# Measure 3-phase Power with Lecroy Waverunner

Please note that experience with a Lecroy Waverunner Oscilliscope is most likely required to answer this question.

I'm attempting to measure the power draw of a brushless 3-phase motor with a Lecroy WaveRunner 204MXi. I've connected two current probes and two differential voltage probes in order to provide two Wattmeters. I've connected the current probes to Phase A (for Ia) and Phase B (for Ib), and the differential voltage probes across Vac and Vbc. This matches the configuration detailed here by Lecroy:

http://teledynelecroy.com/doc/threephase-power-measurements-using-a-fourchannel-scope-to-measure-threephase-power

And from the link, the power draw at any point in time is:

P = VacIa+VbcIb

This is only, I believe, the real power. I should also be calculating the complex power, correct? The complex power will be draw from the battery, and so it is my actual current draw?

S = Vac(rms)*Ia(rms)+Vbc(rms)*Ib(rms)

The issue I'm facing is that to calculate RMS is, for the Lecroy Waverunner oscilliscope, a measurement (P?). RMS is not available as a math function. Then I need to perform three math functions F1=P1*P2, F2=P3*P4 and finally my complex power would be F3=F1+F2. Here, P1,P2,P3 and P4 are my RMS voltages and currents.

The issue I'm facing is that the math multiplication functions only allow for 1 measure to be included as an input? I can have, for example F1=P1*C1 (where C is a channel), but not F1=P1*P2? Am I measuring complex power correctly? Is there a better way to measure complex power with a Lecroy Waverunner?

Any insights are much appreciated.

Thanks

• I was with you all the way to you saying what S is then the sentence below confused me. Then the one after that uses "preform" and that confused me too. – Andy aka Jun 22 '16 at 18:00
• After that, I'm just asking for a way to calculate complex power with the Lecroy Waverunner. It doesn't seem to be easily accessible. I'm also looking for confirmation that my 3-phase power formula is correct. Pardon my spelling. – Brian Dohler Jun 22 '16 at 18:30
• @winny - the standard way IS the two wattmeter method. – Andy aka Jun 23 '16 at 7:14
• Perhaps the better question is how do you multiply two RMS current and voltages together with a Lecroy Waverunner. It doesn't seem to have this functionality. I know that sounds ridiculous... – Brian Dohler Jun 23 '16 at 12:43