1-Ph AC Voltage Controller using two SCR antiparallel

single-phase AC voltage controller / controlled rectifier Simulink models implemented using anti-parallel thyristors (SCRs).

Main Components and Their Functions

1. AC Voltage Source

The sinusoidal source provides the input AC supply to the controller circuit.

2. Pulse Generators

Two pulse generators provide triggering pulses to the two SCRs.

• One SCR conducts during the positive half cycle
• The other SCR conducts during the negative half cycle

The firing pulses are phase-shifted appropriately to ensure alternate conduction.3. Anti-Parallel Thyristors (SCRs)

Two SCRs are connected in opposite directions:

• Upper SCR conducts in positive half cycle
• Lower SCR conducts in negative half cycle

This arrangement enables bidirectional current flow through the load.

4. Resistive Load

The load is purely resistive, therefore:

• Current and voltage are in phase
• Current becomes zero exactly at the supply voltage zero crossing
• Natural commutation occurs automatically

Mathematically,

i(t)=VmRsin⁡(ωt)i(t)=\frac{V_m}{R}\sin(\omega t)i(t)=RVm​​sin(ωt)

5. RL Load Dynamics

The load current follows:

Ldi(t)dt+Ri(t)=Vmsin⁡(ωt)L\frac{di(t)}{dt}+Ri(t)=V_m\sin(\omega t)Ldtdi(t)​+Ri(t)=Vm​sin(ωt)

Because of inductance:

• Current lags voltage
• Current continues even after voltage crosses zero
• Thyristor conduction extends beyond π\piπ

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Operating Principle (For R-Load)

Positive Half Cycle

• Upper SCR becomes forward biased
• When firing pulse is applied at angle α\alphaα, SCR turns ON
• Load receives voltage from α\alphaα to π\piπ

Negative Half Cycle

• Lower SCR becomes forward biased
• Trigger pulse turns it ON at firing angle α\alphaα
• Conduction occurs from π+α\pi+\alphaπ+α to 2π2\pi2π

Key Characteristics of R Load Circuit

Features

• Voltage and current are in phase
• Conduction angle:

σ=π−α\sigma=\pi-\alphaσ=π−α

• Simple commutation
• No energy storage effect

Expected Waveforms

Load Voltage

• Delayed sinusoidal waveform
• Controlled RMS voltage

Load Current

• Same shape and phase as voltage

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Operating Principle (For RL-Load)

Positive Half Cycle

• SCR is triggered at angle α\alphaα
• Current rises gradually because of inductance
• Conduction continues beyond zero crossing

Negative Half Cycle

• Opposite SCR turns ON after receiving trigger pulse
• Current transfer occurs from one SCR to another

This phenomenon is called natural commutation with overlap.

Important Characteristics of RL Load Controller

1. Current Lag

Current lags voltage by angle ϕ\phiϕ:

ϕ=tan⁡−1(ωLR)\phi=\tan^{-1}\left(\frac{\omega L}{R}\right)ϕ=tan−1(RωL​)2. Extended Conduction

Conduction angle becomes:

β>π\beta>\piβ>π

where:

• β\betaβ = extinction angle

3. Energy Storage Effect

The inductor stores energy during conduction and releases it later, causing:

• smoother current waveform
• delayed current decay