Controlled Rectifiers, Commutation & Inductive Loads

A diode rectifier lets source voltage determine conduction. A controlled rectifier replaces some or all diodes with thyristors and delays their gate pulses by firing angle $\alpha$. The gate can start an SCR that is forward biased, but cannot normally turn it off; current must transfer to another path or fall below holding current. That asymmetry is the key to the entire subject.

Resistive and inductive loads are different experiments #

With a resistive load, current follows applied voltage and reaches zero at each source zero crossing. With an inductive load,

$$v_d=Ri+L\frac{di}{dt}+e,$$

so stored magnetic energy keeps current flowing when instantaneous source voltage changes sign. Conduction can extend beyond the resistive interval, and opening the only current path produces overvoltage. A freewheel diode gives current a local path when the source-side bridge is not supplying positive load voltage.

For an ideal single-phase fully controlled bridge with continuous DC current and negligible source inductance,

$$V_d=\frac{2V_m}{\pi}\cos\alpha,$$

where $V_m$ is source-voltage peak. This equation is not universal: discontinuous current, device drop, source impedance, commutation overlap, transformer ratio, and load EMF change it.

Commutation and overlap #

Natural commutation transfers current when the AC source makes the next device pair favourable. Source inductance prevents current from moving instantaneously, so outgoing and incoming phases conduct simultaneously over overlap angle $\mu$. During overlap the bridge effectively connects source phases through inductance, reducing average DC voltage and notching the AC voltage. Higher current and source inductance increase the effect.

Forced commutation uses capacitors, auxiliary switches, or a fully controllable semiconductor to impose reverse voltage or divert current. It is not interchangeable with line commutation: failure to provide the required turn-off time can retrigger or leave a thyristor conducting.

Rectification and line-commutated inversion #

Use voltage and current signs explicitly. In rectification, average DC voltage and current have the same sign and power flows AC to DC. In line-commutated inversion, DC current retains its direction while average bridge voltage reverses, so a motor or DC source returns power to an AC system capable of accepting it. The extinction angle must leave enough reverse-bias time for the thyristor to recover. Weak source voltage, excessive firing delay, overlap, or control error can cause commutation failure and severe current.

Device and system stress #

Calculate each device's average, RMS, and peak current from its actual conduction interval. Check repetitive blocking voltage, $di/dt$ at turn-on, $dv/dt$ while blocking, surge $I^2t$, gate-pulse amplitude/duration, heat-sink sharing, and fuse coordination. The load inductor and supply transformer store fault energy; a gate inhibit does not immediately stop current.

Diagnostic reasoning #

• Correct average voltage but excessive heating suggests RMS current, overlap, imbalance, or cooling rather than firing-angle error.
• Missing pulses can cause discontinuous current or phase imbalance; capture all gate-to-cathode waveforms and phase currents together.
• DC voltage notches and poor power factor can be inherent to phase control, but asymmetric notches indicate firing or source imbalance.
• A bridge that rectifies correctly may still fail in inversion because recovery and extinction margin are stricter.

Never validate a controlled rectifier from an unloaded voltage trace alone. Test the declared inductive or back-EMF load, source impedance, worst firing angles, regenerative states, and fault clearing with isolated measurements rated for the common-mode voltage.