stands for current in amps
    E stands for electromotive force in volts sometimes V
    R is resistance in Ohms
    P is power in Watts

    I=E/R

    This link shows the power wheel and all its relationships
    http://www.etgiftstore.com/ohmslaw.asp
    Mark Parlee
    BESI(building envelope science institute) Envelope Inspector
    EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
    Level one thermagrapher (Snell Training)
    www.thebuildingconsultant.com
    www.parleebuilders.com
    You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

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