Single-Phase Half-Wave and Full-Wave Controlled Rectifier Circuits—Principle of Operation with Resistive and Inductive Loads—Role of Freewheeling Diode: -
Power electronics has
transformed how we control and convert electrical energy. One of the most
fundamental building blocks in this field is the controlled rectifier, which
uses devices such as thyristors (SCRs) instead of simple diodes. Unlike
ordinary rectifiers, controlled rectifiers allow us to regulate the output
voltage by adjusting the firing angle of the SCR. In this blog, we will explore
the single-phase half-wave and full-wave controlled rectifier circuits, their
operation with resistive and inductive loads, and the role of the freewheeling
diode.
1. Introduction to Controlled Rectifiers
Rectifiers are circuits
that convert AC (alternating current) into DC (direct current). Traditional
rectifiers use diodes, which conduct automatically when forward-biased.
However, they offer no control over when the conduction starts.
To overcome this
limitation, silicon-controlled rectifiers (SCRs) are used. An SCR remains off
even if forward-biased until it receives a gate pulse. This property makes it
possible to control the conduction angle and hence the average DC output
voltage. Such rectifiers are known as controlled rectifiers.
2. Single-Phase Half-Wave Controlled Rectifier
The half-wave controlled
rectifier is the simplest configuration. It consists of a single SCR connected
in series with the load and the AC supply.
2.1 Principle of Operation with Resistive Load
When the AC supply is
applied:
- During the positive half cycle, the SCR is forward-biased. It will conduct only when a gate signal is applied at a chosen angle α (firing angle).
- After being triggered, the SCR conducts until the end of the positive half cycle, because the current naturally drops to zero when the supply goes negative.
- During the negative half cycle, the SCR is reverse-biased and remains off.
Thus, the output voltage
appears only during part of the positive half cycle, depending on the firing
angle. The average DC output voltage can be controlled by varying α.
2.2 Principle of Operation with Inductive Load
With an inductive load,
the situation changes. Inductors oppose sudden changes in current. Even after
the input voltage goes negative, the current through the SCR may continue due
to stored energy in the inductor. This causes the SCR to conduct beyond the
natural zero crossing of the supply.
This results in delayed current zero crossing, which can lead to negative voltage across the load when the current continues flowing. This effect reduces controllability and increases losses.
3. Single-Phase Full-Wave Controlled Rectifier
A full-wave controlled
rectifier is designed to make better use of the AC supply. There are two
configurations:
- Center-tap transformer type (two SCRs)
- Bridge type (four SCRs)
Both deliver output during
both positive and negative half cycles of AC, thereby improving efficiency.
3.1 Principle of Operation with Resistive Load
- In the center-tap configuration, one SCR conducts during the positive half cycle and the other during the negative half cycle (based on firing signals).
- In the bridge configuration, pairs of SCRs conduct alternately for positive and negative cycles.
- As in the half-wave case, conduction begins at the firing angle α and ends at the natural zero crossing.
The output voltage is
therefore controllable across both half cycles, giving a smoother DC with a
higher average value compared to the half-wave rectifier.
3.2 Principle of Operation with Inductive Load
When the load is
inductive, current again continues flowing beyond the zero crossing due to
stored energy. In a full-wave rectifier, this means the current from one SCR
pair overlaps with the conduction period of the opposite pair. This overlap must be carefully managed to avoid short circuits.
The main challenge with
inductive loads is that the output voltage may dip below zero due to reactive
energy return, leading to distortions in the waveform.
4. Role of Freewheeling Diode
To address the issues
created by inductive loads, a freewheeling diode (FWD) is added in parallel
with the load.
4.1 Working of the Freewheeling Diode
- When the supply voltage goes negative but the load current still wants to flow (due to inductance), the freewheeling diode provides a path for this current.
- This prevents the current from forcing the SCR to conduct into the negative cycle.
- The result is a smoother DC output, reduced voltage spikes, and protection for the SCR.
4.2 Advantages of Using FWD
- Reduces ripple in the output voltage.
- Protects SCR from high reverse voltages caused by inductive kickback.
- Improves power factor by minimizing negative voltage regions.
- Provides better load performance, especially for motors and inductive devices.
5. Applications
Controlled rectifiers are
widely used in:
- DC motor drives – speed control through average DC voltage regulation.
- Battery charging circuits—where controlled charging is necessary.
- HVDC transmission systems—for controlled power flow.
- Industrial heating systems—where power must be precisely controlled.
More Study Half Wave & Full Wave Go Through Below
6. Conclusion
The study of single-phase
controlled rectifiers provides a foundation for understanding modern power
electronics. The half-wave rectifier is simple but limited, while the full-wave
rectifier offers better efficiency and output smoothness. The presence of
inductive loads introduces challenges such as extended current conduction,
which are elegantly solved by the inclusion of a freewheeling diode.
By adjusting the firing
angle of the SCRs and using freewheeling diodes where necessary, engineers can
tailor rectifier performance to a wide variety of applications. This control
over AC-to-DC conversion has paved the way for advancements in drives, renewable
energy, and industrial electronics.
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