Multiple Analogue Inputs

[Thomas Hunting]

Hi Everyone.

I am trying to read 3 analog voltage inputs representing phase voltages in an electical machine.

The incoming signals have been transformed down by a factor of 100, using an external voltage transformer.

I am reading in the 3 voltages, and scaling them up by 100 to represent the correct waveforms, as well as displaying the RMS value.

However the waveforms are offset, and symmetrical about the origin as they should be, this is meaning that the RMS value is also off.

Also the Phase C value is lower than the other 2, even though it is being supplied with the exact same signal.

If anyone could help, I would greatly appreciate it.

Thanks for your time.

[Mark H]

Hi Thomas,

You say "the signals are offset, and symmetrical about the origin" - I'm not sure exactly what you mean.

If the signals are offset then the RMS value will vary according to the offset.(?)

What is the frequency of the signal - are you sampling enough cycles?

When C phase appears smaller than A and B - are you testing your test system by connecting A,B and C phases to one signal simultaneously? (I think that is what you are saying).

Mark.

[Mark H]

You might try configuring the "Amplitude and Level..." function to "Apply Window" on the "RMS" measurements - this can help to reduce the effect of the waveform end discontinuites (partial periods), giving you a more consistent result accross the three phases.

[Thomas Hunting]

Thanks guys for the response.

What I mean to say is that I would expect the input sinusoidal waveform to have peaks at -1 and +1. This is not the case and it is reading between -0.25 and plus 1.75 on the peaks. Hence the rms value (Sqrt 2 * Peak) is wrong. I am applying the same signal to ai0, ai1 and ai2 by simply looping to each. The exact same signal is physically present on all 3 channels. 

[Thomas Hunting]

PS

The input signal is relatively slow, 50Hz.

[Mark H]

Could it be that you're signal is "floating" i.e. it is not referenced to a ground point, in particular it may not be referenced to the DAQ board's analogue input ground AI GND?

If you haven't allready done so, you may need to connect suitable resistors between the signal inputs and the DAQ board's AI GND connection. This link may help... https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

Mark.

[Thomas Hunting]

I had forgotten to select RSE for the input terminal config. Everything is fine now.

Thanks Mark for your help.

Greatly appreciated.