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

Concept of Full-Control and Half-Control Rectifiers:-

Power electronics plays a key role in modern electrical and electronic systems. One of its most important applications is the conversion of alternating current (AC) into direct current (DC). This process is called rectification. Rectifiers are widely used in power supplies, battery chargers, DC motor drives, and many industrial applications.

In this blog, we will explore the concept of full-control and half-control rectifiers, their working principles, circuit arrangements, and waveforms. By the end, you will have a clear idea of how these rectifiers differ from each other and where each type is used.

1. What is a controlled rectifier?

A rectifier that uses thyristors (SCRs) instead of simple diodes is known as a controlled rectifier. Unlike a diode, which conducts automatically when forward-biased, a thyristor requires a gate signal to turn ON. This allows us to control the output voltage by adjusting the firing angle (α) of the SCR.

• If the firing angle is small, conduction starts earlier, and the average DC output voltage is higher.
• If the firing angle is large, conduction starts later, and the average DC output voltage decreases.

Controlled rectifiers can be classified into two main types:

1. Half-Controlled Rectifier
2. Fully-Controlled Rectifier

2. Half-Control Rectifier

A half-controlled rectifier is a circuit that uses a combination of diodes and thyristors. Typically, half of the devices are diodes, and the other half are SCRs.

For example:

• In a single-phase half-controlled bridge rectifier, two SCRs and two diodes are used.

This arrangement gives partial control over the output voltage because the diodes conduct automatically during certain parts of the cycle, while the SCRs conduct only when triggered.

Circuit Diagram (Insert Photo Here)

Working Principle

• During the positive half cycle, one diode and one SCR conduct, depending on the firing angle.
• During the negative half cycle, the other diode and SCR conduct.
• Since diodes conduct automatically when forward-biased, the control is not complete.

Waveform 

Advantages of Half-Control Rectifier

• Requires fewer thyristors (cheaper and simpler).
• Easy to design and operate.
• Suitable for applications like small motor control and battery charging.

Limitations

• Output voltage control is limited.
• Cannot achieve the full range of control from 0 to the maximum DC.
• Not suitable for regenerative braking applications in DC drives.

3. Full-Control Rectifier

A fully controlled rectifier uses only thyristors (SCRs) in the circuit. All diodes are replaced by controlled devices, giving complete control over the output voltage.

For example:

In a single-phase full-controlled bridge rectifier, four SCRs are used in a bridge arrangement.

Circuit Diagram 

Working Principle

• During each half cycle, two SCRs are triggered according to the firing angle.
• The conduction period starts only when the SCRs are fired.
• By varying the firing angle, the output voltage can be controlled from full positive to even negative (in the case of inversion mode).

Waveform 

Advantages of Full-Control Rectifier

• Provides complete control over output voltage.
• Can be used for both rectification and inversion (power flow from DC to AC side).
• Suitable for industrial drives, HVDC transmission, and large battery chargers.

Limitations

• Requires more thyristors (costlier than half-control).
• Triggering circuits are more complex.
• Generates higher harmonic distortion compared to half-control.

4. Comparison Between Half-Control and Full-Control Rectifiers

Feature	Half-Control Rectifier	Full-Control Rectifier
Devices used	Mix of diodes and SCRs	Only SCRs
Output control	Partial (limited control)	Full (0 to maximum, even negative)
Cost & Complexity	Lower	Higher
Applications	Battery chargers, low-power loads	Industrial drives, HVDC systems
Inversion capability	Not possible	Possible

5. Applications

Half-Control Rectifier:

• Battery chargers
• Small DC motor speed control
• Power supplies for low/medium power

Full-Control Rectifier:

• DC motor drives with regenerative braking
• HVDC power transmission systems
• Large-scale industrial power supplies

6. Conclusion

Both half-controlled and fully controlled rectifiers play important roles in power electronics. While half-control rectifiers are simple, low-cost, and suitable for small applications, full-control rectifiers provide complete control of output voltage and are used in high-power industrial applications.

Understanding these rectifiers helps in designing efficient power electronic systems for real-world applications, from everyday battery chargers to large industrial motor drives and HVDC links.

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