Chapter Two Section (11) Transformer Rectifier Technology

Transformer Rectifier Technology and Diodes

  • Rectifiers convert alternating current (AC) to direct current (DC).
  • Rectifier development evolved from bulky, high-heat vacuum tubes to compact solid-state rectifiers, beginning with selenium diodes and advancing to silicon diodes.
  • Solid-state diodes conduct current in one direction with minimal voltage drop and act as insulators in the reverse direction.
  • Heat sinks are utilized to dissipate heat in high-current diode applications.

Rectification Configurations

  • Half-Wave Rectifier: Employs a single diode to produce pulsating DC with little practical utility.
  • Full-Wave Single-Phase Rectifier: Employs a 4-diode rectifier bridge to capture both half-cycles of the AC waveform.
  • Full-Wave Three-Phase Rectifier: Employs a 6-diode bridge to produce a significantly smoother DC output with minimal voltage ripple compared to single-phase systems.

Transformer Operation and Filtering

  • Transformers are strictly AC devices; voltage reduction must occur prior to rectification on the secondary side at lower voltage and higher current.
  • Direct current provides superior performance over alternating current for most welding processes, excluding SAW.
  • Modern power sources position an inductor before the output terminal as a filter to stabilize the arc:
    • Current is stored as a magnetic field when circuit current increases.
    • Current is released back into the circuit via the collapsing magnetic field when circuit current decreases.

Power Source Control and Phasing

  • Single-phase transformer rectifier units can include a bypass to deliver either AC or DC output, typically regulated using a mechanical reactor.
  • Three-phase AC is preferred for DC arc operation because overlapping phases eliminate most ripple, create a uniform arc, and reduce input line current across the three lines.
  • Three-phase control methods include separable reactors or silicon controlled rectifiers, STR, with transistor control.

Advantages and Disadvantages

  • Advantages:
    • Operates from standard utility AC input.
    • Supplies both AC and DC output when configured with single-phase AC input.
    • Supplies DC output with low voltage ripple when utilizing three-phase AC input.
  • Disadvantages:
    • Heavy and bulky equipment profile.
    • Lower efficiency when utilizing single-phase input.
    • Wear and tear on moving components in mechanically controlled systems.