CTs into analog input

Good Morning,
What is the ideal way to read a CT with an analog input?? On this project, we have several existing CTs. Some are 800:5, 600:5, 250:5. We are checking for a way to monitor these with a plc analog input.

I realize we could buy current transducers and replace the CTs all together, but we were hoping to keep the existing CTs if possible.

I expect there is a gadget out there to do this....just curious if any one can recommend the latest technology. Thanks!

parky said:

https://www.nktechnologies.com/engineering-resources/converting-ct-signal-standard-4-20-ma-output/

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Nice. Thanks!

Since a CT outputs an AC signal you could put a diode on the output and read it with a DC analog input.

Take some measurements and scale the reading in he PLC.

https://www.beckhoff.com/english.asp?ethercat/el3403.htm

Bus coupler of your choice, or just use their PLCs

Putting a diode will not work remember using a diode does not make it DC only rectified AC so still a half wave ripple, also the current produced is 5 amp on most CT's putting it direct into a analogue input could fry it. Even if you convert it to DC the voltage could exceed the safe working limits of the card. remember a CT is just a transformer but only half a winding on the primary.

There's always warning the secondary on a CT has to remain closed or there is a possibility of life endangering voltage.


I don't know whether a diode blocking the half wave constitutes a valid closed circuit or not, but I would want a CT manufacturer to confirm that a diode on the secondary would not damage the CT or create a life endangering situation.

Here's the place that should have what you need

https://www.crmagnetics.com/

GaryS said:

Here's the place that should have what you need https://www.crmagnetics.com/

Click to expand...

I don't think so. There is an existing CT with a 0-5Amp AC secondary and the question is how to convert that 0-1Amp AC to a 4-20mA signal.

The NK Technologies device in post #2 does do the AC to DC signal conditioning.

CR Magnetics has CT's and VT's but I am unaware of a 0-5VAC to DC signal converter to use with an existing CT. And I've asked that question of CR Magnetics as recently as 3-4 months ago and got a negative reply.

The NK Technologies unit is available in the US from one of their listed distributors.

https://www.automationdirect.com/ad..._current_transducers_(true_rms)/actr005-42l-f

Is there any reason we couldn't use a device like this and connect the 2 posts of the CT and loop the wire through this?

Danw
I hate to have to point this out but there are several devices they have that will work very well in this application.
Even without knowing the current ranges of the CT’s as few common things about CT’s
On most Ct’s the secondary / output is 5 AMPs AC 600:5, 800:5, 1200:5 you get the idea

It is a safe assumption that any existing CT’s are connects to some kind of meter or converter because you cannot leave any CT open ended, meaning that it must be terminated to an meter of some kind or shorted together if not the open ended voltage on the secondary could exceed several thousand volts and burn up the CT or worse the arc could start a fire

In this case the CR4120-5 would be exactly what you need 0-5 Amp in 4-20 ma out

I this case the converter selection is simple all the inputs are 0-5 AMP the scaling range in the plc would be selected from the original CT
They also have a split core version that can be installed without interrupting the original CT
With CT’s you can use then read lower current ranges by passing the wire through the CT multiple times
Example : if the range you need to measure is only 0-2 amps but the transducer has a 5 amp input the lop the wire through the transducer 2 times so the sense range is now 0-4 amp just scale you analog input to 20ma – 10 amp (2 X 5)
Also you may want to go back out to the web site and check out some of their Application Guides and Technical References it think it will help clear thing up.

I need to correct my scaling 20ma out should = 2.5 AMP

NK tech as in my previous post do a separate converter specifically forthis application, it connects strait to a standard 0-5A CT (no need to remove original) and produces an analogue signal, I believe it is only 4-20ma.
But converting that into a 0-10v is easy.

parky said:

NK tech as in my previous post do a separate converter specifically forthis application, it connects strait to a standard 0-5A CT (no need to remove original) and produces an analogue signal, I believe it is only 4-20ma.
But converting that into a 0-10v is easy.

Click to expand...

The link you posted is the device that does exactly what you explain here, right?
4-20ma is fine. We are able to buy an input cards we want.

If you read what is on the website that's what it says here is the text:

In some applications, such as monitoring a high voltage system, using a two-piece solution (current transformer and separate signal converter) works better than using a one-piece sensor. A two-piece solution is also a better choice when the system is supplied with bus bars, making installation of a sensor over the conductor extremely difficult or impossible.
In a two-piece solution, the installer connects the secondary of a standard 5 amp current transformer to the input terminals of the CTC device, and then connects the CTC secondary to a nominal 24 volt DC supply and then to the PLC or panel meter input. The converter will then produce 4mA when there is no current through the primary CT, and 20mA when the CT has full range current present[/COLOR]