Controlled Rectifiers & PYQ Solutions

Cheatsheet Content

1. Advantages and Disadvantages of Power Electronic Rectifiers Advantages: Output Voltage Regulation: Can regulate output voltage by changing the firing angle of Thyristors (SCRs). (Page 1) Efficiency: Generally high efficiency due to low power loss in switching devices. Compact Size: Power electronic rectifiers are typically smaller and lighter than traditional rectifiers for similar power levels. Fast Response: Quick control over output parameters allows for dynamic load adjustments. Disadvantages: Harmonic Distortion: Introduction of harmonics into both input current and output voltage waveforms. Complex Control: Requires sophisticated control circuits for precise firing angle adjustment. Cost: Can be more expensive than uncontrolled diode rectifiers. Switching Losses: Power losses occur during the switching transitions of Thyristors. 1.b) Applications of Power Electronic Rectifiers Speed control of DC motors: Used in paper mills, textile mills, and steel mills. (Page 3) AC fed traction systems: Utilizing DC traction motors. (Page 3) Electro-metallurgical and Electrochemical processes. (Page 3) Reactor controls. (Page 3) Magnet power supplies. (Page 3) Portable hand instrument drives. (Page 3) Flexible speed industrial drives. (Page 4) Battery chargers. (Page 4) High voltage DC transmission. (Page 4) UPS (Uninterruptible power supply systems). (Page 4) 1.c) Single Phase Uncontrolled Rectifiers with Resistive Load Uncontrolled rectifiers use diodes, providing no control over the output voltage. They are simpler and less expensive. Half-Wave Uncontrolled Rectifier (with R load) A single diode conducts for the positive half-cycle of the AC input. The output voltage is a pulsating DC with a significant ripple. The diode blocks current during the negative half-cycle. Full-Wave Uncontrolled Rectifier (with R load) Can be implemented using a center-tapped transformer with two diodes or a bridge rectifier with four diodes. Both conduct during both positive and negative half-cycles of the input, resulting in a higher average DC output voltage and reduced ripple compared to half-wave rectifiers. 2.a) Advantages and Disadvantages of Controlled Rectifiers Advantages: Output Voltage Control: The primary advantage is the ability to regulate the output DC voltage by varying the firing angle of the Thyristors. (Page 1) Power Control: Allows for precise control of power delivered to the load. Improved Efficiency: When operated efficiently, they can achieve high power conversion efficiency. Disadvantages: Harmonic Generation: Introduce significant harmonics into the AC supply and DC load, requiring filters. Lower Power Factor: The phase control leads to a lagging input power factor. Complexity: More complex control circuitry compared to uncontrolled rectifiers. Commutation Issues: Can suffer from commutation failures, especially with highly inductive loads or in inverter mode. 2.b) Operation of Bridge Type Single Phase Rectifier with R-L Load A single-phase full-wave controlled rectifier with an R-L load consists of four Thyristors. The operation depends on the firing angle ($\alpha$) and the inductance of the load. Working Principle: During the positive half-cycle of the AC input, Thyristors $T_1$ and $T_2$ are forward biased. When triggered at a firing angle $\alpha$, they conduct. The load voltage follows the input voltage. Due to the inductance $L$, the current builds up slowly and continues to flow even after the input voltage reverses, until the energy stored in $L$ is dissipated or transferred. During the negative half-cycle, Thyristors $T_3$ and $T_4$ are forward biased. When triggered at $\alpha$ (relative to the negative half-cycle zero crossing), they conduct. The output voltage is pulsating DC. The inductor smooths the current, making it more continuous, especially for larger inductance values. The output voltage and current waveforms are affected by the firing angle and the R-L characteristics, leading to continuous or discontinuous conduction modes. (Page 11-12) 3.a) Why Filters are Required in Conjunction with Rectifiers? Filters are required with rectifiers to smooth out the pulsating DC output voltage and current, reducing ripple and providing a more stable DC supply for the load. Rectifiers convert AC to pulsating DC, which is unsuitable for most electronic devices that require a steady DC voltage. Comparison of Filter Types: Capacitive Filter: Principle: A capacitor is connected in parallel with the load. It charges during the peaks of the rectifier output and discharges through the load when the rectifier output drops. Advantages: Simple, inexpensive, effective for light loads. Disadvantages: Poor voltage regulation for varying loads, high peak diode current, not suitable for heavy loads. Inductive Filter: Principle: An inductor (choke) is connected in series with the load. It opposes changes in current, smoothing the output current. Advantages: Better voltage regulation than capacitive filters for varying loads, reduces peak rectifier current. Disadvantages: Bulky, expensive, less effective for light loads, not as good at smoothing voltage as LC filters. Capacitive-Inductive (LC) Filter: Principle: A combination of an inductor in series and a capacitor in parallel with the load. The inductor smooths current, and the capacitor smooths voltage. Advantages: Excellent ripple reduction, good voltage regulation, suitable for a wide range of loads. Disadvantages: More complex, bulky, and expensive than single-element filters. 