Suppose I know nothing about my load, only the voltage and current waveforms.

With this I can calculate apparent power (S), and real power (P) and use P and S to determine my reactive power (Q) for a cycle. However, the Q that is calculated could be positive (implying inductive load) or negative (implying capacitive load) for any given P and S.


Are there any simple tricks I could do with the voltage and current waveforms to determine whether Q is positive or negative?

I know I could see which peaks first, voltage or current, and that would tell me if the current is leading or lagging the voltage but that would involve keeping track of the max/min and just gets complicated. I'm wondering if there is an easier way to do this. I have available to me only the samples in a 60Hz cycle of the voltage and current. I also have sine/cosine vectors available, of which the sine vector is phase-locked with the voltage.


1 Answer 1


CIVIL stands for (amongst other things) Capacitor, I leads V and V leads I in an Inductor (L)

So, given that the displacement between voltage and current can only be 90 degrees max for a reactive load, look to see if the current rises through zero before the voltage waveform. If it is then it's a capacitive load. It's easier doing this than looking for peaks. Clearly you need to know the AC frequency and you need to be sampling at least 5 or 6 samples per cycle to accurately determine C or L type load.

  • \$\begingroup\$ Thanks for the comment. It's a power converter and I'm chopping at about 18kHz, so the current is really noisy with the chop so there will be multiple zero crosses. So for a low Q, it won't be able to tell if it's positive or negative since the zero cross of the current is all over the place near the real zero cross of the fundamental of the current. \$\endgroup\$
    – SPieiga
    Jun 16, 2014 at 16:31
  • \$\begingroup\$ Add a filter - a simple RC stage should be enough but use the filter on both waveforms or you'll get an error. Filter as heavy as you need to because providing both RC time constants are close you'll maintain the same phase angle. \$\endgroup\$
    – Andy aka
    Jun 16, 2014 at 16:40
  • \$\begingroup\$ CIVIL is way better than ELI the ICE man. Thanks for that. \$\endgroup\$
    – Aditya P
    Sep 27, 2018 at 16:31

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