Extinction Angle Control

In AC power systems, extinction angle control is a power electronics approach that improves power factor and reduces harmonic distortion. Usually, AC motor speed controllers and voltage regulators employ it. Using this technique, a thyristor in a regulated rectifier circuit has its turn-on time delayed for each half-cycle of the AC waveform. The delay is referred to as the extinction angle, and it can be adjusted to limit the power supplied to the load, improving power factor and lowering harmonic distortion.

Extinction angle control is an effective approach for controlling the output voltage of AC voltage regulators, especially in high-power applications. It allows the regulator to maintain a constant output voltage despite variations in the load while keeping power factor and harmonic distortion within acceptable limits. This approach is an important tool for power electronics engineers because it increases efficiency, improves power quality, and reduces electromagnetic interference, all of which are critical for contemporary power systems.

Symmetrical Angle Control

The use of symmetrical angle control in power systems can improve power factors while decreasing harmonics. Essentially, this approach includes regulating the timing of the power converter's switching components (such as thyristors or IGBTs) to ensure that the input current waveform matches the input voltage waveform. This method improves power factor and reduces harmonics in the system.

Adjusting the on-time and off-time of the switching devices is critical in symmetrical angle control to get the desired input current waveform. This adjustment is generally made by changing the phase angle or delay between the voltage and current waveforms, often known as firing or conduction angles. The firing angle can be precisely controlled to bend the input current waveform into a more sinusoidal shape, decreasing harmonic distortion.

Symmetrical angle control is a commonly used approach in high-power applications such as motor drives, primarily to improve system efficiency and reduce electromagnetic interference. It is a complex control system that requires sophisticated control algorithms and exact timing management for the switching devices. Nonetheless, when properly implemented, symmetrical angle control can significantly improve power factor and reduce harmonic distortion.

Constant Voltage Control

Constant voltage control is a technique for regulating the rectifier's output DC voltage to a set value, regardless of changes in input AC voltage or load circumstances. This is performed by a feedback loop that continually monitors and adjusts the output DC voltage to maintain the desired level. Constant voltage control is usually achieved by means of a voltage regulator, which compares the DC voltage output of the rectifier to a reference value that signifies the desired output voltage. The voltage regulator modifies its output to correct any deviation and bring the output DC voltage back to the intended value if there is one between the output DC voltage and the reference voltage.

There are two main categories of voltage regulators: switching and linear regulators. While switching regulators use pulse width modulation (PWM) techniques to alter the output voltage, linear regulators use a transistor to regulate it. Each type of voltage regulator has benefits and limitations, and the choice is based on individual application needs. In the linear regulator, the Zener diode provides a constant voltage at the transistor's base, necessitating a continuous flow of current. The collector current equals the total of the load current and the Zener diode current, resulting in excessive power losses via a series limiting resistor.

Figure 20: PWM voltage regulator circuit (block diagram)

A pulse-width-modulated circuit is required to remove a linear regulator's built-in inefficiencies.

The oscillator generates the triangle/trapezoid wave at the reference frequency. The reference voltage is compared to the output voltage. Both signals are sent to an error amplifier, which magnifies the disparity between the values. The resultant signal (that comes from the error amplifier) is passed to the Pulse Width Modulator. The PMW determines the length of the pulses by comparing the resultant signal amplitude to the trapezoid signal generated by the oscillator.

The following equation is used to calculate the output voltage:

$$ V_{\text{out}} = V_{\text{in}} \cdot \frac{T_{\text{on}}}{T_{\text{on}} + T_{\text{off}}}$$

Where:

• V_in is the input voltageV_in
• T_on is the time interval (in one cycle) for which the error amplifier's output signal is less than the oscillator's output signal.
• T_off denotes duration (inside a cycle) during which the signal output from the error amplifier exceeds the oscillator's signal.

