PLC - SCR Heater Control - Introduction Needed (Analog)

[mk42]

They do exist, Ron.

To take those more complex heater controllers a step further, I've not only seen 4-20 mA devices, I've also seen some with fieldbus comms. We only used the heater elements, but supposedly they had thermocouple modules that could be plugged in, and all the data was passed back to the PLC together.

 

[g.mccormick]

Ron, Have a look at the following.
http://www.watlow.com/downloads/en/brochures/windin0909.pdf

This is watlow's Din-A-mite series of 4-20MA controlled SCRs. The 4-20MA signal to the SCR controls % on time of the SCR. The document shows how at 20%, the SCR would be on for 3 ac cycles and off for 12. 50% would be on 3 off 3, 80% is on 12 off 3.

I would take the clients "insisting" on meaning that the 4-20MA control signal would come from the PLC and be the output of logic in the PLC (PID loop, etc) as opposed to using a separate stand-alone PID controller.

Aren't most all SSR (solid state relay) actually SCR (silicon controlled rectifier) componnents? The SCRs such as the Din-a-mite have additonal circuitry built in to do the PWM control of the scr from a 4-20MA signal.

 

[naidanac]

Ron, Have a look at the following.
http://www.watlow.com/downloads/en/brochures/windin0909.pdf

This is watlow's Din-A-mite series of 4-20MA controlled SCRs. The 4-20MA signal to the SCR controls % on time of the SCR. The document shows how at 20%, the SCR would be on for 3 ac cycles and off for 12. 50% would be on 3 off 3, 80% is on 12 off 3.

I would take the clients "insisting" on meaning that the 4-20MA control signal would come from the PLC and be the output of logic in the PLC (PID loop, etc) as opposed to using a separate stand-alone PID controller.

Aren't most all SSR (solid state relay) actually SCR (silicon controlled rectifier) componnents? The SCRs such as the Din-a-mite have additonal circuitry built in to do the PWM control of the scr from a 4-20MA signal.

This is actually very interesting - I have learned so much in this thread.
Thank you very, very much.

I will have to ask - how does the SCR Power Controllers implement the logic behind SCR control?
Is it hardware or software implemented? By that, I mean will there be software embedded and/or a processor in the Power Controller that analyzes the current input and does PWM to the actual SCR? Or is it an electrical/hardware implementation?

 

[Ron Beaufort]

thanks, g.mccormick ...

I would take the clients "insisting" on meaning that the 4-20MA control signal would come from the PLC and be the output of logic in the PLC (PID loop, etc) as opposed to using a separate stand-alone PID controller.

and you're probably correct ... personally I tend to get too hung up on specific words (insist, SCR, SSR, direct, etc.) ... sometimes that's helpful to me – other times not so much ...

I hope that the OP will keep us informed as to the final outcome of this project ...

 

[naidanac]

I would take the clients "insisting" on meaning that the 4-20MA control signal would come from the PLC and be the output of logic in the PLC (PID loop, etc) as opposed to using a separate stand-alone PID controller.

That is correct. I sometimes fail to articulate myself well.
There should be no intermediary between the PLC and the SCR itself.
They insisted on it as it is a sensitive application.

Thank you.

 

[Ron Beaufort]

I have one more quick question - then I have to get back to work ...

would any one of the devices that have been linked (by other posters above) meet the demands of the client? ... specifically, does the device HAVE to say "SCR" on the label - or could a device labeled "SSR" also be acceptable? ...

back to the OP's post #5 ...

unfortunately, SSRs are out.

based only on a QUICK reading, the client probably won't be too accepting of any of the devices linked so far ... it looks to me like they're all going to either be labeled "SSR" - and not "SCR" ... or else they're going to fall into the "extra components" category that the client is (apparently) frowning upon ...

honestly I'm not trying to overly complicate this project ... it's just that I get a queasy feeling whenever a client/customer starts dictating HOW a project should be done - rather than specifying what the final results should be ... there might be perfectly good reasons for the client's restraints - but (so far) we don't know what those reasons are ...

 

[GaryS]

What you are looking is called a phase controlled solid state relays
and there are several companies that make them. they as designed to do exactly what you are trying to do [heating application]
check the link I posted earlier.
also keep in mine most local codes require a high temperature cut off that independent of the controller and also requires a manual reset. It must disconnect all heating sources

 

[kamenges]

This simple phase angle controller pucks are a hardware implementation. The more advanced heater power controllers have some type of processor in them.

None of these give you direct control over the SCR in the device. The simple phase angle controllers are closer to direct control but they are not direct. I know of no plc that would be able to do what you want if the customer is REALLY asking for direct control of the SCRs in an SSR. Even the digital controlled on/off models have internal conditioning circuitry.

All this really begs the question whether the customer really knows what the want or what they are asking.

Keith

 

[Phil Buchanan]

OP there is no direct control of an SCR from a 4-20 ma signal. Make sure but I am 99.9% certain your customer is asking for an SCR based solid state relay that will accept a 4-20 ma signal so that you can control phase angle / On/off conducting time in portions of a second in most cases.

This method gives more heater life and more precise control. As kamenges laid out there are hardware implementations and more advanced units normally have an embedded control system that you can't touch.

Almost all SSR for this type application will be double backed SCR's and that is likely where your customer is getting that terminology.

You may want to consider a few other things in your design though. It is a good practice to protect each zone with fusing and semi conductor fusing is best as it will protect the SSR also.

You also need a limit contactor ahead of the SSR because when SSR's fail they almost always fail closed and will cause the heat to rise even when there is no control output so it's a good idea to have a limit controller or have limiting in your plc that will open this contactor if a certain temp is exceeded or if there is a loss of temperature signal.

