Input delay causing weird result when measuring voltage and current from PWM signal?

[GusTTShowbiz]

Hello, I'm measuring PWM signals coming from a VFD, and I want to calculate the efficiency: 3 phase (L-N) voltage using 9244 and use 9239 to read external CTs.  One thing I noticed was the efficiency value of the drive was off (100+%).  The waveforms look clean.  Both are on the same task in LV, so it should have the same count.  I was wondering if the input delay on the module will cause the power factor to be off for the measurement.  My sampling rate = 10kS/sec

9239 - (40+5/512)/fs+3.3us

9244 - (40+5/512)/fs+1.7us

Want to see if anyone else experience this before.  

[ZYOng]

How did you synchronize both modules in your DAQmx task? Please share your code.

[santo_13]

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@GusTTShowbiz wrote:

Hello, I'm measuring PWM signals coming from a VFD, and I want to calculate the efficiency: 3 phase (L-N) voltage using 9244 and use 9239 to read external CTs.  One thing I noticed was the efficiency value of the drive was off (100+%).  The waveforms look clean.  Both are on the same task in LV, so it should have the same count.  I was wondering if the input delay on the module will cause the power factor to be off for the measurement.  My sampling rate = 10kS/sec

9239 - (40+5/512)/fs+3.3us

9244 - (40+5/512)/fs+1.7us

Want to see if anyone else experience this before.  

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Indeed possible, if both modules are different digital filter delay, it is possible for phase mismatch resulting in incorrect measurements.

There are ways to match digital filter of mix of DSA cards but not easy for C-series cards even if you put all into one task.

Please read - Synchronized Data Delayed When Using NI DAQ Devices with Delta-Sigma-ADC - NI

related read - How Can I Compensate for Different Group Delays with C Series Modules in LabVIEW FPGA? - NI

[santo_13]

The right way and best way is to calibrate your system against known resistive load, in the sense, use a resistive load, you must read a PF 1.0, now, anything not closer to PF 1.0 is due to the difference in digital filter delay between your Voltage and Current DSA modules.

Use this value as a calibration value to adjust your final reading with your VFD (either as a PF or calculate the phase of the current to adjust in terms of phase).

[santo_13]

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@santo_13 wrote:

The right way and best way is to calibrate your system against known resistive load, in the sense, use a resistive load, you must read a PF 1.0, now, anything not closer to PF 1.0 is due to the difference in digital filter delay between your Voltage and Current DSA modules.

Use this value as a calibration value to adjust your final reading with your VFD (either as a PF or calculate the phase of the current to adjust in terms of phase).

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Even better, feed in a single sinewave to all 6 channels across two modules, capture synchronously, you can calculate the delay in each channel, now, offset these delays during your capture