Firing circuit contoled by PLC

HI There!
Anyone can help me with firing circuits that can be connected to a plc??

Please be more specific! What do you want to fire:

• firecrackers
• missiles
• people
• ...

Regards,

i need to control the output current of a rectifier, so it came to the idea to control the firing angle of the two anti-parallel SCR there are in the primary transformer on the rectifier!
What do you think of this?

Why not use a DC motor control?

I'm not sure a PLC is right tool for what you're trying to do. PLC scan times are typically measured in milliseconds. To accomplish what you seem to be trying to do, you need to be able to gate the SCR at the appropriate time during the positive half-cycle of the AC sine wave. Each positive half-cycle lasts for 8.333 milliseconds. You'll need a resolution at least an order of magnitude better than that to have any kind of control at all, and two orders of magnitude for barely adequate control.

Here in portugal we have 230v 50hz.
it means that a single complete sine wave last for 20 ms, so one half wave last for 10ms.
as an example: my plc can output current signals each 5 ms
that means in each 5 ms my SCR is ON until the sine wave crosses 0, and then it turn on 20ms after, this way if we concider the begining of a sine wave the point 0, every 5ms after it begins the SCR turns on
Please apreciate this thinking and give me your feedback
Thanks

Whether the AC frequency is 50 Hz or 60 Hz doesn't matter. I don't think a typical PLC is fast enough to give you reliable, repeatable, accurate control of the gating pulse for an SCR rectifier.

Consider your example of a PLC with a 5 mS scan time. That means you turn on an output connected to the SCR gate with 5 millisecond resolution. Let's assume you're trying to gate the every time the voltage crosses zero from negative to positive. Let's also assume that you have a sensor connected to a PLC input that can detect that event.

Best case: The sensor detects the zero cross one microsecond before the PLC reads the inputs, so the ladder logic commands the output to turn on. Elapsed time from sensing the zero cross to firing the gating pulse is something less than 5 milliseconds. Even if the time is as little as 2 milliseconds, you're at 18 degrees before gating the SCR.

Worst case: The sensor detects the zero cross oe microsecond after the PLC has serviced the inputs. One complete PLC scan cycle of 5 milliseconds must pass before the PLC reads the inputs again. If we use the 2 millisecond throughput from the best case scenario, 7 milliseconds will pass before gating the SCR (at 126 degrees).

So, unless you can figure out some way to synchronize the PLC scan to the waveform, the best you can hope for is to be able to gate the SCR at somewhere between 18 and 126 degrees of each cycle.

Zezito,

You posted this to me privately:

SKPC 200-240

Hi Doug
i saw the SEMIKRON SKPC 200-240 datasheet, and it seem to me that it would may work!
have you ever used this component in any aplication?
Think it is possible to connect the PLC's anologue output to the skpc 200-240 and control the frequency that a signal is given in the Thyristor gate? this way i think it's possible to control the SCR output current, Here in portugal we have 230v 50hz.
it means that a single complete sine wave last for 20 ms, so one half wave last for 10ms.
as an example: my plc can output current signals each 5 ms
that means in each 5 ms my SCR is ON until the sine wave crosses 0, and then it turn on 20ms after, this way if we concider the begining of a sine wave the point 0, every 5ms after it begins the SCR turns on
Please apreciate this idea and give me your feedback
Thanks

Click to expand...

First, to use the Semikron unit.
I have never used this unit before. I picked it out of an RS catalogue. If I wanted to control SCRs from a PLC, this would probably be what I would try first.
Referring to the data sheet that Eric Nelson kindly linked to, from figure 1 it appears that you can link a 0 to 5V analog input directly to the unit. In this case you would connect the analog to pin 5 and the common to pin 6. You can probably leave pin 4 off altogether.
If you flick forward to figure 8, you can see this circuit set up for closed loop control. If you replace the analog PID controller with a PLC, you will have your conceptial design done.

Second, about your PLC control idea, I think you have the wrong idea. The PLC will output an analog signal constantly. This signal will generally be updated and possibly modified once per scan of your PLC. This analog signal will then be converted by the firing circuit to the correct firing angle. 0V should be firing angle of 180 degrees, or too late to catch the wave. 2.5 volts should give you a 90 degree firing angle, which would give you an output of about 50% full load. 5V should give you a firing angle of 0 degrees, or 100% full load output.

Don't try to use a PLC to control the firing angle directly, its like trying to use a 12mm flat blade screwdriver to screw down the terminals for your PLC IO cards. It is too big and clumsy for this type of job.

Hope this helps,

Doug

zezito

I think your trying to re-invent the wheel.

you said: "I need to control the output current of a rectifier" You did not specify the application, voltage or current range, so I cant point you to a spicific unit. However there are many many many small and large dc control boards, drives, etc.. that will provide what you want.

Tyristor

Bem Vindo Zé

É sempre bom encontrar mais um português aqui. Escrevo em Inglês, pois á a lingua cá do sitio.

Had you tried Eurotherm (www.eurotherm.com). They have tyristors and controlers.

João

Thank you ALL for your repliess! I realy appreciate your concern on this!

Doug: i think i will experience the skpc 200-240 opn this one! if you want i'll post to you my feedback on that product, and thanks for your advices!

elevmike: as i told before i need to have my scr output controlled by a PLC, scr output is 20V/50A,it has a transformer with 2 thyristors in anto-parallel positions, Doug told to use the skpc 200-240 as an interface between PLC and the firing circuit, what do you think?

Jcurado: muito obrigado pela tua resposta! espero que possamos discutir...na nossa lingua...assuntos relacionados com a nossa profissao!!

Thank you all
Ze

zezito said:

elevmike: as i told before i need to have my scr output controlled by a PLC, scr output is 20V/50A,it has a transformer with 2 thyristors in anto-parallel positions, Doug told to use the skpc 200-240 as an interface between PLC and the firing circuit, what do you think?[/B]

Click to expand...

zezito,

I truly dont know what to think. I dont know anything about the application, or what type of waveform, or pattern your looking for as an end result, or whats going to be powered by your circuit. However I do know that you will not be able to directly control the firing of the SCRs directly from the PLC. I really thing you need to step back and provide a broader view of the application to determin if your idea for controlling the output of your circuit is actually the best way to go about it. Personally, to better facilitate future maintenace and service I prefer to design my system so that various components can be easily located, purchased, and replaced by the people maintaining the equipment, long after its been placed in service.

the scr in question is for a plating process. i need to control the output of the scr so that i can know when a certain A/dm2 value it's reached in a time previously established! the best way i found was using Semikron skpc 200-240 as an interface, do you have other ideas?

zezito,

As I'm totally unfamillar with your applaction, electro-plating, I really cant offer any further advice that would be worthwhile to you.

Best wishes on you project.

Mike.

Hi LAN,

Thought abour using a comparator to sense the zero crossing? It can then output a "clock signal" of say 5V to a microprocessor such as a PIC. Starting a timer with a variable preset, hence firing the gate when ever the timer is done? So easy and so cheap. The same applies with a PLC, but you would really be better of with a very fast PLC, at hight cost, some you would use in motion control apps e.g. AB 175-L63 Contrologix.

Cheers
Dieter