Heater Diagnostics

[Brijm]

I'm looking to add some diagnostic logic, to some three phase heaters. The elements our SCR controled (2-leg) at about 100A. We are mostly looking for current imbalance, to indicate that a element is blown, or there is a problem with an element. I did a quick google search for 3ph current imbalance, and also heater burnout, but didn't find much. I'm thinking about using analog cards to compare the current values in the PLC.

Can anyone reccmend a device for this application? How much do I need to worry about the 2-leg controled SCR if I take the signal into the PLC?

Thanks

 

[sthompson]

You might find something along the lines you need in thsi post.

http://www.plctalk.com/qanda/showthread.php?t=63849

Stu....

 

[wolverine1981]

Hello

You could also try the link below.

http://www.automationdirect.com/adc...C)/Current_Transducers/AC_Current_Transducers


And here for current switches.

http://www.automationdirect.com/adc...s_(AC_-a-_DC)/Current_Switches_-a-_Indicators
Tom

 

[Lancie1]

Brijm,

Several different approaches come to mind, with advantages dependent on how many heaters you need to measure:

1. If there are a bunch (50 or more?) then you may want to minimize the number of analog inputs, so a multiplex switching arrangement may be more economical. This is where you use only a few analog inputs, with a PLC switcher that steps (in a serial fashion) the CT inputs from many heaters into one, two, or three analog inputs. After all, you probably do not need instantaneous real-time current levels, only a fairly recent measurement for each heater. For each reading, the PLC knows which heater is being read because it controls the switching contactors.

2. If there are only a few heaters, then put CTs on each heater leg, run the CTs to PLC analog inputs, one for each heater. Read the currents, compare each to some known expected current value, and set up alarms when the current level is below the expected value at certain operating levels.

 

[Brijm]

Ok... I think that what I am looking at, is the AD true RMS current XDCR, into an analog card. Just didn't know if there was anything off the shelf, to preform this function.

 

[iant]

are they 3 phase phase to phase voltage or Phase to Neutral.

If they are a Balanced Load you can either measure the current in the neutral
Or look at the curent on all three phases (total should be 0)
then is dependant of your current transformer

 

[Lancie1]

No, if you have SCR heater controllers, the currents will not be true sine waves, so current readings will be off, prbably lower than actual RMS value.

You could scale (using scaling card or module) so that the currents are close to actual, but easiest is to just read some to see what is typical (at lowest operating level) for each heater, then set your alarm point below that level.

I am assuming here that you don't need the real, true currents, but only need to know if the heater is operating normallyl, or not.