PLC - SCR Heater Control - Introduction Needed (Analog)

[naidanac]

Hello, everyone.

I've been trying to wrap my brain around this for a few hours now - doing all the research.

I am looking to control a heater through an SCR (as per the requirements) with a 4-20mA output from a PLC.
The PLC also has a temperature feedback from the heater.
The platform is a Schneider Electric M340 PLC.

I am new to SCR control - should I be incorporating a PID controller?

I do understand that the 4-20mA output determines the percentage "on-time" of the heater, but I have seen applications where SCRs are mainly used as just latching circuits.
I am confused.

Any and all your help is greatly appreciated.

 

[GaryS]

You may want to look at one of these
http://www.nuwaveproducts.com/index.html
https://www.gavazzionline.com/pdf/RM1EAAdatasheet.pdf
they should work

 

[Phil Buchanan]

There are 2 basic types of SCR based Solid State Relays that most people use for doing this which are On /Off and Proportional (Phase Angle)

With the proportional it would be best to have a PID and the output of the PID giving 4-20 ma signal that will determine the on / off time and you may need something like a split range time proportional block depending on the PID functions you have in your PLC.

Or you could us a PID block with a split range time proportional block with standard on /off Solid state relays.

I like to use the Proportional solid state relays with a PIDE block and a SRTP block.

The proportional solid state relays have a better temp control and respond better to change IMHO.

 

[Phil Buchanan]

As far as a brand I like Watlow Din-a Mite series best.

http://www.watlow.com/products/controllers/din-a-mite-a-power-controller.cfm

 

[naidanac]

Thank you very much for the replies. I really do appreciate it.

There is a hard requirement that a SCR be used and only the PLC to do the logic - I am unable to use any other embedded type of solution.

Basically the 4-20mA output of the PLC is connected to a SCR - unfortunately, SSRs are out.

 

[Ron Beaufort]

is this a school project - or something for work? ... we'll try to help either way - but it's a lot easier to nail things down if we know where these restrictions/constraints are coming from ...

going one step further ... if it is a project for school, does it really have to WORK - or is it just a design on paper that will never actually be built? ...

 

[naidanac]

is this a school project - or something for work? ... we'll try to help either way - but it's a lot easier to nail things down if we know where these restrictions/constraints are coming from ...

going one step further ... if it is a project for school, does it really have to WORK - or is it just a design on paper that will never actually be built? ...

Hello Ron,

It is indeed for work. It is a sensitive application that the client insists that only the PLC do the direct SCR control.

Thank you very much in advance.

 

[edw1983]

What is this SCR component exactly? Do you have a datasheet?

I presume you are not trying to connect to a discrete SCR component directly, right?
(if the insulation fails, your PLC will be fried)

 

[Phil Buchanan]

Basically the 4-20mA output of the PLC is connected to a SCR - unfortunately, SSRs are out.

Most SSR's for this application are made from 2 back to back SCR's

Also as my friend Ron Beaufort said if you give us all the details and limitations of what you are doing upfront we can help you in a more precise manner and much faster also.

 

[naidanac]

What is this SCR component exactly? Do you have a datasheet?

I presume you are not trying to connect to a discrete SCR component directly, right?
(if the insulation fails, your PLC will be fried)

We are still in the discovery phase - they have not specified the SCR component.
We have requested the datasheet by end-of-day today.

Thank you!

Most SSR's for this application are made from 2 back to back SCR's

Also as my friend Ron Beaufort said if you give us all the details and limitations of what you are doing upfront we can help you in a more precise manner and much faster also.

Thank you so much. I should have most of the details by end-of-day today.
I'm sorry about the early posting - I wanted to get a glimpse on how to control these components.
My initial thoughts of it being a straightforward control is not exactly correct.
I'll have to drill down on the requirements soon.

Thank you and please do subscribe to the thread.
Much appreciated.

 

[g.mccormick]

Hello, everyone.

I've been trying to wrap my brain around this for a few hours now - doing all the research.

I am looking to control a heater through an SCR (as per the requirements) with a 4-20mA output from a PLC.
The PLC also has a temperature feedback from the heater.
The platform is a Schneider Electric M340 PLC.

I am new to SCR control - should I be incorporating a PID controller?

I do understand that the 4-20mA output determines the percentage "on-time" of the heater, but I have seen applications where SCRs are mainly used as just latching circuits.
I am confused.

Any and all your help is greatly appreciated.

I may be missing something here, but this seems like a fairly straightforward application. The 4-20MA signal to the SCR will control on time of the voltage to the heater, this will control kw output of the heater.
You would do the control inside the PLC (PID or other control scheme if you think that would work better). The output will be to the SCR to increase/decrease temperature.
One note/caution you stated

"The PLC also has a temperature feedback from the heater"

You will want to be sure what this temperature is. Most electric heaters that I have seen have a temperature that is meant to be a safety and it may not be in the best location for sensing your process temperature. You may want to put another temperature in that is in a better place to measure the actual temperature you are wanting to control.

