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Why VA and not W?

  1. Dec 16, 2008 #1

    es1

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    I thought a watt by definition was a volt * ampere.
    However many safety documents use the unit VA and not W where presumably VA also equals volt * amp.
    I am just curious, why the new unit?
     
  2. jcsd
  3. Dec 16, 2008 #2

    berkeman

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    That definition of a watt only works for a resistive load. When the load is reactive, the current and voltage are not in phase (in general), and multiplying their instantaneous values won't give you the real power picture.
     
  4. Dec 16, 2008 #3

    mgb_phys

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    It's used by electrical engineers because for some AC power applications with a phase difference between the current and voltage the power varies.
    For the simplest case of purely resistive loads, rms power = rms V * rms A
     
  5. Dec 16, 2008 #4

    es1

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    Oh I get it. The symbols change because the units change. W is an instantaneous unit and VA is average power over a cycle? So why VA and not J? :)
     
  6. Dec 16, 2008 #5

    f95toli

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    No, the units are the same.
    The difference is that VA is used for cases where the load is or can be reactive, it is more a convention than anything else.

    Joule is the unit for the time integral of power; I.e. 1 J =1 Ws or 1 VAs (s is second).
     
  7. Dec 16, 2008 #6

    MATLABdude

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    http://en.wikipedia.org/wiki/Volt-ampere

    VA is not the average power, but the product of the RMS voltage and RMS current (each a sort of average--see the Wikipedia or Mathworld articles on RMS for more details), and will be higher than the 'real' power used. Also, you typically pay for your Volt-Amperes, and not your Watts (because the generator has to work harder, and you need additional capacity).
     
  8. Dec 16, 2008 #7
    Not quite es1. The difference in watts and VA is that at every point along the sine wave of a voltage driving a resistive load ohms law holds true and of course averaged it would too. But, when driving a reactive load the current is out of phase with the voltage by up to 90 degrees. So yes the average current and the average voltage do in fact agree with ohms law. But since the current and voltage are out of phase it is impossible for ohms law to hold true during every instant of the sine wave. This is why it is called VA. Real power is not being dissipated as it would in the watt. So your average power over a cycle statement is correct and to be honest I'd never thought of it that way until now, EXCEPT the part about power. It isn't really power since the current and voltage are not in phase.
     
  9. Dec 16, 2008 #8

    Residential customers do NOT pay for reactive power.
     
  10. Dec 16, 2008 #9

    dlgoff

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  11. Dec 16, 2008 #10

    es1

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    Thanks Guys. I think this clears it up for me. Post #7 makes a lot of sense too. Thanks Averagesupernova.

    I also found this link.
    http://en.wikipedia.org/wiki/Volt-ampere

    And learned a new term. "Apparent Power" whose unit is the VA.
    What a day! :)
     
  12. Dec 16, 2008 #11

    mgb_phys

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    Another question - why does the UPS I just bought claim to be 650VA (300W) on the box?
    Is there something very odd about the phase characteristics of battery + sine wave I'm not aware of?
     
  13. Dec 16, 2008 #12

    MATLABdude

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    That's true (I'm billed per kWh). Having just checked, in my neck of the woods, you only get dinged if you have low power factor, and most residences don't have low enough power factor to get dinged. No clue what that number is, however.
     
  14. Dec 16, 2008 #13
    Yes es1, apparent power. Somehow reactive power didn't look right when I typed it out. Apparent would be the correct wording.
     
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