Estimating PF of 24 AC system using A/D

Discussion in 'The Projects Forum' started by donaldm444, Aug 8, 2011.

  1. donaldm444

    Thread Starter New Member

    Jul 16, 2010
    23
    0
    Hi,
    I am trying to estimate the power factor of a 24 V AC system. I am planning on using the A/D channels of an Atmel UC3A0512 microprocessor (EVK1100 board) to sample the AC voltage and current waveforms, then write code on the micro to detect rising edges (and then estimate PF).

    If anyone has any advice on circuits that would enable me to measure the AC current waveform for this purpose it would be greatly appreciated. For the AC voltage measurement I am planning on using a transformer (from 24 V ac to ~ 3 V ac) and sampling its output.

    Any other advice or suggestions would be greatly appreciated.

    Thanks,
    Don
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    632
    I suppose you have considered using a resistor and possibly and amplifier to convert the current into voltage. Just remember to bias up your signal at the input of the A/D so that (being AC) it does not try to go below zero volts. That implies that there will also be some kind of offset adjustment, either manual or automatic.

    If, for some reason, your controller is cannot connect directly to the 24VAC line (as implied by the use of a voltage sense transformer), then you can use a current transformer. Be careful if you do this because many current transformers sold for monitoring purposes do not have sufficient low frequency response to, for example, reproduce a 100 Hz square wave. You can imagine what that would do to some times of AC current waveforms (I am thinking about lamp dimmers; you have not described your load). The transformer may need frequency compensation.

    Also, please check the linearity of your voltage transformer; some transformers can made a nice, clean sine wave look pretty ugly.
     
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  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    There are multiple true RMS converter IC's available. They are not cheap starting at about $10USD each. Two of those and you have the voltage and current values. Just ask Digi-keys search engine for "RMS converter" and you can spend the rest of the day picking one.

    A pair of zero crossing detectors could give you the phase angle between to generate true RMS power.
     
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