NI 9215 signal deformation

[AASakhno]

Hi,

I have some problem with NI 9215 module, I use Ni9215 with NI-ISB-9162, the six measurements schemes and ni output are in the enclosure. I have signal deformation when I tried to measure signal from one phase of the 3-phase system, and don't have this deformation when I tried to measure two or three phases simultaneously. Another trouble is when I tried to measure simultaneously two signals from different voltage splitters - I have terrible deformation (scheme_6).

Could You kindly explain me, why I have such deformation of signal on NI-output.

I hope for Your help.

[AASakhno]

schemes attachments

[AASakhno]

schemes attachments

[AASakhno]

schemes attachments

[TimmTheEnchanter]

AAsakhno,

The wiring for your first schematic looks fine, but I don't believe this is going to be a problem with the 9215 DAQ device.  Are you absolutely sure the diagram you included is exactly how it's being wired?  If you have a short between one of the other transformers, this may be the cause for your deformation.  You might be able to confirm if you have an oscilloscope or if you wire into another channel to see what waveform you get.

Also, be wary of how you're connecting your signals up.  If you are using a breadboard, for instance, there could easily be an unseen, unexpected connection on the PCB.

Finally, I might make a recommendation--you don't need a 1MOhm resistor connecting each signal to ground.  Doing so reduces the effective resistance by a factor of however many resistors you have (2 means 500kOhms, three means 333kOhms, etc.).  Instead, since all of your negative signals are connected together, you should only need one 1MOhm resistor.

I hope this helps--please let me know what you find.

[AASakhno]

Hello TimmTheEnchanter,

Thank You very match for Your answer.

I'am absolutely sure that diagram 1 is exactly how it's being wired. I have such deformation one all four channels when I connect one signal as on the diagram #1, it is no matter what transformer I connect and it is no matter what channel of 9215 I use. But, if I connect two signals from different phases, no matter in which configuration: A and C, A and B, B and C etc. (example in diagram 2) the result is fine (result of diagram 2). The signal has true sine form, it measured by parallel connected device with input 50 MOhm. The signal is clear. I don't understand deformations (diagram 1 and 6). You are absolutely right about 1Mohm resistance dividers, but the deformation is when I connect only one signal (in such scheme resistance = 1MOhm), but it disappear when I connect two signals (resistance=0.5 MOhm). It seems to me that something wrong in 9215 connection, do you have more ideas about these deformations?

Thank you for Your help. Waiting for Your answers.

[John_P1]

Hi AASakhno,

Try connecting your cirucit like this:

That is, instead of using the 1 MOhm resistors, directly connect COM to your Ground Reference. 

The 9215 is isolated from earth ground so you need to provide a reference yourself (in this case the reference would be to ground).  My hunch is that the 1 MOhm resistor is too high of a value and is causing unexpected behavior due to any capacitance and inductance that is present in the system.  As you add more resistors in parallel the connection would get better and better.  You do not have to worry about a ground loop (the 9215 is isolated from earth ground) provided that all of your signals sources are referenced to the same ground.

Best Regards,

[AASakhno]

Hi John,

Thank You very match for Your reply. This is very useful suggestion. I suspect that there is something wrong with 1 MOhm resistor. We have experience of using 9215 as shown on scheme_4 and scheme_5, but when we tried to add some signals as shown on scheme_1 and scheme_6 we have these deformation. Unfortunately the substation where our equipment with NI9215 is installed is far from me. So I haven't opportunity to try to move 1 MOhm resistor right now. This will take 1-2 month. I will inform You about the results. If You have any more ideas about this problem, I will be glad to try it when I will on substation.

Thank You very match.

Best regards.

Alexandr Sakhno 

[AASakhno · Accepted solution]

Thanks for all replies, there is no problems in NI9215, the problem was in high potential on "ground". That caused unnormal measurements. So be careful with potential on your ground and common pin of NI9215.