current devider

Hello,

My suggestions are based on the understanding that you already have a 4-20 mA level transmitter that is used to control a valve, you also want to use it to control the speed of a pump motor, but you cannot do anything that interferes with the function of the valve controller.

If your level transmitter has a sufficient supply voltage and your controllers have sufficient common-mode voltage range, you can simply route the signal wire to the valve controller through the VSD input (wire it in series with the valve control input). Both will get the same current.

If, for some reason, electrical modification of the valve control circuit is not permissible, you can use a magnetic current sensor such as the NVE AAV003-10E as the input to another voltage-controlled current source. Information on this part is at

http://www.nve.com/webstore/catalog/product_info.php?cPath=27_28&products_id=544

You would still have to route the signal through the sensor chip, but the small change in impedance should not be significant for what you are doing.

I need source current 4- 20 Ma from level transmitter to control VSD of delivery pump from tank

By this, my understanding is that you are trying to control a pump on the outlet of the tank. No control other than 0% level=0% pumping (what happens to the motor at that point?, and 100% level=100% pump capacity? Think about what will happen to the downstream process.

I presume that DSC (no idea in this context) and DCS (Distributed Control System) are the same?

I can't utilize sorurce of transmitter from DSC because I don't get permited to make addition program to take source Ma from DCS

From that, and the next line, tells me that someone else has control of the programming. Talk to those people. If the concept is valid, you should be able to convince them that the additional programming is worthwhile. Since you have a VSD, there is no reason it could/should not be controlled. But it has to be controlled in such a way that it does not hurt the downstream process.

If there are no objections, then a I/I (current to current converter), can be installed that will supply both the DCS, and your circuit.

Use an isolator that gives two outputs...

there are a good amount of vendors who manufacture these.

http://www.pepperlfuchs.com/products/product.jsp?product_id=18381 MODEL : KFD2-STC4-EX1.2O This will act as an isolator for field transmitter and provides two isolated 4-20mA outputs

http://www.pepperlfuchs.com/products/product.jsp?product_id=18381 MODEL : KFD2-STC4-EX1.2O This will act as an isolator for field transmitter and provides two isolated 4-20mA outputs

hi,

you really need to get this signal from your DCS where you can implement PID control or something similar over the VSD. By using the ma source of the transmitter direct to the VSD (bypassing the DCS) makes it a hidden function which when it goes wrong no one will know about. On the DCS you will have graphics, trends and diagnostics.

Convince the programmers that this is the way to go. I am a DCS programmer and i wouldn't refuse this request. This is what the DCS is for.

hi, sumardi

what you need for vfd drive, is not only lvl xmtr signal but also a controller and a isolator in this condition campare with the dcs requirements ie; one output in dcs and isolator /barrier to connect to vfd so make a comparison with this two options and added features of with dcs ie ;auto/manual station,alarm , trend and diagonistic features .Finally campare with time and cost for installation.

Select what is best to process .