PXI-6251 Phase Angle

Good Day,
I am currently using a PXI-6251 module and I need to measure Phase Angle between two sinusoidal waveforms. I have done a lot of searching and can not find much. If anyone is able to help I would appreciate it.
TIA,
TomJr

Hi TomJr-
I'm not sure if it will be very significant, but there will be some phase difference introduced into your sampled signals due to the fact that the input lines are MUX'd through a single PGIA and ADC. This means that there will inherently be some skew between channels that you may want to account for.
The easiest way to accomplish this is to query the AI.Convert.Rate propery as shown below.
This value is the frequency of the clock that initiates samples from each channel in succession, so the reciprocal will represent the system-introduced phase difference in the sampled values. If the amount of time introduced by this multiplexing is significant you might want to manually add it to the value you calculate with lrallen's suggestion. Alternatively, you could use a simultaneously sampled S Series board to remove these effects if they are very significant in your system.
Hopefully this helps-
Message Edited by Tom W [DE] on 03-06-2006 01:15 PM
Tom W
National Instruments
Attachments:
untitled.GIF ‏3 KB

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Echo_Mode_PXI_5122.vi ‏49 KB
Pulse_Echo_Mode_PXI_5122.vi ‏49 KB

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Hello,
I require some help with a phase angle measurement.
The circuit I have to measure is attached. I would like to measure the current and phase angle through R3 and C1.
I believe that if I use a wattmeter and measure across them (R3 and C1) then cos^-1 the power factor, I will only get the angle associated with their impedance. (about -37 degrees). I want the angle of current relating to the original voltage not just the shift resulting from that branch.
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Message Edited by dan1988 on 12-07-2007 04:38 AM
Attachments:
EEP a1.ms10 ‏48 KB

I calculated either 98.97 degrees or -82.87 depending on whether I used a thevenin or super position analysis. I believe the 98.97 degrees is to be taken from the 180 degree point, thus making them them as good as same answer. (Can't go more than 90 degrees anyway so it can't be taken from 0).
I found an interesting result from playing around with the measurement probes you suggested, whereby I was receivng two readings in the load branch from seperate measurement probes, each referenced to a different supply. The readings were 120 degrees and -60.1 degrees.
Now the combined thevenin and load impedance is -20.93 degrees.
120 - 20.93 = 99.07
-60.1 - 20 = -80.1
Meaningful results? I don't know, I do not have enough understanding of the subject to prove or dissprove it.
Any way its still not what i was looking for because Ive had to use my calculations which is what I'm trying to prove in the first place.
If your interested ive attached the circuit and my calculations. As you can probaly tell its part of an assignment, for my HNC. I handed it in yesterday and just had to explain why i couldn't prove the angle (not that we were expected to). I credited you, Lacy, for the measurement probe idea.
Best regards,
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Attachments:
EEP a1_ measurement probes.ms10 ‏45 KB

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Required specification::
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Are you working with Nigel91 who posted a similar question in the Electrical Power Suite board?
You have the same problem that he has. For dt = 0.02 (equivalent to a 50 Hz sampling rate) you cannot measure anything about a signal with a frequency higher than 25 Hz. And you could not get meaningful phase information at frequencies above a few hertz.
To get good phase information at typical power line frequencies sampling rates of 1000-1200 Hz or higher should be used.
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2_counter_outputs.vi ‏32 KB

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Hi all,
I have recorded a same signal several times for two minutes and trying to perform the spectral analysis. If I take any two data samples, as far as the waveform is concerned the wave is very similar to each other for both the samples as can bee seen from attached screenshot (waveform). Same is true for amplitude spectrum. But when it comes to phase spectrum the data between different samples is not same as can be seen for a frequency of 2150 in the screenshot attached.
Why is there the difference in phase spectrum only for different data samples while waveform and amplitude spectrum is same which the case should be since the same signal is recorded at different intervals???????
Anxiously waiting for the response.
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Attachments:
Waveform.JPG ‏38 KB
phase.JPG ‏65 KB

Don't appologize for being ignorant or unexperienced! your learning
and asking questions! I grew up as an electronics technician and have
worked in electronics every day for over 25 years (I'm getting old) and I
STILL learn new stuff every day!
OK so you have a few
misconceptions:
1) MYTH: the power company provides a
stable regulated voltage and frequency with no distortions. REALITY:
the power company has no interest in providing a stable pure 50 Hz. (in
fact, it would be nearly impossible) They do attempt to regulate
frequency and voltage to 48-52Hz at 220 - 240 VAC but, surges, spikes and
failures occur often. Additionally, few -if any- line powered devices fail to work properly if the power source is within 10% of "ideal".
2)MYTH: the Fluke i200s current clamp introduces 0 phase distortion. REALITY: The phase distortion is unspecified in the literature (Follow the link.) while the clamp is "usable" up to 10KHz if you used controlled sources and graphed out the accuracy in amplitude, and spectral purity vs input frequency you'ld see an interesting curve with several non-linear components.
3)MYTH: there is no "noise" on the line. REALITY: TV, Radio, Local radar systems, cell phones and microwave relay towers all broadcast RF that can induce currents on the transmission and distrubution lines (not to mention solar events). Additionally PLC technologies actually intentionally introduce modulation of the line frequency.
so in conclusion: I'm not sure there should be any spectral relationship between random samples of line currents from a residential or comercial power provider near 2KHz. I would suggest you limit your experiment to the first 3-5 harmonics as there seems to be no rationale to compare the phase relations way outside of the 50hz that is of interest.
Perhaps you do need to gain information about these higher freq components. If so you will need some type of AC-AC converter with much tighter regulation than the power company provides.
Jeff

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Hi All,
I'm trying to produce a chopped sine wave to simulate an SCR controlling
mains voltage. The output
only needs to be a few volts with an positive offset.
I've thought of making a table of normalized values to represent the fixed
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value. That way I can control
the trigger angle. Of course the trigger is reset when passing through the
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I'm I on the right track here or is there an easier way with a prebuilt VM?
TIA for you help
Nick

Greetings Nick:
You should be able to get good results with your technique. (You're essentially doing a poor-man's
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Regards,

Are you performing any processing on your data? Are you performing a continuous or finite acquisition? Do you see this behavior in the Test Panel as well? How is your load cell wired? Typically load cells need external excitation. If this is the case for your load cell, what is providing the excitation? If this is a bridge-based sensor, then this document would be good to review. Hopefully this helps!
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How to acquire correctly three-phase signals (Problem with the gap between signals)

Greetings
I need to analyze three sinusoidal signals 120 degrees out of phase by nature including my version of LabVIEW 7.1 and the acquisition card I used is the Measurement Computing USB 1208FS. In principle the acquisition and display them individually does not represent any kind of inconvenience, the problem arises when I try to observe the three signals in the same waveform graph. At this point to run the test continuously (Continuously RUN) the signals lose their phase shift and this becomes variable along the process of signal acquisition occasionally taking its original phase shift (in Figure 2 shows how to vary the phase angle between the signals), just watch the same channel form using 3 distinct stages and the signal (in theory the same) is outdated fig 4. How can this problem be corrected? There is some appreciable time lag between data collection in the different channels that would alter the correct acquisition of the signals. You can correct varying the sampling frequency (rate) and the count?
Pd = vi attachment used for the acquisition
Thanks
Attachments:
figure 2.jpg ‏70 KB
figure 4.jpg ‏72 KB
3 channel.vi ‏388 KB

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Cory K

PXI-6251 Phase Angle

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