Can any one explain why there is no voltage (24VDC) is present in analog output circuit to the valve or VFD. I am able to measure 4-20mA when I send the output from HMI/Controller, but I cant see any voltage if I measure with multimeter.
Please explain.
Just like there has to be a pressure differential to create flow, there has to be a voltage potential difference to drive the current.
A control signal from a PLC to a VFD is an analog input to the VFD. That VFD analog input will have a shunt/dropping resistor built into it. The ABB ACH550 manual on my desk says it uses 100 ohms across the current input so it will drop 2.0V at 20mA across its input terminals, proportionally less at lower currents. The remainder of the AO's supply voltage is dropped by the current regulator in the analog output in order to modulate the loop current.
Analog outputs do not necessarily drive an output with 24Vdc. The Allen Bradley 1756-OFx module uses 20V to drive the output.
https://s24.postimg.org/wvlthi2hx/A-_B_AO_1756-_OF4-8.jpg
On the field instrument side, some installations do not ground one side of the loop's DC power supply. Putting a voltmeter probe on what might appear to be a 'ground' is not likely to provide a voltage reading if the DC supply is floating. There's no ground connection. But you can always measure whatever voltage drop the transmitter is modulating across the transmitter's (+) and (-) terminals.
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