The switch is closed for the duration of T_on. A magnetic field is produced by the inductor when it starts to charge. The capacitor also starts to generate its electric field and charge. Now, the inductor's magnetic field will collapse when the switch is switched off (during the time period T_off), causing current to flow through the Zener diode and into the ground. The energy stored in the inductor will power the load from ground to V_out potential. The capacitor's electric energy will be discharged, and the current will flow through the load in the same direction. With a constant load current, the output voltage will remain steady. So the full pulse was enough to deliver the appropriate output voltage value.

When the load increases, the output voltage value decreases due to higher current demand. The difference between the reference and output voltages will be higher than the error, resulting in the error voltage at the error amplifier's output. In comparison to the trapezoid signal in PWM, the overall voltage value will be decreased. As a result, the time interval T_off will also be reduced. T_on will grow. This indicates that the inductor's magnetic field and the capacitor's electric field will be sufficient to fulfill the load requirement during the new T_off period. This manner, the output voltage remains steady as the load grows. The same procedure is used when the input voltage varies.

Constant voltage control is an important feature of regulating a rectifier's output DC voltage because it ensures stability and dependability even when the input AC voltage fluctuates or the load varies. This feature is especially important in applications that demand accurate and constant DC voltage supply. Constant control of voltage can enhance energy efficiency and cut long-term operating costs by minimizing power losses and improving the power factor.

To summarize, knowing the concepts and applications of continuous voltage control is essential in designing and maintaining efficient and high-quality power electronics systems. Choosing the right kind of voltage regulator is crucial in obtaining the appropriate level of consistent voltage control. The rectifier circuit must be properly designed and controlled in order to function reliably in a range of applications.

Constant Current Control

Constant current control is another popular control approach for three-phase diode rectifiers, with the goal of maintaining a constant output DC independent of changes in the input AC voltage or load conditions.

One method for achieving constant current control is to use a feedback loop that monitors the direct current output and makes alterations to keep the desired value. This feedback loop can be implemented using a current regulator, which compares the output direct current to a reference current representing the desired output. If the output DC deviates from the reference current, the regulator changes its output to rectify the discrepancy and restore the output DC to the desired value.

Depending on the specific requirements of the application, there are several approaches for achieving constant current control. A frequently used technique is to utilize a shunt regulator, which is a linear regulator that controls the output current by varying the resistance of the shunt resistor. Another option is to employ a switching regulator, such as a buck converter or boost converter, which alters the duty cycle of a switching transistor to more effectively control the output current.

When compared to alternative control strategies, constant current control is a more advantageous approach, particularly in conditions requiring precise current regulation or involving highly fluctuating loads. Constant current control lowers the danger of overloading or overheating, improves system stability and reliability, and provides greater protection against errors like short circuits by controlling the rectifier's output current. Constant current management can also raise the system's power factor, which lowers total power losses and raises energy efficiency.

In conclusion, three-phase diode rectifiers need continuous current control as an essential control strategy, particularly for applications requiring accurate current regulation or highly fluctuating loads. It takes knowledge of the fundamentals and practical application of constant current control to build and maintain effective, high-quality power electronics systems.

Voltage-Fed PWM Control

Voltage-fed pulse width modulation (PWM) control is a common method for controlling three-phase diode rectifiers, and its operation is simplistic. The control system uses a high-frequency pulse generator to generate a PWM signal that drives the rectifier's switching components. The pulse generator generates a signal with a constant frequency and changing duty cycle, which determines when the switching devices turn on and off. The voltage delivered to the load grows in proportion to the duty cycle of the PWM signal.

Implementing voltage-fed PWM control consists of two major stages: the pulse generator and the switching devices. The pulse generator can be implemented as an analog or digital circuit using a microcontroller. The analog circuit generates a sawtooth waveform, which it compares to a reference voltage to generate the PWM signal. In contrast, the digital circuit generates the PWM signal using a microcontroller and predefined algorithms.

Voltage-fed pulse width modulation (PWM) control is commonly utilized in power electronics applications. The switching devices utilized for PWM control are critical components in this system since they regulate the voltage output. These switching devices can be implemented using a variety of technologies, such as bipolar junction transistors (BJTs), metal oxide semiconductor field-effect transistors (MOSFETs), and insulated-gate bipolar transistors (IGBTs).

