Measuring signals with different levels using common AI in Fieldpoint

[Gurdas]

Hi!,

I am trying to configure a Fieldpoint system to measure the following (its for a DC motor test station):

Motor Voltage: 0-240 VDC
Motor Current: 0-50 ADC
Generator Voltage and Current: same as for motor

Options which come to my mind are:

1) Convert the voltages into 0-10 VDC and use FP-AI-110
and simultaneously convert currents into 4-20mA and use FP-AI-111.

2) Convert both voltages and current into 4-20mA and use only FP-AI-111

My questions:

1) Which is a smarter config?
2) Would there be any issues of isolation that I need to consider?

Note:
A) There are other measurements like thermocouple and counter and I will be using the signal specific modules for those.
B
) It is important that total system hardware cost be kept to the minimum.

Platform is Fieldpoint using FP-1601
OS is Win200
Coding is in NI-PDS-6.1

Thanks and best regards,
Gurdas

[Bill B]

Hello Gurdas,

You could use the FP-AI-110 to measure both your voltage and current signals. However, you will need to create an attenuation circuit so that you can measure your signal in the 0-10VDC range as you have described. You can also use a current shunt resistor to measure the current by running the current through this resistor and then measure the voltage across it. You will most likely need a resistor near 0.01Ohms that can handle up to 50A. (50^2*0.01)= 25W. That is a big resistor.

It would still be best to convert both to voltages since the modules measure voltage. Even the FP-AI-111 uses current shunts and measures a voltage.

Another thing to know about taking motor measurements is Flyback voltages generated from the coils. These v
oltages can reach several thousand volts and could damage your hardware. Check to make sure that you motor or motor drive has circuitry to limit flyback voltages. Zener diodes are commonly used to limit flyback voltages.

Regards,
Bill B
Applications Engineer
National Instruments

[Gurdas]

Hello Bill,

Thanks for the reply.

I want to use the FP-AI-111 for two reasons:

1) I can convert voltage signals into currents at the source and thus ensure good noise immunity while the signals travel from source to Fieldpoint node.

2) It is a 16 channel module and thus allows for significant cost savings.

Please let me know if using the AI-111 compromises the integrity of my measurements and/or results in a 'weak' config. In that case, I'd be happy to look for alternatives.

We would be using suitable isolation circuits to protect DAQ from spikes etc.

Best,
Gurdas

[Bill B]

Gurdas,

If you can convert your high-voltage signals into current signals that can be measured by the FP-AI-111, then I do not see any reason why you should not use it.

Regards,
Bill B