measuring current density!

i need to control a rectifier with a plc. the most concerning fact is that a need to create a ramp from 0 Amp/dm2 to a maximum value that may change in order to do other process(maximum value=variable)! anyone know how to do that?
are there any sensors that can read current density?

I'm not sure what current density is! What are the engineering units?

it's A/dm2

I understand current density to be the current that an electrode draws divided by the true electrode suface area (actual suface area, including pits, dimples, etc.) Usually used for large electrodes used in melting glass. I don't know of any way to actually measure the suface area, so any device used will simply have to measure the current, then do the division using an educated guess for the surface area. Electrodes wear fairly quickly (4 to 8 weeks, depending on current) in glass furnaces.

Rectifier for what type of process?

What is the process that you are using rectification for?

In plating and/or anodizing we use a.s.f. (amps per square foot) with other math of course, to control rectification. Other process' use differing equations and formulations.

At least in my business, current density is a known value which is then acted upon through various formulations.

A bit more info would be helpful.

Regards,
Chris

Zezito,

Something was bugging me about your questions, and I now know what it is. Have you considered ripple current. If you use the thyristor circuit that you seem to be proposing, you are going to get large, low frequency, ripples that need a lot of filtering to remove.

Before you go further with your design, I suggest that you calculate your ripple content and then discuss it with someone who has excelent knowledge about your process.

Recently I was involved with the installation of a large cathodic protection system, which is little more than a type of electrolysis process. With this device, ripple would destroy the anodes, so a linear system was used.

It also had an adjustment from 0A to 30A.

The setup was a conventinal step down transformer and bridge rectifier. This was then filtered. It was then run through a variable linear voltage controller with a large transistor amplifier and very large heat sinks. Adjustment was via a trim pot with an ammeter to allow the operator to check the current. It was not PLC controlled, although it had an alarm wired back to a PLC.

To answer your questions:
I would avoid doing this task with a simple thyristor rectifier.

You can control the current for different processes by setting up your system to be fully adjustable from 0 to a maximum value, with the maximum value being at least as high as the highest current you need. The adjust the set point to lower values for other processes.

Current can be measured using a hall effect DC current transducer, and from that you need to calculate the current density. You may be able to even out the current density across the electrode by connecting it at two (or more) ends.

Hope this helps,

Doug

Since you cannot change size of conductors on fly, this value
will be constant (area of the cross section). By regulating current
through conductor, you will get what you ask for.
Controling SCRs is easily done with microcontrollers or even
discrete components by controlling trigger delay for SCRs.
In order to do this, you need to detect zero crossing of the mains
and start counting. Once you reach particular delay, trigger SCRs.
By altering delay from 0-x where x is duration of halfperiod of your
sine wave, you can control the duty from 0-100%.
The period in 60Hz mains is 16.66ms so halfperiod is only 8.333ms.
To get resolution of 0.1% for example, this would mean response time
better than 8.33us (microsecond). This is not what PLCs are designed for meaning you would need to add some external circuitry.

Zezito,
I remember years ago using a controller from a company named Entron. We used them for firing the thyristors in our flashbutt and spot welders. We also used them to control the 'heat' percentage on a normalizing process that used brute force inductance. We controlled them with both PLC's and relay logic controls. These units may do what you are looking for and I'm sure they now have much more to choose from. You can use the following link to surf their website www.entroncontrols.com

Good Luck,
Bob

thank you all for your replys
I think Panic Mode had a good idea!
what do you think if i could have a circuit that allows me to change the number of impulses in the thyristor?
all i need is a circuit that can change frequency of the pulses in the thyristors gates right???
and that circuit could be controled by a plc!....i hope
what do you think of thisidea? give me your feedback
thank you all!!

You have to control the firing angle, not number of pulses. It has to be timed carefully. The frequency must match your fundamental supply frequency (50 or 60Hz).

Look up the data sheet for this module:

Semikron SKPC200-240

Data sheet is available from RS electronics, sheet number 232-3282.

The control circuit you want is pre-built and takes a 0 to 5V analog input signal.

Once again, I wish to repeat my mis-givings about using this control method for this task.

Regards,

Doug

thanks doug!
as i understand you are saying that this kind of circuit it's a pre built circuit that can be controled by a plc with analogue output correct??

Yes, I think this may be able to do your job.

Most circuits for common tasks are pre-built, because for most applications you are generally not the first person to need it.

Just a word of warning, I am giving you pointers in the right direction. Only you can tell if this will work in your application.

Regards,

Doug

A Link Might Help...

The data sheet for the SKPC200-240 is HERE.

beerchug

-Eric

Look up these guys..

Zezito

For our rectifiers, we use a package that includes the firing circuit and SCR bridges. It's made by Phasetronics . In a nutshell, our PLC's do all the necassary math to calculate current density and send the appropraite 0-10V reference signal to the packages to achive it. I agree with Doug, dont try and reinvent the wheel in regards to the package. Plus a PLC is not fast enough to control current. Analog controls yes, a PLC no.


Andrew

thank you Andrew
you said that your PLC's do all the math to send the analogue signal in order to control the outoput of the scr's, right??
i have one rectifier that has this firing circuit but the output currrent is controled by a var. resistor (in the circuit), and i need to make that adjustements automaticaly with a plc. do you think it's possible??