480V heating element control

Snap25 said:
So a contactor for all 6 and a SCR on one heating element (after the contactor)?

Toggle the other 5 heat elements at different deviation stages and use the SCR to maintain?
Click to expand...

Yep - one smaller SCR and 6 smaller contactors is going to be far more cost effective than one larger SCR and one Contactor

The output of your control loop will be 0-100% master signal /6 = 16.67% per stage. The first 16.67 % gets scaled to 0-100% PWM signal to modulate the SCR controlled heater. When the output of your loop passes 16.67% the second contactor pulls in and the PWM signal is back to 0% - 100% for the next section 16.67%-33.33% of the master if that makes sense.

What PLC are you using? I'll dig out some sample code and post it on here, I think its in RSLogix500
 
There is a 10 degree range for this tank, so SCR may not be necessary.

If not, I could use a 3 pole solid state relay for each element and element the mechanical contactors all together correct?

Do solid State relays generate a lot of heat? I'm alright fighting heat as it is!
 
raffles said:
Yep - one smaller SCR and 6 smaller contactors is going to be far more cost effective than one larger SCR and one Contactor

The output of your control loop will be 0-100% master signal /6 = 16.67% per stage. The first 16.67 % gets scaled to 0-100% PWM signal to modulate the SCR controlled heater. When the output of your loop passes 16.67% the second contactor pulls in and the PWM signal is back to 0% - 100% for the next section 16.67%-33.33% of the master if that makes sense.

What PLC are you using? I'll dig out some sample code and post it on here, I think its in RSLogix500
Click to expand...


Yes, that makes sense! Thanks. I'm using a compactlogix.. But I have a Rslogix 500 license laying around here somewhere, just have to reinstall it on my laptop. I was hoping I would never have to use it again!
 
James Mcquade said:
The rslogix 500 software will not work on a compact logix or control logix.
you need the logix 5000 software and its $3,600 +/- a little.

regards,
james
Click to expand...

I'm aware. I was just stating I have 500 and I'm able to open up the sample program he mentioned.
 
I've just pulled out the relevant section and made a .pdf file of it so anyone can read it. The rest of the PLC program isn't relevant. This was a Micrologix from an OEM air handler manufacturer that I had the joy of being asked to fix in Fort Mac a couple of years ago

Given that you're using CLX this will be easier for sure.

N90:17 is the CV from the PID loop, N7:35 goes to the SCR controller (this one had a 0-10Vdc signal input - you can roll your own PWM easily as it doesn't need to be high speed in this application)

The compare instructions are there to provide a hysteresis for the outputs so they weren't short cycling. This can be wrapped into an AOI in logix

There are many ways to skin this cat though
 
James Mcquade said:
I would recommend a mercury contactor rated higher than your contactor. no contacts to weld or contactors to replace.
Click to expand...

I can't begin to estimate how many mercury contactors that we replaced in the injection molding area of Saturn. Hundreds at least.

They do eventually go bad, which sometimes means that they weld closed, which has resulted in at least one fire in the plant.

One of the problems was that a well-meaning electrician would replace just the bad pole and leave the other two in place. When I pointed out that the other two poles were not long for this world, and that he should be replacing the entire contactor instead, the look of surprise was hilarious. He had never considered that...
 
For most of our applications we use groups of elements. One element bundle is typically 50 kW and consists of 3x 3phase elements.
Depending on the total power, we typically have:
1x16.7kW,1x33.3kW,1x50kW,1x100kW etc...
This way you can step up with small steps, maintaining a pretty good temperature/pressure.

We have used thyristor too. All groups are equal there. We ramp up thyristor to 100%, then simultaneously set thyristor to 0% and start element. This way you can ramp the entire power spectrum of the heater. Costumer has been very happy with the results. I believe it was a JUMO unit we last used, I can look up the part if you want.

As mentioned, you will need contactor for each group. We also add fuse to each group and a main circuit breaker to the incoming line. This breaker will trip in case of safety break (H.High Temp/Press, L.Low Level/Press, etc..)
 
boneless said:
For most of our applications we use groups of elements. One element bundle is typically 50 kW and consists of 3x 3phase elements.
Depending on the total power, we typically have:
1x16.7kW,1x33.3kW,1x50kW,1x100kW etc...
This way you can step up with small steps, maintaining a pretty good temperature/pressure.

We have used thyristor too. All groups are equal there. We ramp up thyristor to 100%, then simultaneously set thyristor to 0% and start element. This way you can ramp the entire power spectrum of the heater. Costumer has been very happy with the results. I believe it was a JUMO unit we last used, I can look up the part if you want.

As mentioned, you will need contactor for each group. We also add fuse to each group and a main circuit breaker to the incoming line. This breaker will trip in case of safety break (H.High Temp/Press, L.Low Level/Press, etc..)
Click to expand...


I like this idea! I wasn't involved when they purchased the heating elements so all 6 are sized the same. 15kW each.
 
For every element use a SCR, and 6 outputs on a simple PLC (or arduino)No need for fancy stuff, switch on a SCR when temperature is deviating for 1 degrees, the next one on 2 etc.
 
We don't. Most of our cabinets next to the boiler, plus all elements are in a single connection box.
 

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