It is also a good practice to make this limit control have it's own temp sensing device TC, RTD, etc. so that it can be compared to the temp sensing device that is doing the control this comparison is easy to do in the PLC logic.

Many of the controllers like the watlow Din A Mite can detect a shorted SCR which provides more diagnostics.

You can also look at your output and the temperature and it's rate of change to determine if there may be a problem that needs attention and this works well on processes that take a long time to het up and to soak in to correct temperature.

Just some other points to consider in your design.

 

[mbartoli]

+1 for the Watlow DIN-A-MITE http://www.watlow.com/products/controllers/din-a-mite-c-power-controller.cfm

We use these on a wide range of sizes of electric heaters for process heating, and their performance is outstanding, and they will accept either a voltage or current analog control signal.

 

[naidanac]

Thank you very much, everyone for all your help.
This has been a great learning process for me.

The client has gotten back to me with the following data sheet. It looks like they are SSRs? :facepalm:
They insist that they are SCRs.

http://instrumentation-central.com/SCRPowerControls/hdr/Manual/ZF1-3_15-70.pdf

They have selected the ZF1 that is to be controlled by the PLC directly.
Reading from the literature, it says that it does take the 4-20mA current as an input and scales it to the voltage output (albeit, not linearly).

I have one more quick question - then I have to get back to work ...

would any one of the devices that have been linked (by other posters above) meet the demands of the client? ... specifically, does the device HAVE to say "SCR" on the label - or could a device labeled "SSR" also be acceptable? ...

back to the OP's post #5 ...



based only on a QUICK reading, the client probably won't be too accepting of any of the devices linked so far ... it looks to me like they're all going to either be labeled "SSR" - and not "SCR" ... or else they're going to fall into the "extra components" category that the client is (apparently) frowning upon ...

honestly I'm not trying to overly complicate this project ... it's just that I get a queasy feeling whenever a client/customer starts dictating HOW a project should be done - rather than specifying what the final results should be ... there might be perfectly good reasons for the client's restraints - but (so far) we don't know what those reasons are ...

What you are looking is called a phase controlled solid state relays
and there are several companies that make them. they as designed to do exactly what you are trying to do [heating application]
check the link I posted earlier.
also keep in mine most local codes require a high temperature cut off that independent of the controller and also requires a manual reset. It must disconnect all heating sources

This simple phase angle controller pucks are a hardware implementation. The more advanced heater power controllers have some type of processor in them.

None of these give you direct control over the SCR in the device. The simple phase angle controllers are closer to direct control but they are not direct. I know of no plc that would be able to do what you want if the customer is REALLY asking for direct control of the SCRs in an SSR. Even the digital controlled on/off models have internal conditioning circuitry.

All this really begs the question whether the customer really knows what the want or what they are asking.

Keith

OP there is no direct control of an SCR from a 4-20 ma signal. Make sure but I am 99.9% certain your customer is asking for an SCR based solid state relay that will accept a 4-20 ma signal so that you can control phase angle / On/off conducting time in portions of a second in most cases.

This method gives more heater life and more precise control. As kamenges laid out there are hardware implementations and more advanced units normally have an embedded control system that you can't touch.

Almost all SSR for this type application will be double backed SCR's and that is likely where your customer is getting that terminology.

You may want to consider a few other things in your design though. It is a good practice to protect each zone with fusing and semi conductor fusing is best as it will protect the SSR also.

You also need a limit contactor ahead of the SSR because when SSR's fail they almost always fail closed and will cause the heat to rise even when there is no control output so it's a good idea to have a limit controller or have limiting in your plc that will open this contactor if a certain temp is exceeded or if there is a loss of temperature signal.

It is also a good practice to make this limit control have it's own temp sensing device TC, RTD, etc. so that it can be compared to the temp sensing device that is doing the control this comparison is easy to do in the PLC logic.

Many of the controllers like the watlow Din A Mite can detect a shorted SCR which provides more diagnostics.

You can also look at your output and the temperature and it's rate of change to determine if there may be a problem that needs attention and this works well on processes that take a long time to het up and to soak in to correct temperature.

Just some other points to consider in your design.

+1 for the Watlow DIN-A-MITE http://www.watlow.com/products/controllers/din-a-mite-c-power-controller.cfm

We use these on a wide range of sizes of electric heaters for process heating, and their performance is outstanding, and they will accept either a voltage or current analog control signal.

 

[dginbuffalo]

We have several furnaces with these types of controllers that are PID controlled from the PLC. Some have a 2nd PID loop that is current limiting if needed. I suggest a controller with built in current limiting if possible.

 

[Highland Controls]

I think most things have been covered now, but just a few comments. Some confusion about the term SCR from the true electronics guys here. SCR to me now means anything from a true SCR, up to a very smart power controller(like the Watlow Power Series). Usually means a device that can take a logic signal of some sort from a PLC or discrete controller. You must be aware of the firing type for your application. Zero crossing is very common for the smaller SCR's. This works fine for resistive loads. If you are firing into a transformer(very common if using silicon carbide heaters), then you must use phase-angle firing, unless you want to smoke an expensive transformer. Phase-angle firing SCR's can produce quite a bit of noise, so be aware of that.

A high limit controller should be incorporated that is completely separate from the PID controller. I always use a discrete controller for this. Could be one with a display, or one buried in the panel that you need to adjust a dial that is on it.

 

[osmanmom]

i don't know schneider PLC,

check this link,
http://www.plctalk.net/qanda/showthread.php?t=100102&highlight=watlow