Power factor calc, in 3phase with line voltage and currents

[xj25]

I had some scope measurements from a motor of line voltages and currents:
Ia, Vab, Vac and want to get the power factor.

From thread238-222240 I concluded that the power factor can be got doing this with the waveforms:

The voltage must be phaseshifted 30º lead or lag, depending if you take Vab or Vac and the reference of measurement. Do it to get The current lagging the voltage waveform some angle that makes sense (from 0 to 90º)

Once done, it can be measured the lagging time of the current (ms) and do:
PF = (ms of 90º - ms lagging)/(ms of 90º)

where (ms of 90º) is 5ms for 50Hz and 4.1666ms for 60Hz.

Please correct me if I am wrong in any sense

 

[electricpete]

I would say it's right although which one shifts 30 degrees which way needs to be considered closely depending on phase sequence and terminology.

I start out with assumption A / B / C sequence (A leads B leads C).... needs to be verified.

I clarify terminology Vxy means + lead hooked up to x and - lead hooked up to y.

I assume of course balanced system.

Then I would say that Vab needs to be shifted 30 degrees in LAG direction and Vac 30 degrees in lead direction in order to align with Van (and we can then compare to Ia to compute power factor).

At least that's what I came up with scribbing on my paper. Anyone feel free to double-check.



=====================================
(2B)+(2B)' ?

 

[electricpete]

Another minor note on terminology:

I interpretted that shifting Vab 30 degrees "in the lag direction" means shifting it 30 degrees backward in time. (that seems straightforward definition in general, but I wanted to clarify because in this particular case talking about using shifted voltage to compute power factor... pushing voltage vector in time-deltay = lag direction can tend to push computed power factor angle more leading direction!).

=====================================
(2B)+(2B)' ?

 

[electricpete]

Wow. I just realized "backwards in time" can itself be ambigous. Let me substitute "later in time".

Sorry. I'm really done now.

=====================================
(2B)+(2B)' ?