AC Voltage Controllers

AC voltage controllers convert a fixed alternating voltage to a variable alternating voltage without changing the frequency. You see them heavily used in industrial induction motor soft starters and domestic fan regulators.

Core Concept

The circuit uses phase control to vary the RMS output voltage. Thyristors block current until a gate pulse arrives at a specific firing angle, typically between 30 and 150 degrees.

Once fired, the thyristor conducts until the load current falls to zero. A larger firing angle reduces the conduction interval and lowers the delivered power.

For highly inductive loads, the current lags the voltage, requiring careful gating strategies. The switching frequency matches the line frequency of 50 Hz.

Key Formulas

Vrms = Vs * sqrt((1 / pi) * (pi - alpha + (sin(2*alpha)/2))). Input power factor = sqrt((pi - alpha + (sin(2*alpha)/2)) / pi). Vavg = 0 V for full wave.

Given:\nVs = 230*sqrt(2) V Peak, alpha = 90 degrees (pi/2 rad), R = 10 ohms.\n\nFormula:\nVrms = Vs_rms * sqrt((pi - alpha + (sin(2*alpha)/2))/pi)\n\nSteps:\nVrms = 230 * sqrt((pi - pi/2 + 0)/pi)\nVrms = 230 * sqrt(0.5)\nVrms = 162.6 V\n\nFinal Answer:\nThe RMS output voltage is 162.6 V.

Exam Tip: GATE trap: The average output voltage of a full-wave AC voltage controller is always zero. Do not confuse it with a controlled rectifier.

Performance Parameters

• RMS Output Voltage: Highly dependent on firing angle and load power factor.
• Input Power Factor: Decreases as the firing angle increases.
• Harmonic Distortion: Generates odd harmonics like 3rd, 5th, and 7th in the supply current.
• Efficiency: Very high, typically above 98 percent due to minimal switching losses.
• Peak Inverse Voltage (PIV): Equals the peak supply voltage, Vm.

Quick Revision

• Converts fixed AC to variable AC.
• No change in fundamental frequency.
• Uses two antiparallel thyristors or one TRIAC.
• Natural commutation is utilized.
• Commonly used for induction motor speed control.
• Exam trap: Assuming the input power factor equals the load power factor. Phase control inherently worsens the input power factor.