Does anyone have this formula already written in AD PLC?

Gentleman:



Does anyone have this formula already written in AD PLC?



Let’s assume I have an Amp meter (4-20ma) connected to an analog card in a CPU 260 rack.



Questions:



What is the correct formula in ladder logic for figuring out how much electricity the control panel is consuming base on the amp meter analog signal feedback?



Thanks all



Andy

Since you didn't list the analog card you are using, you can start with this chapter from the manual.

http://web1.automationdirect.com/static/manuals/d2anlg/ch2.pdf

Page 2-16 talks about scaling.

I am assuming you are wanting to use the ampmeter as a watt-hour meter. Metering electrical consumption is not as easy as one might at first suspect. At unity power factor, watts = amps * volts. This is an instantenous reading. Inductive and capacitive loads however will affect the power factor, which is the cosine of the phase angle between the voltage sine wave and the current sine wave. This could vary from .7 to 1, depending on the nature of the loads in your plant. What this means is that in reality watts = amps * volts * cos t, there t is the phase angle. If you aren't measuring this angle then you cannot meter your electrical consumption. Once this is known, then simply totalizing a sereies of samples over a known period of time will give you a good reading on electrical consumption.

The answer to the question you asked is:

Single Phase: kW = I x V x PF / 1000

Three phase: kW = I x V x 1.73 x PF / 1000

In both cases PF = the Power Factor expressed as a decimal

The answer to the question you should have asked is: You can approximate the power consumption from an amperager reading, but the power factor and voltage are constantly varying so your answer will not be correct. To get true power readings you need voltage and power factor measurement. The best bet is to use an actual power transmitter, available from Load Controls Inc., Cutler HAmmer, Rochester Instruments, and many other sources.

The answer to the question you thought you were asking is: The ladder logic is fairly straight forward, and can be done with standard multiply and divide functions. You may need to use either double word or floating point math to get the accuracy you need. If you are not using floating point make sure you pay close attention to the ranges of transmitter so you don't have overflow or underflow of the calculation results.

Thanks all for your asnwers, I think I will go for a the transmitter.


Andy