an Isolator transformer output energizing PC, DAQs. Grounding?

[askay]

Hi friends,

I need you helps regarding a floating signal source and its measurement.

I have my  4 AC motors connected to main power supply of 380V. Torque sensors are attached to it with NI DAQ and PC controller test bench.

Now what I have is a 220V AC/AC UPS with built in isolator transformer. (this UPS always isolate input although there is a live main supply) The UPS has its clear AC power and an isolated new ground.  I use the new (ground'n power) to supply  industrial PC (NI DAQs are installed) and small AC/DC converter (220AC/24DC V) to supply torque amplifier. Basically everywhere within control cabinet (signal level)

The question is:  Can I safely use the isolated ground in my measurements without touching the field ground?without attaching them together

The same power'n ground is used PC, used in shield ground. Basically I use the ground everywhere as a Signal ground within control unit, i.e. PC, sensor Amps. Etc.)

I read in the NI helps about floating signal (I am supposing that I have floating signal now by having a UPS, I know that it has ground, makes me confusing )

Do you think that I have a floating signal ? I have a new ground reference. I think it is not a floating signal, i.e. floating ground. I measure the signal differentially.

Last question, Do you think isolated  signal grounds and Field ground points should be connected somewhere at the mains supply, so that all of them flowing to the earth? or not at all touch to isolated signal ground?

Is there any risk/recommendation that I should follow?

I have attached a schematics of what explained above.

I need you help.

Best regards,

Desmond,

[GarPar]

Hi Desmond-

     From your diagram, it looks to me like the signal you are trying to measure and your Industrial PC share a common ground.  The AC/AC power supply has its own (Earth) ground.  As long as the industrial PC is getting its power requirements, it look to me like it is blind to Earth ground.  To be safe, however, it would be best to measure your signal differentially, as you are doing.  This will remove any ground loops or offsets in your system.

     I am not sure if you have a already encountered it, but the Field Wiring and Noise Considerations for Analog Signals Tutorial is an excellent resource for grounding questions.

     I hope this helps.  Have a happy holiday season!

[askay]

Hi Gary,

Would you suggest powering the control &sensor devices via isolator transformer  that is not providing ground isolation but power isolation. I still use the same ground as in mains grounding.(My aim is just to supply a clean AC to the instrumentation(sensors, industrial PC) side since I have power swtc. devices like motor drives connected to the same mains in a test bench).

Also,  Do you think it is good idea to have a seperate grouding bed (ground termination bar) for signal and power grounding, then finally getting them together somewhere at a star point. Should I keep these grounding bed as close as possible to eachother in a test bench with a 1x1x2 m size.

One more question, Considering an industrial PC controlling a motor,  If I drive enable an actuator  externally, and then power up the industrial PC, I see an unexpected behavior of the motor (like PC DAQs sending 3.3V or something  as a reference signal) . This is happening before everything is under control of a test bench GUI. I heard that it is due to the fact that before DAQs chips are energized and control its I/Os, DAQ's I/O s are not controllable. One of my friend suggested me to to consider an external power supply for DAQs live 5V. So that DAQs are energized via ups, battery, before the control PC.

Thanks for your helps,

Desmond.

thanks for your help.

[Efrain_G]

Please avoid duplicate threads. Let's keep working on this issue on this forum.

Regards,

Efrain Gutierrez

Applications Engineering

National Instruments