# Single-Phase Thyristor Rectifier

## Working principle

A single-phase thyristor rectifier converts an AC voltage to a DC voltage at the output. The power flow is bidirectional between the AC and the DC side. The circuit operation depends on the state of the AC source and the firing angle α of the 2-pulse generator. The source inductance Ls is neglected for simplicity.

• α = 0°: When the firing angle of the thyristors is zero, the given circuit reduces to the diode rectifier with inductive load.
• 0° < α < 90°: A firing angle larger than zero means a thyristor will also carry positive blocking voltages. This leads to negative DC voltages and also to a smaller average load voltage of Vload = 0.9 · Vs,rms · cos(α).
• 90° < α < 180°: The circuit is in inverter mode with power flowing from the DC to the AC side.

### Influence of source inductor Ls and load voltage Vload

• As with the single-phase diode rectifier, a source inductance Ls larger than zero leads to a current commutation interval between the thyristor pairs T1/T2 and T3/T4. This interval adds to the thyristor firing angle α and leads to a further reduction of the average load voltage.
• When increasing the load voltage the current through the inductor is reduced. As with DC-DC converters, during steady state, the circuit is said to operate in discontinuous conduction mode if the current through Ld reaches zero, and in continuous conduction mode if the current never reaches zero.

### Experiments

• Set the grid inductance and thyristor firing angle α both to zero and run a steady-state analysis. Check that the results correspond exactly to the ones obtained for the diode rectifier with inductive load and source inductance L= 0 H.
• Set L= 100 μH, α = 45° and Vload = 150 V. Observe how the load current reaches zero during steady-state operation - the converter now runs in discontinuous conduction mode.
• Check that for increasing firing angles α>45° and source inductance Ls>100µH the output voltage is reduced.