Heater current monitoring

[RoTaTech]

Has anyone here a lead on monitoring AC current? We are rebuilding a machine that has approx. 70 heater zones, and in the past, each has been monitored with a CT and an analog panel meter. The various zones draw 12 to 30A at 240VAC.

I would like a system to read the CTs into analog channels so we can do burnout detection and stuck contact detection. The current Siemens 505 PLC does not have room for 70 analog channels. It can be stand-alone with its own panel display to emulate the 70 gauges, or it could feed over Profibus to our PLC.

We have a few conditions that I want to be able to react to:
Heater burnout - no current when there should be; stuck output (current when there should not be; I can turn off the master Heat control circuit); stuck contactor (current when there should not be; I can turn off the master Heat power circuit with a shunt trip breaker). The panel meter at present require someone to notice; I want to be preemptive and catch the faults to annunciate status.

 

[Nova5]

CTs, a PLC and a stack of 16 channel analog input cards?

If you have a current PLC and it supports Ethernet rack communications you can likely just add a new remote rack.

 

[ndzied1]

Googling around I found this ......

https://youtu.be/ZYC7fzFqOUs

Not totally industrial but easy on the wallet....

 

[TWS]

If you don't care what the heater amperage is then use a CT with dry contacts.
Dwyer CCS-131100. around $20.00. Switch closes when there is current flow.

 

[BryanG]

Another option is to look at energy meters, these give you a load of information you don't want, but importantly do give current. I know you can get them with Modbus output. Maybe with more searching you would find a Profibus output, but I can be sure you would pay extra.
Like this:
https://www.metermarket.co.uk/produ...ngle-phase-100a-direct-connected-energy-meter


Search for : single phase energy meter

 

[lfe]

Years ago we used in an electric kiln with total power over 1200kW a lot of transformers as the link, connected to analog inputs.

The heaters were controlled by SCR, in short ON-OFF cycles of about 15 seconds, a few tenths of second after switching to ON it was checked that the intensity was within the correct range, a few tenths of second after switching to OFF it was checked that the intensity had dropped.

On a SCADA screen you could see the intensity of about 50 different heaters, with the problems in red.

Faulty heater warning was almost instantaneous

In addition, there were contactors that disconnect the entire zone if the temperature was excessive.



https://www.gavazzionline.com/pdf/A821050.pdf

 

[Onstege]

A fieldbus-connected energy meter which monitors the total consumption of the machine? Since you know how many outputs you have activated, you also know what the total power is supposed to be. There are of course pitfalls if you get one broken heater and one welded contactor at the same time, but...

 

[RoTaTech]

Addendum

Per the Ethernet connection - no, our current PLC does not support it.

Per the bunch of analog cards - no physical room for another chassis.

Per the CT with switch - no, we do want to display the current -- each zone may have multiple heaters, so if the zone had three heaters and should draw (e.g.) 30A and one burns out, we would see only 20A. I know the on/off CT would notify that, but staff is accustomed to bouncing needles on meters.

And on that note: Measuring the overall total draw will not work as each zone can cycle on/off independently.

But I thank you for the thoughts; I will check on two of your suggestions.

 

[the_msp]

Something like this perhaps:
https://www.janitza.com/umg-20cm.html

Manual mentions ModBus RTU with up to "20 subscribers", need to check if that means you can poll it on RTU from the PLC.

 

[TheWaterboy]

Assuming the CT's are analog, Use the existing CT's into a single analog channel through 70 relays using 70 DO points to engage one CT at a time through the relay.

Not ideal but it would work.

 

[g.mccormick]

I would do RMS CTs with 4-20MA output and use something like Beckhoff IO through a profibus buscoupler

 

[Saffa]

You'll still need seventy 4-20mA CTs, but you could cut your PLC analogs down to 5 with one of the below. Will need some digital outputs though.

https://www.intech.co.nz/product/2100-m/

This is a NZ made product but I'm sure if you search for 4-20mA analog multiplexer / MUX you'll find something similar.

Down side is of course it takes a few seconds to switch through your 16 channels per unit, but if you can tolerate 60 seconds or so of alarm condition, this is a cost effective option.

 

[TWS]

A handful of these: https://www.acromag.com/shop/signal...annel-single-ended-input-profibus-dp-modules/.

 

[JesperMP]

it could feed over Profibus to our PLC.

You mean that the CTs should have profibus built-in, because there is no space for an expansion rack ?
70 CTs with profibus, expensive !

Isnt this the perfect reason to migrate the obsolete 505 PLC to something newer ?
Then you can probably get a higher channel density so that you can fit the extra 70 analog channels in the same space. The new PLC will also support more modern fieldbuses rather than profibus which is getting long in the teeth.

You have to migrate the 505 sooner or later.

 

[Puddle]

Isnt this the perfect reason to migrate the obsolete 505 PLC to something newer ?

Motion seconded.
Newer PLC is most likely smaller than the old S5, allowing enough room for analog modules or it will be able to communicate on modbus, profibus or whatever protocol the choice of CT can put out.
Assuming you're sticking with Siemens it would need to be an S7-1500 CPU for analogs as the 1200 can't support enough modules. I like the ET200SP mounted CPU's as they're cheaper.


That said... I do like that multiplexer idea. It's going to be slower but it's going to alarm faster than a gauge.