Sensors for current

hi

i need some kind of sensors to detect the current and give a digital feedback to the plc.

i'll explain u the application. the plc has been installed on a furnace and some feedback is needed in case of heating element failure. this feedback needs to be immediate and so using temperature sensors is not a good idea. i believe the most suitable feedback would be to sense the current on the three phase supply of the heater and so as soon as the element fails there wont be any current flow and hence the failure may be detected.

could any one please give some information about this.

pkansara

You're exactly on the right track.

http://web1.automationdirect.com/adc/Shopping/Catalog/PLC_Hardware/DirectLogic_405/Specialty_Modules/F4-CT

Read the specifications and the online manual. Run a google search on heater burnout and you'll turn up a wealth of information for the application.

As Marc said, you're taking the right approach. You probably need more than just the current transformer though. HERE'S one source. Go to Products-->Current Switches.



-Eric

Pretty good, CroCop... a CT might be right on target... or not.

A set of CTs (Current Transformers) might be the right answer. However, he asked for a digital response... I wonder if maybe he does not have an analog capability in his current system. If not, this might required an independent smart-module like a multi-phase-detector module (Red Lion, or some such) which provides a digital output (all phases present or all phases not present). Where phase-present is detected by current flow on each phase.

A phase-module would provide a single digital input and not require a set of analog inputs. Much more cost effective... no?

pkansara,

If you need to know that a failure has occurred in a particular heating element, what better way is there than to monitor the current going through the heating element? A phase-detector that monitors each phase by current will do the job. This device will provide a single digital signal indicating that all phases are well or at least one phase is not well.

However, bear in mind, if the heating elements are controlled by temperature feed-back, or some such, and it is turned off for a period of time because the temperature is high enough, or maybe too high, you also need to monitor the status of the output to the heater. "It depends" *** on your particular situation.

*** "It Depends" = The Classic PLCnet answer!

If the heater is turned off, there would be, of course, no heater current. Would a no-current condition absolutely indicate a failure? If so, then you don't need to monitor the status of the output. If not, then you need to also monitor the output status.

That's the most popular response with PLCs. It depends. The digital response would be the most simple, cost effective method. The benefit to the CTs is the flexibility, but the Analog IO can be pricey.

i think Carlo Gavazzi is making them with digital output too...

pkansara,

If you want to "detect the current and give a digital feedback to the plc" as you stated, then you only need a current sensing relay, or a universal AC current sensor as made by SSAC, a unit of ABB Controls. SSAC has MANY models available, but the TCS series is made to interface with PLCs. It has a bult in CT coil, a current range of 2 to 20 amps, input operating voltages of 24 vdc, or 230 vac, with NO and NC outputs. The trip point is adjustable form 2 to 20 amps, with switch-selectable for undercurrent or overcurrent. I have used these several times and have been pleased with them. They cost about $75 U.S.

Try this link:

http://www.ssac.com/standard/ff-sen.htm

Check with Ohio Semitronics. I have used their products to monitor power, amps, volts, and watthourts of three phase vacuum heating elments. The Model PTB will do it all in a single unit, but they have other models as well. The PTB is particularly well suited for accuately measuring the non-sinusoidal waveforms from SCR controlled power supplies and saturable core reactor type power supplies.

http://www.ohiosemitronics.com/

You can find some good devices at http://www.nktechnologies.com/


I've been using their current transmitters for years. A very good value.

panic mode said:

i think Carlo Gavazzi is making them with digital output too...

Click to expand...

Yes, I use 3 of them to detect phase loss on a motor.
You can wire the signal from each unit in series so that you only spend one digital PLC input per motor.
I think the Carlo Gavazzi type is EIS..
They are cheap, compact and I am satisfied with them.

hi woods

u r right i do not have an analog capability in my present system and so i need something digital.

also ur apprehension about the heater being cutoff by a temperature controller is correct. the heater shuts off when it is at a particular temperature.

ur advice will prove useful

thanks a lot

regards

pkansara

Use a current transformer I believe RS Components do one that will give out 4-20ma to interface to an analog input but I bet there would be others like 0-10v or use a CT + isolation/converter, most control system manufacturers have them like Klippon pheonix etc.