How does a three-phase controlled rectifier work?

A three-phase controlled rectifier uses six thyristors instead of diodes, with firing angle alpha controlling DC output. At alpha=0, it behaves like diode rectifier with Vdc = 1.35*Vll. As alpha increases, output reduces: Vdc = 1.35*Vll*cos(alpha). For alpha > 90 degrees, Vdc becomes negative (inverter mode, power returns to AC). Firing pulses must be synchronized to AC voltage and spaced 60 degrees. Commutation occurs naturally. Input current has harmonics (5th, 7th, 11th, 13th); higher pulse numbers reduce harmonics.