I am measuring induction motor current via current transformer (CT-Hall Type). Line current has 50 Hz frequency. Sampling rate is 2.5 KHz and I collect 2.5 K Samples. DAQ Card is NI PCI 6281, LabVIEW 2012. The problem I face is that the phase of the measured waveform appears different in every run of my vi. I could not understand that why phase of the waveform keep varying?? (attach are the some graphs and vi) while magnitude remain same. (Motor running under no load conditions and at constant speed).
Also I am calculating cross correlation between the measured waveform and simmulated waveform. But I could not understand why x-axix scale of the corelation garph Rxy has fixed range from 0 to 2.
Can anyone please explian me about the above stated two problems. I shall be thankful to you for your attention and consideration.
do you know "line frequency" can vary depending on many conditions in the power distribution network?
Do you know your sampling frequency can be off your target number by a smalll amount, either because the desired frequency doesn't fit for an integer base clock divider or because the base clock may have some variations?
You cannot depend on measuring full wave cycles just because you try to sample multiples of (average) line cycle periods!
Dear Mr GerdW
Then what is the solution. How I can estimate the phase angle of the fundamental signal.
you could either trigger your data measurement (to start always on zero crossing) or you could apply some data processing later on…
a phase can only be measured in relation to a reference. If you start your measurement at an arbitrary time, you will get arbitrary phase.
In your case I guess you would typically take the motor voltage as reference and look at the phase shift of the current signal. Just a wild guess because you haven't explained what the purpose of the setup is.
Have I missed something? A simple reading of you post implies that you are surprised that starting the measurement at an arbitrary time means that the a.c. sinewave starts at an arbitrary point.
With induction motors, normally "measuring phase angle" means measuring the phase angle between voltage and current. If this is true in your case, then you need to measure instantaneous voltage as well instantaneous current.
Note that the noise on the A.C. line means that trying to trigger on the voltage zero-crossing is likely to lead to a noisy phase measurement.
Dear Mr. Edgar
My objective is as follow:
I want to calculate the Correlation Degree between two signals. One signal is the motor line current having fundamental frequency of 50 Hz (environmental noise and some harmonics will also be present in real time measurement) second signal is a refernce signal that I generated at 50 Hz frequency. Correlation Degree will be maximum when both signals have same phase angle. Now if I could know the phase angle of line current signal then I could simulate a reference signal of same phase angle as of measured motor current signal.
Then determine the zero crossing of the motor signal and give the phase value to the sine wave generator. But I don't understand what the procedure would be good for. You still don't explain what the intention of the setup is. What do you want to find out about the motor?
Dear Mr. Edgar
Can you please give some idea how to determine the zero crossing of real time measured signal?
Actually I am working on fault analysis of the motor. And for that purpose I am trying to measure phase of the motor current signal.