i realy need some help!

[zezito]

HI there!
i'm new in this plc bussiness, so thank god i found this forum and i hope for someone to help me!
is it possible to control a tyristor rectifier with a plc? shall i connect an analog output (4-20ma) to the tyristors gates?? and how can i make the readding of the output current of the rectifier, and adjust the current in the tyristor gate?
if some one can help me i'l be very appreciated!
Thanks

 

[JesperMP]

What does this "thyristor rectifier" do ?
It would help if you would state the exact make and type designation of the thing.

edit: AND state what it is that you are trying to achieve with the thing.

 

[zezito]

a thyristor rectifier is one rectifier wich is output current is controled by a group of 2 thyristors in oposition. as i refer, i need to control the rectifier automaticaly with a plc, and this is nothing i've ever done! can any one help me?

 

[Doug_Adam]

I know you want to control it, but what EXACTLY do you want to do?

I would suggest that you do not connect the 4-20mA to the thyristor. Not yet anyway.

We need more information to help you, so please answer these questions.

What PLC outputs do you have? You mentioned analogue 4-20mA. What about digital?

What input does your rectifier take? Thyristors generally take a timed, pulsed, input if you want to control the output voltage. If you don't, then they just act as a solid state relay for AC to DC.

What do you want to do?
Do you want to control the voltage output for the rectifier? Or do you just want to switch on and off a fixed DC voltage?

Answer these questions and we can help you.

Doug

 

[zezito]

hi again!
it also has digital outputs, but i thougt that analogue output would serve better...
i need to control the current density in an anodize process!
the thyristors are in the primary of the rectifier transformer, so i thought that if i could control their gate's curent that would be sufficient, what do you think of that? you think it's possible to have an dinamic adjust in the thyristors while the process is ocuring?

 

[rdrast]

You don't need to control gate current of the inverse thyristor pair, you need to control the firing angle.

There are several firing circuts out there that can control firing angle based on a current (or voltage) analog input.

(Google search for some)
http://www.google.com/search?hl=en&ie=UTF-8&q=SCR+FIRING+CIRCUIT

In any event, you will still need an external regulator loop, controlling your actual process, to drive the firing circuit.

 

[Doug_Adam]

I agree with rdrast,

The direct thyristor control circuit is a specialised device.
This would interface with the PLC, probably through analogue control. I think you need to study more about thyristors and their control circuits. You cannot use them directly as amplifiers like you can (in theory) use transistors.

If you want more help, how about draw a circuit diagram of what you have and what you want to do.

Doug

 

[waggs]

Zezito,
I remember years ago using what we called an Entron to fire thyristors in flash-butt and spot welding machines. The Entron (company name), I believe, had all the features you may be looking for and can be controlled externally via plc, relay logic, or what have you. Also it could be used for setting 'heat' percentage on a normalizing process that we did with brute force induction. I'm sorry that I don't have any specifics for you, but you may want to start by looking for Entron...try this link. www.entroncontrols.com Good luck.
Bob

ok, I promise to look up how to post links...lol

Ohhhh.....the link actually worked!!!

 

[zezito]

thanks a lot for your replys
as far as can understand, i realy need to have a firing circuit for the scr's and then i can control that circuit with a plc right??
waggs : entron sells does kinds of circuits? is that it??
rdrast : that i think is the very best way to do what i want, do you think it can be possible to control that firing circuit with a plc??

Doug_Adam : thanks a lot for your help! i think i will need more help to draw that kind of circuit, do know any software that i can use to help me on this one??
thanks guys!!!!

 

[Doug_Adam]

Zezito,

For drawings you can use:

Pencil and paper, plus scanner or digital camera (Easiest).

MS paint brush, easy to get hold of but more difficult to use.

Or you could use a CAD package.

I use AutoCAD LT, but if you are on a tight budget you could use Qcad Version 1.

It is available here:

http://prdownloads.sourceforge.net/qcad/

It is only fair that I also provide the link to the company that wrote this software as well:

http://www.ribbonsoft.com/qcad.html

Like all CAD packages, this one requires some practice.

Hope this helps,

Doug

 

[JesperMP]

zezito,

how about using a standard DC motor controller. It can generate adjustable DC currents.

HERE is an example of a type that can generate up to 106A at 460VDC. Same company has models that can generate a lot more than that.

I would contact such a company, explain them what I want to do, and ask them if their product can be used for such a purpose.

 

[zezito]

thanks Jesper
i had that idea already, this standard current controls are designed for motor controls, their microcontolers have algoritims that were spec. designed for that purpose! the experience i have tells me not to use one of those controlers, there are many info's that will not reach the mic. beacause it is not a motor whos being controlled!
sorry for my "not so good" english!