What are you heating? Are you heating a stream, container, air, liquid?

 

[naidanac]

I may be missing something here, but this seems like a fairly straightforward application. The 4-20MA signal to the SCR will control on time of the voltage to the heater, this will control kw output of the heater.
You would do the control inside the PLC (PID or other control scheme if you think that would work better). The output will be to the SCR to increase/decrease temperature.
One note/caution you stated

"The PLC also has a temperature feedback from the heater"

You will want to be sure what this temperature is. Most electric heaters that I have seen have a temperature that is meant to be a safety and it may not be in the best location for sensing your process temperature. You may want to put another temperature in that is in a better place to measure the actual temperature you are wanting to control.

What are you heating? Are you heating a stream, container, air, liquid?

Thank you! Once we drill down on the specific SCR (model, info, etc.), I'll be able to let you know.

We have two temperatures - outlet and sheath. The PLC is to control the outlet temperature of the heater at a setpoint.
The sheath temperature is for over-temperature and shutdown.

We are doing saturated silicate heating in one process and water heating in another.

 

[Ron Beaufort]

DISCLOSURE: I don't get around as much as some of the other guys on here do – so feel free to ignore what follows ... frankly I'm 100% prepared to learn something new ...

Basically the 4-20mA output of the PLC is connected to a SCR

personally I've never seen an SCR that would directly accept a 4 to 20 mA signal – and then give a proportional output in response ... does such a thing even exist? ...

specifically, is there a commercially available component (SCR) that will take an input of (for examples):

8 mA and give you 25% of heating capacity ...

12 mA and give you 50% of heating capacity ...

16 mA and give you 75% of heating capacity ...

and so on ...

now I have seen – and worked with – something like this PCM4 component from Omega ...

http://www.omega.com/pptst/PCM4.html

this little gimmick mounts directly on top (piggyback style) of a commercially available SSR (not exactly the SCR that you've specified) and accepts a 4 to 20 mA signal from a PLC, etc. ...

the PCM (Pulse Control Module) then provides an output signal which pulses on and off ... these pulses then trigger the relay on and off ...

as I said earlier – feel free to ignore this – but as far as I know, you're going to need more components than you've told us about to get this working ... (I understand that you're still working on providing that list ... no problem there) ... my biggest issue (based on what's been posted so far) is that I'm not sure that you're going to be able to just connect a 4 to 20 mA signal DIRECTLY into an SCR and then expect to get a proportional amount of drive to your heater ...

now that would be a new trick for this old dog to learn ...

 

[kamenges]

They do exist, Ron. Take a look here:

http://www.gavazzi-automation.com/nsc/HQ/EN/solid_state_relays

Under Select A Product you will find a filter option that talks about control type. If you select phase angle control you will get the stuff we are looking at.

Companies like Watlow and Emerson also make larger, more involved versions of these devices. The user can select the the output format for a given analog input. the output format might be direct phase angle control, it may be a direct time proportioned output or it may be a burst mode type of operation, which is basically time proportioned but with a number of AC cycles per "burst" determined by the analog signal. The more involved of the heater controllers have internal compensation for voltage fluctuation and will have a true power output based on command.

If the customer doesn't specify the solid state control you are to use and money isn't a big object I would go with one of the the solid state heater controllers from someone like Watlow as opposed to a simple phase angle controller from someone like Gavazzi.

Keith

 

[Ron Beaufort]

thanks, Keith ... I've only had a few seconds to browse through the link you've posted – but I'll definitely spend more time with it soon ... this is interesting because it looks like the little piggyback device – and the SSR that I mentioned earlier – are both incorporated into one commercially available part ...

hopefully the OP will be able to use the ideas that you've made available ...

I'm pretty sure that I'm (personally) just getting needlessly hung up on the following statement ...

the client insists that only the PLC do the direct SCR control.

am I just getting confused by the alphabet soup terminology? ... SCR versus SSR ...

if the client is "INSISTING" that the PLC connect DIRECTLY to an SCR (specifically, not to an SSR) then I'm not 100% sure that the client would accept one of the components that you've linked ...

but chances are pretty good that the client would say, "Oh yeah, that's what I really meant to say" if you presented with this approach ...

I'm afraid that a lot is going to depend on exactly who is doing the "insisting" – and whether that person has any real/valid reasons for the constraints that are being imposed ... if it's just to shave a buck or two off of the final cost of the components – at the expense of a lot of extra design and build time – then things can quickly become less than attractive ...

thanks again ...