3.b) Operation of Single Phase Full Converter with RL Load A single-phase full converter (fully controlled bridge rectifier) with an RLE load (Resistive, Inductive, and back EMF load) uses four Thyristors. Its operation depends on the firing angle ($\alpha$), load parameters, and the presence of a back EMF (E). Working Principle: During the positive half-cycle of the input AC, $T_1$ and $T_2$ are forward biased. When triggered at $\alpha$, they conduct, and the input voltage is applied to the load. The inductance $L$ stores energy, causing the current to flow even when the input voltage becomes less than the load EMF or reverses polarity, until the current through $T_1, T_2$ falls to zero or opposite Thyristors ($T_3, T_4$) are triggered. During the negative half-cycle, $T_3$ and $T_4$ are forward biased. When triggered at $\alpha$ (relative to the negative half-cycle zero crossing), they conduct. The output voltage and current waveforms depend heavily on the firing angle and the RLE load characteristics. Continuous or discontinuous conduction modes are possible. (Page 11-12) The presence of back EMF $E$ means the Thyristors will only turn on if the instantaneous input voltage exceeds $E$. The converter can operate in both rectifier (power flow from AC to DC) and inverter (power flow from DC to AC) modes, depending on the firing angle and the magnitude of $E$. 4. Operation of Three Phase Half-Wave Controlled Rectifier with R and RL Loads A three-phase half-wave controlled rectifier uses three Thyristors ($T_1, T_2, T_3$) and a neutral connection. It is supplied by a three-phase AC source. Working Principle (R Load): Each Thyristor is connected to one phase. A Thyristor conducts when it is forward biased and a gate pulse is applied. The Thyristors are triggered sequentially at a firing angle $\alpha$ with respect to their respective phase voltages' positive zero crossings. For a resistive load, each Thyristor conducts for $120^\circ$ (or $120^\circ - \alpha$ if $\alpha > 0$). The output voltage is the phase voltage of the conducting Thyristor. The output is pulsating DC, and its average value can be controlled by varying $\alpha$. (Page 22-26) Working Principle (RL Load): With an inductive load, the current through a Thyristor may continue to flow even after the phase voltage becomes less than the next phase voltage, or even negative, due to the energy stored in the inductor. This can lead to continuous conduction, where the output current is relatively smooth. The output voltage waveform is smoothed by the inductor, and the average output voltage is still controlled by $\alpha$. The inductance helps to reduce ripple in the output current. (Page 22-26) 5.a) Effect on Input Power Factor after Rectification of AC Voltage When AC voltage is rectified using controlled rectifiers (Thyristors), the input power factor generally decreases (lags). This is due to: Phase Displacement: The firing delay ($\alpha$) introduced by the Thyristors causes the input current to lag the input voltage, even for a resistive load. The current starts flowing only after the firing angle, creating a phase shift. Harmonic Content: The non-sinusoidal nature of the input current (due to switching action) introduces harmonics. These harmonics contribute to distortion power factor, which further reduces the overall power factor. The power factor for rectifiers is often expressed as $PF = \frac{True \, Power}{Apparent \, Power} = \frac{V_{rms} I_{rms} \cos\phi}{V_{rms} I_{rms}} = \frac{P}{S}$. However, for non-sinusoidal current, it's more accurately $PF = \frac{P}{V_{rms} I_{rms}} = (DF) \times (TD_{PF})$, where $DF$ is the displacement factor and $TD_{PF}$ is the total harmonic distortion power factor. The firing angle directly impacts the displacement factor. (Page 15) 5.b) Applications of Dual Converters Dual converters are capable of providing four-quadrant operation (forward motoring, forward braking, reverse motoring, reverse braking) and are used where reversible DC output voltage and current are required. (Page 37-38) Direction and Speed control of DC motors: Allows for precise control of motor speed and direction. (Page 40) Applicable wherever reversible DC is required. (Page 40) Industrial variable speed DC drives: For applications requiring high performance and dynamic response. (Page 40) Regenerative Braking: Can return energy from the load back to the AC source. 5.c) Performance Specifications of Power Rectifiers Key performance metrics for power rectifiers include: (Page 15-16) Average Output Voltage ($V_{avg}$): The DC component of the output voltage. RMS Output Voltage ($V_{rms}$): The effective voltage of the output waveform. Average Output Current ($I_{avg}$): The DC component of the output current. RMS Output Current ($I_{rms}$): The effective current of the output waveform. Ripple Factor ($R_f$): A measure of the AC content in the DC output, indicating how smooth the DC output is. Defined as $R_f = \frac{V_{ac(rms)}}{V_{dc}}$. (Page 16) Form Factor ($FF$): Ratio of RMS value to average value of the output voltage. $FF = \frac{V_{rms}}{V_{avg}}$. (Page 16) Transformer Utilization Factor (TUF): Measures how effectively the transformer is used. $TUF = \frac{P_{dc}}{VA_{rating}}$. (Page 16) Rectification Efficiency ($\eta$): Ratio of DC output power to AC input power. $\eta = \frac{P_{dc}}{P_{ac}}$. (Page 16) Peak Inverse Voltage (PIV): The maximum reverse voltage that a diode or Thyristor must withstand. (Page 16) Input Power Factor (PF): Measures the efficiency of AC power utilization from the source. Total Harmonic Distortion (THD): Measures the distortion of the input current or output voltage waveform.