Constant current control
The main task of a welding controller is the time and power specification for a resistance welding machine. The control unit specifies the welding time and the welding current setting via the current controller (thyristor power unit), or the user can programme these values. The time is fixed with the current digital control and is always worked off. The welding current is determined via a phase angle, usually as a scale division, depending on the control in per cent or in parts per thousand from 0 to 99 or 0 – 999 scale divisions. Due to the not always ideal ambient conditions, the resulting current value can of course fluctuate. In order to compensate for this, a control of the welding current is used in higher-quality control systems.
In principle, a controlled variable is kept constant in a closed-loop control by means of feedback. This is done by comparing the actual value with the specified setpoint. With constant current control, the welding current is measured on the secondary side using a Rogowski belt and compared with the specified value. The manipulated variable for constant control is the phase angle, the change in the firing angle of the thyristors. In the process, such disturbance variables as mains voltage fluctuations, secondary impedance changes or surface scattering are detected and compensated for.
With constant current control set, the welding current can then be programmed directly in kiloamperes. In 50 Hz applications, the controller works periodically, the dead time is 20 ms. In inverters, the controller works correspondingly faster with a death time of only 1 ms. Constant current control is already available with controllers of the MPS200 series, recognisable by the designation R1. The inverters of the HWI2000 series always have the option of constant current control.