MOSFETs and IGBTs are the most often employed switching devices in voltage-fed PWM control among the available technologies. One reason for this is their great efficiency, which makes them an attractive alternative for applications requiring minimal power loss. MOSFETs and IGBTs also have quick switching rates, which allow them to function at high frequencies.

Another reason for the extensive usage of these devices in voltage-fed PWM control is their ease of installation. MOSFETs and IGBTs are available in a wide range of sizes and voltage levels, making them applicable to a variety of applications.

Voltage-fed PWM control offers various advantages for controlling three-phase diode rectifiers. The high-frequency PWM signal reduces harmonic distortion in the output voltage waveform, leading to increased efficiency and lower noise. The variable duty cycle of the PWM signal governs the output voltage, allowing the control system to maintain a consistent output voltage even when the input voltage and load circumstances fluctuate.

Voltage-fed PWM control is extremely effective at regulating three-phase diode rectifiers, and its benefits, such as increased efficiency, lower harmonic distortion, and accurate output voltage regulation, make it a preferred choice for a wide range of power electronics applications. Understanding the fundamentals and application of voltage-fed PWM control is critical for creating and sustaining high-quality, efficient power electronics systems.

Current-Fed PWM Control

Current-Fed Pulse Width Modulation (PWM) Control is a common control strategy for three-phase diode rectifiers that allows for accurate control of the output current. It regulates the output current using a feedback loop, which changes the input voltage to the rectifier circuit.

In Current-Fed PWM Control, a series resonant circuit regulates the input voltage to the rectifier circuit. This resonant circuit serves as a buffer, connecting the source and the rectifier. It is tuned to the frequency of the input voltage and uses the voltage across the resonant circuit to regulate the rectifier's switching. This control technique improves the management of the input current and voltage, resulting in increased efficiency and less electromagnetic interference. Furthermore, the resonant circuit can be designed to allow for gentle switching, which reduces strain on the switching devices and increases their lifespan. Current-fed PWM control is commonly used in high-power applications like motor drives and power supplies. To provide a consistent output current, the PWM signal modulates the duty cycle of the rectifier's switching signal.

Current-Fed PWM Control generates the PWM signal by continually monitoring the output current and modifying the duty cycle of the switching signal in response to a reference current. This feedback loop ensures that the output current remains constant even when the input voltage or load varies.

Compared to alternative techniques for regulating three-phase diode rectifiers, Current-Fed PWM Control provides high efficiency, minimal harmonic distortion, and precise output current control, reducing the rectifier circuit's influence on the power system. However, this approach has several disadvantages. To control the switching of the rectifier, a resonant circuit acting as a buffer and tuned to the frequency of the input voltage is required. Implementing the resonant circuit might be difficult and expensive. Furthermore, fluctuations in the load might have an impact on the system's output current stability, thereby limiting its reliability in specific applications.

In conclusion, current-fed PWM control is an excellent control technique for three-phase diode rectifiers that provides precise output current control while being very efficient. However, its application might be complex and subject to load fluctuations. As a result, understanding the concepts and application of current-fed PWM control is essential for designing and maintaining efficient power electronics systems.

SPWM Control

Sinusoidal Pulse Width Modulation (SPWM) is a prominent technique for boosting power factors and lowering harmonic distortion in power electronics. SPWM compares the reference waveform to a high-frequency triangular waveform, and the difference between the two is utilized to create a sequence of pulses with varying widths. These pulses are then sent to switching devices like MOSFETs or IGBTs, which regulate the output voltage or current waveform. By changing the amplitude and frequency of the reference and carrier waveforms, SPWM can produce high-quality sinusoidal waveforms with low harmonic distortion and a high power factor. SPWM is very effective in applications like as motor drives and renewable energy systems because it significantly improves the efficiency and dependability of power electronics systems.