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Reactive power experiment

  1. Feb 21, 2013 #1
    Hi everyone! I always use this forum to consult but I had never had the need to post something.

    I need to create an experiment in which I measure reactive power with a meter (see attachment).

    I was thinking on using a transformer with a resistive load on it's secondary, but I'm not sure if the current generated by the resistor on the secondary would be percieved as reactive by the meter.

    Would this work?

    Besides connecting a motor to the line, and using a transformer, is there any other way I could generate reactive power for it to be perceived by the meter?

    Thanks for your help.

    PD: My native language is Spanish, sorry if I made any mistakes while writing this.

    Attached Files:

  2. jcsd
  3. Feb 21, 2013 #2


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    Staff: Mentor

    Welcome to the PF.

    To effectively observe reactive effects, you will need a 2-channel oscilloscope. Do you have access to such an instrument?
  4. Feb 21, 2013 #3

    I don't have an oscilloscope at hand, however, the purpose of the experiment is to test the meter's capability to measure the reactive power consumed by a load connected to it, I just need to connect something that consumes reactive power as a load.
  5. Feb 21, 2013 #4


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    Staff: Mentor

    A load that contains an inductor or capacitor will have a reactance, but I'm not sure how to use the meter to measure the reactive power.

    BTW, in the circuit you posted, the first 100 Ohm resistor will be dissipating close to 500 Watts. I'm not sure that's such a good idea...
  6. Feb 21, 2013 #5
    Yes, I actually have really big resistors that can dissipate that sort of power as heat, that is really not an issue, it's not very efficient but this is for test purposes only, not something permanent.

    I read this:
    Transformers - Transformers produce magnetic fields and therefore absorb reactive power.
    The heavier the current loading the higher the absorption

    Here (Page 2). Is that correct? and if so, is the reactive power absorbed by it comparable to the reactive power absorbed by an electric motor?
  7. Feb 21, 2013 #6

    The Electrician

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    Gold Member

    If you want to use a single meter to measure reactive power, what you need is a "VAR meter"; see:



    VAR meters are not commonly found outside of the electric power industry.

    An alternative method of measuring reactive power would be to measure VA (volt-amperes) with two meters--an ammeter and a voltmeter, and also measure the real power with a wattmeter. From those two measurements you can calculate reactive power.

    See these references:



  8. Feb 21, 2013 #7


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    A little off-topic........but interesting.

    A vintage way:



    A modern way:


  9. Feb 21, 2013 #8

    jim hardy

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    Your proposed circuit consumes both real and reactive power. So does a motor or transformer.
    You haven't said what your "meter" consists of.... is it able to separate real from reactive power?

    In an experiment one usually tries to make it clearly demonstrate the principle being investigated. To that end you'd like to have a fairly 'pure' reactive load to start with.

    A large transformer operated at say half voltage should be a fairly 'pure' inductive reactance, because the iron losses will be small at half voltage and the copper losses will be nil with only magnetizing current flowing. Your trouble will be finding one cheap..

    A motor run capacitor should make a fairly pure capacitive reactance. One of those should cost around twelve bucks at an electrical supply house. Often you can pillage one from an abandoned appliance in a scrap pile - airconditioners and furnaces have one for the fan motor. Beware motor start capacitors, they are not suitable for continuous energization.
    Motor run caps usually have a metal case and are labelled "motor run". Most start caps are plastic case.

    Once you have a fairly pure reactive load, you can parallel it with resistive and observe the pythagorean relationship of VA to watts and vars.
    That'd be a neat experiment.

    good luck

    old jim
  10. Feb 22, 2013 #9
    I'm using a digital electrical meter, the kind of meter you would have at you house measuring your monthly consumption for the electrical company. It is able so separate real power from reactive power and shows them in different fields, the reactive one is just marking zero because I haven't been able to connect anything that absorbs reactive power as a load.

    A motor run capacitor sounds like a great idea! I found these in my area:



    (Is MFD another way of writing uF? for a moment I thought that was Mega and thought it was odd)

    However, is there any consequence to undervolting one of these capacitors? The only ones I could find are for 440 VAC and I have a 220 VAC connection.

    You have been of great help, thank you!
  11. Feb 22, 2013 #10

    jim hardy

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    yes, it's often used. I guess some labelmakers just dont have Greek letters.

    And your capacitor will be happy at reduced voltage.
    Feel it after a few minutes, it should not be heating up.
    Capacitors do have small resistance because their plates are made from thin foil.
    And the dielectric does absorb a little energy which comes out as heat.
    When you study capacitors, that's what the tan δ parameter is - a measure of the deviation from pure capacitance.
    A motor run cap should be pretty good, especially at reduced voltage where you aren't working the dielectric very hard.

    Have you yet calculated expected current with that 40 uf? I dont know your meter, just plan ahead so you don't let the smoke out of your test equipment.....

    Here's some info on motor run caps

    i've found CDE's site to be an excellent source of technical information on capacitors. They kinda "wrote the book"...

    Have fun and learn a lot !

    old jim
  12. Feb 23, 2013 #11
    The other option - and good experiment, will be to connect a 220V AC induction motor and have a way to vary the mechanical load.

    These have poor power factor when unloaded - when looking at Reactive power it will stay about the same as you increase the mechancal load, but the real power will increase. Then add a capacitor to the circuit and see how you can "cancel" out the reactive power ( from the sources perspective).

    Your original circuit will have relatively low reactive power. Unless you replace the resistor with a suitable sized inductor.
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