Power Factor Measurment

The PM3000A measures power by multiplying the voltage and current waveforms together, and VA by multiplying the RMS voltage and current measurements together. Power Factor is then computed by dividing the watts by the VA. It does not do this continuously but in chunks over an exact number of cycles. Each chunk results in a single result, and a number of results are then averaged (by the number programmed in the AVERAGING setting) to present the number you see on the display. To determine the exact number of cycles and therefore the time over which to take each result it needs to measure the frequency accurately. If the frequency is not measured then the results will be unstable.

The formulae for the measurements concerned are shown on pages 3-1 and 3-2 of the PM3000ACE user manual available for download from here: https://www.voltech.com/Support/Forum.... I hope that this answers your question. If you have any more questions or need any help understanding your measurements then please provide more details of your test setup and the waveforms being measured. Files can be attached in a reply to this thread if necessary.

Best regards,

Tim.

Hello,

Thanks for the waveform. I still don't have enough information to verify the measurements you are seeing, but immediately I would expect quite a low power factor. This is because power will only be produced by harmonic components on the same frequencies. I can see that the voltage waveform is very sinusoidal - not much distortion so therefore no harmonics to speak of. All of the power must therefore be carried at the fundamental frequency - 400Hz. The current waveform is very distorted - only on for half a cycle with a large DC component. The DC component and harmonics cannot generate any power hence the low power factor. Are you expecting a different power factor or think that the PM3000A is measuring it incorrectly in some way?

Best regards,

Tim.

I've performed a simulation of your signals and I can see that the power factor does indeed seem low. The maximum theoretical power factor of a rectified system with no diode drop is 0.707. With the diode drop, or dead zone, I get a power factor of about 0.64 (from looking at your waveform and guessing at what voltage the current starts to flow). If I then introduce a phase shift such as you may get through inductance in the circuit then even with 30 degrees the power factor is only down to 0.52. Do you have another means of measuring the power, and if so, what is that measuring?

I have a few questions for you about how the PM3000A is setup:
- Is the PM3000A set to defaults?
- has it been calibrated/verified recently?
- Is it detecting the frequency correctly at 400Hz?
- Are you using the PM internal shunt or also using a CT, external shunt, or other current measurement module?
- How are you measuring the PF, for example, are you reading it from the PM3000A or software?
- Are you taking fundamental measurements (FUND key) or using AC only coupling?

I have attached a copy of the mathematics I used to perform the calculations. This may be useful to you or someone else reading this thread to help clarify how the PM3000A computes the parameters.

I look forward to hearing from you soon.
Best regards,

Tim.