How it works
For a single-phase AC voltage controller with resistive load, the output RMS voltage is Vo = Vs·√[(π−α+sinα·cosα)/π], where Vs is the supply RMS and α is the firing angle (0 ≤ α ≤ π). At α = 0, Vo = Vs (full output); at α = π, Vo = 0. A TRIAC (e.g., BTA16-600) is preferred over two anti-parallel SCRs for single-phase loads up to about 25 A because it uses a single gate signal. For inductive loads, the extinction angle β > π and minimum firing angle is restricted to α ≥ φ (load power factor angle) to avoid loss of control. Three-phase AC voltage controllers use three TRIACs or six SCRs in anti-parallel for star or delta-connected loads; control range is more complex and depends on load configuration.
Key points to remember
The output voltage of an AC voltage controller is varied by changing firing angle α, not frequency — the supply frequency and output frequency remain identical at 50 Hz, distinguishing it from a cycloconverter or inverter. For resistive loads, harmonic content includes odd harmonics (3rd, 5th, 7th…); even harmonics cancel because of half-wave symmetry. Total Harmonic Distortion (THD) increases as α increases beyond 90°. Applications include lamp dimmers, fan speed controllers (with the TRIAC firing IC like the BT136 triggered by a diac), heater temperature control, and soft starters for small motors. Integral cycle control (burst firing) is an alternative to phase-angle control that switches complete cycles on and off, reducing harmonics but causing flicker.
Exam tip
Every Anna University power electronics paper asks you to derive the RMS output voltage of a single-phase AC voltage controller for resistive load and sketch the output waveform for α = 90° — derive the RMS formula from the integral of v² over the conduction interval and draw both half-cycles of the output clearly.