Industrial Electronics – I (Single-phase & polyphase controlled rectifier)

 Three-Phase Half-Wave and Full-Wave Controlled Rectifiers: Operation with Inductive and Resistive Load & the Role of Freewheeling Diode: -

When it comes to power electronics, controlled rectifiers form the backbone of converting AC power into controlled DC power. In industrial drives, battery chargers, and HVDC systems, these rectifiers play a critical role. Among them, three-phase controlled rectifiers are widely used because of their higher efficiency, reduced ripple, and better utilization of the AC supply compared to single-phase rectifiers.

In this blog, we will explore three-phase half-wave and full-wave controlled rectifiers, understand their operation with resistive and inductive loads, and highlight the importance of the freewheeling diode in such circuits.

1. Basics of Controlled Rectifiers

A controlled rectifier is essentially a diode rectifier where diodes are replaced with thyristors (SCRs). Unlike diodes, thyristors can be triggered at a desired angle (firing angle, α), which allows us to control the output DC voltage.

1. Three-phase rectifiers are more efficient than single-phase ones because they provide a smoother DC output with less ripple.
2. The main configurations are

• Half-wave controlled rectifier – uses one thyristor per phase.
• Full-wave (bridge) controlled rectifier—uses two thyristors per phase.

2. Three-Phase Half-Wave Controlled Rectifier

Circuit Description

The three-phase half-wave rectifier uses three thyristors, each connected to one phase of the AC supply, with a common load return.

Operation with Resistive Load

• Each thyristor conducts for 120° when it is forward biased and triggered.
• The output voltage is simply the instantaneous phase voltage of the conducting phase.
• The average DC output depends on the firing angle (α).

Waveform:

For resistive loads, the output voltage follows the input instantly, since there is no energy storage element.

Operation with Inductive Load

• With inductive loads, current does not drop to zero immediately when the input voltage goes negative.
• This causes overlap in conduction, and the thyristor keeps conducting until the current naturally reduces to zero.
• As a result, the output current is continuous, but the voltage waveform gets modified.

3. Three-Phase Full-Wave (Bridge) Controlled Rectifier

Circuit Description

In the full-wave rectifier (also called a six-pulse bridge rectifier), six thyristors are connected in a bridge configuration.

• At any instant, one SCR from the positive group and one from the negative group conduct.
• Each SCR conducts for 120°.
• This gives a much smoother DC output compared to the half-wave version.

Operation with Resistive Load

• When the firing angle α = 0°, the rectifier behaves like a diode rectifier, giving maximum DC output.
• As α increases, the output voltage decreases.
• At α > 90°, the average output voltage can even become negative (inversion mode).

Waveform:

Operation with Inductive Load

• With an inductive load, the current remains nearly constant.
• When a new thyristor is triggered, the old one does not turn off immediately due to inductive current.
• This creates a commutation overlap where two thyristors conduct simultaneously for a short period.
• The output current becomes smoother compared to the resistive case.

4. The Role of the Freewheeling Diode

When the load is inductive, sudden changes in current can create high voltages (back emf) and distort output. To manage this, a freewheeling diode (FWD) is connected across the load.

Functions of Freewheeling Diode:

• Provides a path for inductive current when SCRs turn off, preventing abrupt current interruption.
• Reduces voltage spikes across the load and devices, improving reliability.
• Smooths output voltage by maintaining current during non-conduction intervals.
• Protects thyristors from reverse voltage stress.

5. Comparison – Half-Wave vs Full-Wave Rectifiers

Feature	Half-Wave Controlled	Full-Wave Controlled
Number of SCRs	3	6
Pulses per cycle	3	6
Ripple	Higher	Lower
Efficiency	Low	High
Applications	Small power applications	Industrial drives, HVDC, battery charging

6. Applications

Three-phase controlled rectifiers with freewheeling diodes are widely used in:

• DC motor drives (for speed control).
• Battery chargers (for controlled charging).
• HVDC transmission (converting AC to DC at the sending end).
• Electroplating and welding equipment.

Conclusion

Three-phase half-wave and full-wave controlled rectifiers are key elements in power electronics for obtaining controlled DC power. While the half-wave rectifier is simple, the full-wave (six-pulse bridge) rectifier is preferred in practice due to its smoother output and higher efficiency.

The load type plays a major role: resistive loads produce discontinuous current, while inductive loads ensure smoother current but require careful commutation. The freewheeling diode becomes essential in handling inductive loads, reducing voltage spikes, and ensuring reliable operation.

By understanding these concepts, engineers can design robust rectifier systems for industrial and commercial applications.

----------Industrial Electronics – I (Single-phase & polyphase controlled rectifier)-5.3 Next Page --------