Current control using SCR and arduino in seam welding plant

[hanif]

Hi,
i have to design current control system for a seam welding plant. The available resources are two SCRs ST330C12 (disk type), an arduino board for SCR gate triggering pulses and a potentiometer for operator to input current set point. How do i design the code and what other accessories i required to acomplish this task.

 

[KlausST]

Hi,

at the beginning of each design one need specifications.
Values with units... and tolerance. And other dechnical informations about the function.

What voltage, what current (range), what timing, what precision..
What is the load impedance (range)? What type of load?
Isolated control unit or not?
Is DC at the output a problem? (like when transformers are connected)

Klaus

 

[hanif]

Hi,

at the beginning of each design one need specifications.
Values with units... and tolerance. And other dechnical informations about the function.

What voltage, what current (range), what timing, what precision..
What is the load impedance (range)? What type of load?
Isolated control unit or not?
Is DC at the output a problem? (like when transformers are connected)

Klaus

The inout voltage is 440 V AC. The desired current range is from 10 to 225 A selected through a potentiometer. When operator select some value through pot say 10 A the arduino should read through ADC and then set the gate pulse accordingly. This is an open loop control so no feedback is required at this stage.

 

[FvM]

The available resources are two SCRs ST330C12 (disk type), an arduino board for SCR gate triggering pulses and a potentiometer for operator to input current set point.

Means you have only Arduino or a special Arduino shield providing trigger pulses?

Usual trigger method for high power applications is trigger transformer. To provide the 3V/200mA trigger pulses, respective transistor drivers and a sufficient power supply, e.g. 12V is required.

How about SCR heat sinks and mounting accessory?

If the intended control method is phase angle control, you'll also need a zero crossing reference to the arduino.

If you want true current control instead of steering a precalculated welding current, you need current measurement.

- - - Updated - - -

O.K., last point answered.

 

[KlausST]

Hi,

so indeed it´s more a "voltage steering" than current control.
(in your case: if load resistance varies then the current will vary, too)

If 225A (RMS) is at full wave (load resistance = 440V/225A = 1.95 Ohms),

Then 10A (RMS) is at 12.1° (for ohmic load and ideal switching)

Expect about 110W of power dissipation per SCR with 180°, 225A RMS sinusoidal.

You could use pulse transformers to control the SCR.

Klaus