Controlled Rectifiers

A thyristor conducts only after a gate pulse and only while forward-biased. Delaying the pulse by a firing angle α discards the first part of each half-cycle, so the average output follows V_dc = V_dc0·cos α — continuously adjustable with nothing dissipating the difference.

That is why controlled rectifiers replaced Ward-Leonard motor-generator sets in DC drives: one control variable, no rotating machinery, and no rheostat burning the surplus.

Beyond α = 90° the average output voltage becomes negative while current continues in the same direction — so power flows from the DC side into the mains. The hardware is unchanged; only the firing angle moved.

Inversion requires the DC side to be a source — a battery, or a motor spun by the load's inertia. A descending lift or decelerating train drives its motor as a generator, and regenerative braking recovers 10–20% of a metro system's traction energy.

With a large DC-side inductor, the line current is a rectangular block rather than a sinusoid, producing harmonics at 6n ± 1 — the 5th, 7th, 11th and 13th. They flow back into the supply, distorting the voltage for other customers and overheating transformers.

Delaying the firing angle delays the current block, so its fundamental lags the voltage by α — the control mechanism and the phase shift are the same thing. Displacement power factor is cos α, so at half output the converter draws heavily lagging current.

Total power factor is worse still once the harmonic distortion factor is included. A drive running at reduced output is doing considerable damage to the supply's power quality.

Thyristor rectifiers persist where power is extreme and ruggedness matters — HVDC at gigawatt scale, electrochemical plant, large generator excitation systems.

Below that, PWM active front ends with IGBTs give unity power factor, near-sinusoidal input current and bidirectional flow, at the cost of more switches and a controller. Harmonic regulations have made that trade increasingly compulsory.