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Thread: Power Factor

  1. #46
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    Quote Originally Posted by A J in WA View Post
    VA is the "apparent" power in an AC circuit. If you put a voltmeter across a load, and an amp meter in series with the load you would get a voltage and a current reading. Multiplying these together is the "VA" or "apparent power."
    VA (apparent power) = Volts x Amps.
    See post#3 for the relationship between these.
    Look at the second part: cos q = adjacent divided by hypotenuse.
    I was under the understanding that volts times amps gives you watts. Not volt amps. It is the reason why you do not pay for reactive power, because it is more difficult to measure, because of electricity is being returned to the source, that would cancel out the same quantity of electricity going to the load.
    Gold, the colour of choice for the discerning person.

  2. #47
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    Quote Originally Posted by .RC. View Post
    I was under the understanding that volts times amps gives you watts. Not volt amps. It is the reason why you do not pay for reactive power, because it is more difficult to measure, because of electricity is being returned to the source, that would cancel out the same quantity of electricity going to the load.
    Yes you are right it would be watts. Unfortunately without knowing the phase angle difference between the voltage and current you only have the manufactures PF value which will change with loading.
    Best Regards:
    Baron J.

  3. #48
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    Quote Originally Posted by .RC. View Post
    I was under the understanding that volts times amps gives you watts. Not volt amps. It is the reason why you do not pay for reactive power, because it is more difficult to measure, because of electricity is being returned to the source, that would cancel out the same quantity of electricity going to the load.
    Putting BaronJ's reply another way ......

    Apparent Power is called just that because the calculation does not contain only Watts. It also contains a component that relates to the "reactive power." Remember "Watts" is a way of saying "work" - like horsepower, where one form of energy is converted to another form. Remember, energy can neither be created or destroyed - except in an nuclear reaction.

    With a reactive load not all of the electric current entering the load is converted to another form of energy.

    If the load was a pure resistor, which is what an incandescent light globe is (pretty well), you could say that the Real Power (Watts) and the "Apparent Power" are the same, and the calculation is the same, and would yield the same answer.

    But not all loads are purely resistive, and this is where the "Apparent Power" and "Watts" calculations differ in the result they yield.

  4. #49
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    Hi AJ, Guys,

    Thanks for that explanation, you put it far better than I !

    Its a lot of years past since I did any of this at Uni, I confess up front that I hated the maths but it was something that you/I had to do to get through. I got my BSc but it was a struggle !
    Best Regards:
    Baron J.

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