They told me P=IV so watts a volt amp?!

Discussion in 'Electricians' Talk' started by Bumbling Diyer, Jan 30, 2004.

  1. eawr89

    eawr89 New Member

    CIVIL - blimey, that takes me back further than I care to admit - capacitive current leads voltage; voltage leads current inductive
  2. Bumbling Diyer

    Bumbling Diyer New Member

    Thanks all for your interesting and extremely well informed answers to my post!

    Perhaps I should get specific: I've got 5 50w LV downlighters run from 1 Aurora 50-250W dimmer switch. Here's the blurb from screwfix about it:

    2G 2W DIMMER 2X40-250W Suitable for use with mains and low voltage dimmable light fittings.

    * For use with resistive and inductive loads
    * Compatible with dimmable electronic transformers and tungsten lamps without de-rating
    * Integrated over-temperature protection fuse
    * Module case ultrasonically sealed to reduce buzzing
    * etc

    I can't find the power factor for the transformers (also from Screwfix), but from the sound of the blurb (ie. suitable for use with dimmable txformers without derating) I should be (just) OK. Which doesn't really explain why the switch buzzes fairly loudly and feels warm to the touch...

    Thanks again for all the expert advice... Any thoughts?


  3. The Trician

    The Trician New Member

    Could be the quality of the dimmer switch?
    If it is one where the electronic circuitry 'chops' the voltage then you will get a certain amount of heat. Am not well up on the design of these things. Not sure if the frequency of the supply is changed too. One for the electronics bods methinks.

    On the subject of Transformer lighting, I am just beginning to wonder how conventional low quality light switches will stand up to regular switching of large inductive loads? Much arcing I suspect. (They'll be starting 100HP motors with them next :)
  4. supersparky

    supersparky New Member

    dimmers always buzz, its normal
    they sometimes throw some heat aswell, its normal

  5. Screw-It-All

    Screw-It-All New Member

    The Trician is confusing "0" and "unity" (unity = 1)

    When the phase angle is 0 degrees (resistive load & VA = W) then the power factor is 1 (cosine of 0 degrees)

    As the phase angle increases from 0 towards 90 degress, the PF decreases from 1 towards 0
  6. unphased

    unphased Screwfix Select

    The full formula is P=VICos"thi" (havent got the greek symbol) where "thi" is the phase angle. VA (or kVA) is what the REC charge you for your electricity (the hypotenuse of the power triangle also called apparent power). W (or kW) is the active power measured in kW (the adjacent side) and the reactive power (kVAr or Wattless power) has no effect on the actual power of a circuit other than to give power correction. Power factor is kW/kVA and, as Engineer rightly points out, the closer to unity it gets the cheaper the electricity!!!

    If you understand all this then become a sparky!! ;)
  7. The Trician

    The Trician New Member

    Already am....I think!!
  8. eawr89

    eawr89 New Member

    Actually, domestic consumers only get (directly) charged for the kWs' that they use, hence the kilowatt hour meter by your consumer unit. The greek symbol in the VI Cos formula is phi by the way.
  9. unphased

    unphased Screwfix Select


    thi should be phi, sigh

  10. Dr Who

    Dr Who New Member

    Phi ..... When I was tought all this stuff, circa end of 80's, I remember using only theta for the phase angle!

    Dr Who
  11. eawr89

    eawr89 New Member

    Theta is the angular displacement of, for example, the voltage vector e.g. V sin (theta),measured in radians. Phi is used in phase angle formula to avoid confusion over this (theta used for magnitude, phi used to determine the inphase component of, for example, the voltage vector)

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