Firing angle: controlling DC power

Lesson notes

Key points

• In phase control, the SCR is fired at a delay called the firing angle α, measured from the voltage zero crossing.
• Delaying the firing removes the start of each half-cycle, so less voltage reaches the load on average.
• For a single-phase half-wave controlled rectifier with resistive load, average output voltage follows this equation.
• Mine winders, conveyors and DC drives use this principle to vary motor speed efficiently.

Definitions

Firing angle α

Delay in electrical degrees from the natural start of conduction to the moment the gate pulse is applied.

Takeaway: Later firing, lower average voltage: Vdc ∝ (1 + cos α).

Firing angle αAverage voltageOutput equationDC drives