No Voltage to Analog Output

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kishore

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.
 
You should see a Voltage across the load, it won't be 24 Volts though, the module puts out current. It will be 1 to 5 Volts across a 250 Ohm load, 0.4 to 2 across 100 Ohms etc. Because it controls current not Voltage the signal at the far end of long cables is compensated for the wire resistance.
 
I have two Caterpillar generators with ABB VFD (ACH550) controlling the fan motor. The control cables to the VFD comes from an Automatic Genset Controller (AGC-4 manufactured by DEIF). The two generators, VFDs were working initially . Once you start the generator the VFD automatically comes up from the display panel. While we were test running one of the generators the VFD refused to start automatically as it should. We measured the input voltages of both generators in the terminal tagged "10- 24VDC" on the VFD circuitry using a multimeter, the second generator read 24VDC but the one that refused to come up did not read on the multimeter. The AGC-4 kept displaying Since the VFD controls the fan motor we are unable to run the generator for long because of heat building up in the package. What could be the issue here?
I have attached the pictures of the VFD and the AGC-4 that is dispalying "3430 VFD 4-20mA loss"
 

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