Abstract: A 90° phase-shifted quadrature detection method addresses zero-crossing detection limitations in single-phase AC motor thyristor speed control systems. Conventional zero-crossing triggering suffers from grid disturbance sensitivity in optocouplers, motor inductance-induced phase lag degrading power factor, and high costs of improved alternatives. The solution generates a precise 90° phase-shifted reference signal using DSP-based grid frequency tracking, replacing direct zero-crossing detection. This approach suppresses harmonic interference and compensates load current phase lag automatically. Experimental results demonstrate stable triggering accuracy during voltage fluctuations, with enhanced noise immunity and lower cost versus conventional methods. The reliable technique proves valuable for appliance motor control applications like AC fans.

Key words: Conduction angles, Load-induced phase offsets, Dynamics emphasized, Voltage phase