25:29
Lec 12 Full Wave Controlled Bridge Rectifier & its Harmonic Analysis
Gatematic by Sohail Sir
Overview
This video explains the operation and analysis of a single-phase full-wave controlled bridge rectifier. It details how four thyristors (T1-T4) are arranged in a bridge configuration to control the output voltage by adjusting the firing angle (alpha). The lecture assumes a constant load current, typical for highly inductive loads, and analyzes the output voltage, thyristor voltage drops, and source current waveforms. It also covers key performance parameters like average output voltage, circuit turn-off time, average and RMS thyristor currents, and delves into the harmonic analysis of the source current, leading to calculations of distortion factor, THD, and input power factor. The concept of bidirectional power flow and the converter's dual role as a rectifier or inverter based on the firing angle are also highlighted.
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Chapters
- A full-wave controlled bridge rectifier uses four thyristors (T1-T4) in a bridge configuration.
- The input voltage is sinusoidal (Vs = Vm sin(omega*t)).
- The analysis assumes a constant load current (I_not), implying a highly inductive load (RL or RLE).
- Thyristors are triggered at a firing angle 'alpha' to control the output voltage.
Understanding the basic structure and assumptions is crucial for grasping how the controlled rectifier operates and how its output can be manipulated.
The arrangement of four thyristors T1, T2, T3, and T4 forming a bridge with the load connected across it.
- When T1 and T2 are triggered at angle alpha, they conduct from alpha to pi + alpha.
- During this conduction period, the output voltage (V_not) follows the input supply voltage (Vs).
- Even after pi, T1 and T2 continue to conduct due to the constant load current, despite the supply voltage reversing.
- Thyristors T3 and T4 remain off (open-circuited) and experience a voltage drop equal to -Vs.
This explains the initial phase of output voltage generation and highlights the critical role of the load current characteristic in thyristor conduction.
The output voltage waveform follows the positive half of the input sine wave from alpha to pi + alpha, even when the input voltage becomes negative after pi.
- When T3 and T4 are triggered at angle pi + alpha, they conduct from pi + alpha to 2*pi + alpha.
- During this period, the output voltage (V_not) follows the reversed input supply voltage (VBA).
- Thyristors T1 and T2 turn off and experience a voltage drop equal to -V_not.
- This completes one cycle of operation, with T1/T2 and T3/T4 conducting alternately.
This section details the second half of the output voltage generation, ensuring continuous conduction and completing the full-wave rectification process.
The output voltage waveform follows the negative half of the input sine wave (but appears positive at the output terminals) from pi + alpha to 2*pi + alpha.
- The average output voltage is given by V_not_avg = (2*Vm / pi) * cos(alpha).
- The circuit turn-off time for a thyristor is (pi - alpha) / omega.
- Each thyristor conducts for a period of pi radians (half a cycle).
- If alpha < 90 degrees, power flows from source to load (rectifier mode).
- If alpha > 90 degrees, power flows from load to source (inverter mode), enabling bidirectional power flow.
These parameters quantify the rectifier's performance and reveal its ability to act as both a rectifier and an inverter, allowing for flexible power control.
When alpha is 120 degrees (greater than 90), the output voltage becomes negative, indicating power is being returned to the source.
- The source current is not sinusoidal but a rectangular waveform repeating with a period of 2*pi.
- It consists of the load current (I_not) during T1/T2 conduction and -I_not during T3/T4 conduction.
- The Fourier series of the source current contains only odd harmonics (n=1, 3, 5...).
- The fundamental component of the source current is Is1 = (4*I_not / pi) * sin(omega*t - alpha).
Harmonic analysis is crucial for understanding the impact of the rectifier on the power supply and for designing filters to mitigate unwanted harmonics.
The source current waveform is shown as a series of positive pulses of magnitude I_not from alpha to pi+alpha, and negative pulses of magnitude -I_not from pi+alpha to 2*pi+alpha.
- The Distortion Factor (G) is calculated as the ratio of the fundamental RMS source current to the total RMS source current.
- The Total Harmonic Distortion (THD) quantifies the overall harmonic content in the source current.
- The fundamental displacement factor is cos(alpha), representing the phase shift between fundamental voltage and current.
- The Input Power Factor (IPF) is the product of the Distortion Factor and the fundamental displacement factor (IPF = G * cos(alpha)).
These metrics are vital for assessing the efficiency and quality of power drawn from the source, influencing system design and compliance with power quality standards.
The input power factor is calculated as (2*sqrt(2) / pi) * cos(alpha), combining distortion and phase shift effects.
Key takeaways
- The firing angle 'alpha' is the primary control parameter for a full-wave controlled bridge rectifier, determining the output voltage and power flow direction.
- Constant load current (highly inductive load) is a key assumption that simplifies the analysis of thyristor conduction periods.
- Thyristors in a full-wave bridge rectifier conduct for half a cycle each, and the output voltage waveform is a controlled segment of the input sine wave.
- The rectifier can operate as an inverter when the firing angle exceeds 90 degrees, enabling bidirectional power transfer.
- The source current is non-sinusoidal and contains odd harmonics, necessitating harmonic analysis for power quality assessment.
- Input power factor is a composite measure of harmonic distortion and phase displacement, directly impacting system efficiency.
Key terms
Full-wave controlled bridge rectifierThyristor (T1-T4)Firing angle (alpha)Constant load currentBidirectional power flowHarmonic analysisDistortion Factor (G)Total Harmonic Distortion (THD)Input Power Factor (IPF)Fundamental displacement factor
Test your understanding
- How does the assumption of constant load current affect the conduction of thyristors in a full-wave controlled bridge rectifier?
- What is the relationship between the firing angle (alpha) and the direction of power flow in this rectifier circuit?
- Why is harmonic analysis of the source current important for a controlled rectifier?
- How is the input power factor calculated, and what are its components in this rectifier configuration?
- Explain the significance of the average output voltage formula V_not_avg = (2*Vm / pi) * cos(alpha) in controlling the rectifier's output.