All questions of Principles of Thyristor Choppers and Inverters for UPSC CSE Exam

Introduction:
Inverters are electronic devices that convert DC (Direct Current) power into AC (Alternating Current) power. They are widely used in various applications like power supplies, motor drives, renewable energy systems, etc. Inverters can be designed using different types of electronic components, including BJT (Bipolar Junction Transistor).

Active Region vs Saturation Region:
BJTs operate in two main regions - active region and saturation region. In the active region, the BJT acts as an amplifier, and the output voltage is linearly related to the input voltage. In the saturation region, the BJT acts as a switch, and the output voltage is either fully ON (saturated) or fully OFF (cutoff).

Reasons to Prefer Saturation Region:
In the context of inverters designed from BJTs, the saturation region is preferred over the active region due to the following reasons:

1. High Efficiency: In the saturation region, the BJT operates in a fully ON state, which means it has a low resistance. This leads to lower power dissipation and higher efficiency of the inverter. In contrast, in the active region, the BJT operates as an amplifier, which introduces losses and reduces overall efficiency.

2. High Power Factor: Power factor is an important parameter in AC power systems, and it measures the phase relationship between the voltage and current waveforms. In the saturation region, the BJT acts as a switch, and the output waveform closely follows the input waveform, resulting in a power factor close to unity (1). A high power factor is desirable as it reduces reactive power losses and improves the overall system efficiency. On the other hand, in the active region, the BJT operates as an amplifier, which can introduce phase shifts and distortion in the output waveform, leading to a lower power factor.

Conclusion:
Inverters designed from BJTs are preferably used in the saturation region rather than the active region due to their high efficiency and high power factor. The saturation region allows the BJT to operate as a switch, resulting in lower power dissipation, improved efficiency, and a power factor close to unity.

Explanation:
DC choppers are electronic devices that are used to convert a fixed DC voltage into a variable DC voltage. The waveform for input and output voltages in DC choppers is as follows:

Input Voltage Waveform:
The input voltage waveform is continuous in DC choppers. It means that the voltage is present at the input of the chopper throughout the operation.

Output Voltage Waveform:
The output voltage waveform is discontinuous in DC choppers. It means that the output voltage is present only for a certain period of time during the operation.

Reason:
The reason for the continuous input voltage waveform is that the input voltage is connected to a DC source that provides a constant voltage. On the other hand, the output voltage waveform is discontinuous because the chopper switches ON and OFF at a certain frequency. During the ON state, the output voltage is present, and during the OFF state, the output voltage is not present.

Conclusion:
Hence, we can conclude that the waveforms for input and output voltages in DC choppers are respectively continuous and discontinuous.

Is controlled by adjusting the modulation index of the switching signals. The modulation index represents the ratio of the amplitude of the modulating signal to the amplitude of the carrier signal. By varying the modulation index, the output voltage of the inverter can be adjusted.

CSIs use a high-frequency carrier signal to switch the power devices (such as transistors or thyristors) in the inverter. The carrier signal is usually a sinusoidal waveform with a fixed frequency. By modulating this carrier signal with a low-frequency sinusoidal waveform, the output voltage of the inverter can be controlled.

When the modulation index is increased, the amplitude of the modulating signal is increased relative to the carrier signal. This leads to a higher peak value of the output voltage waveform. Conversely, when the modulation index is decreased, the output voltage waveform has a lower peak value.

The modulation index also affects the harmonic content of the output voltage waveform. Higher modulation indices result in more harmonics, while lower modulation indices result in fewer harmonics. This is because the modulation process introduces additional frequency components to the output waveform.

Overall, the modulation index in CSIs plays a crucial role in controlling the output voltage magnitude and harmonic content. It allows for precise control of the output voltage, making CSIs widely used in applications such as motor drives, renewable energy systems, and power grid interfaces.

In current source inverters, an L filter is used before the CSI (input side).

Introduction
Current Source Inverters (CSIs) are power electronic devices used to convert DC power into AC power. They are commonly used in applications such as renewable energy systems, motor drives, and uninterruptible power supplies. To improve the performance and efficiency of CSIs, filters are often employed to reduce harmonics and provide a smooth output waveform.

Purpose of Filters
Filters are used in CSIs to eliminate or reduce the harmonics generated by the switching action of the power electronic devices. Harmonics are unwanted frequencies that can distort the voltage and current waveforms, causing problems such as overheating, increased losses, and interference with other sensitive equipment. Filters help in achieving a cleaner and more sinusoidal output waveform.

Location of L Filter
In current source inverters, an L filter is used before the CSI on the input side. This means that the L filter is connected between the DC source and the CSI. The purpose of the L filter is to smooth out the current waveform supplied to the CSI, reducing the harmonics and providing a more sinusoidal current input.

Working of L Filter
The L filter consists of an inductor (L) and a capacitor (C). The inductor acts as a current source, while the capacitor acts as a voltage source. When the current flows through the inductor, it smooths out the current waveform by storing energy during the periods of high current and releasing it during the periods of low current. This helps in reducing the harmonics and providing a more sinusoidal current input to the CSI.

Advantages of Using L Filter
1. Reduced Harmonics: The L filter helps in reducing the harmonics generated by the CSI, resulting in a cleaner and more sinusoidal current waveform.
2. Improved Power Quality: By reducing the harmonics, the L filter improves the power quality of the system, preventing issues such as voltage distortion and interference with other equipment.
3. Increased Efficiency: The smoother current waveform provided by the L filter results in reduced losses and improved efficiency of the CSI.

Conclusion
In current source inverters, an L filter is used before the CSI on the input side. This filter helps in reducing the harmonics and providing a more sinusoidal current input to the CSI, resulting in improved performance and efficiency of the system.

Ms, find the duty cycle of the chopper.

The duty cycle of a chopper is defined as the ratio of the ON time of the thyristor to the total time period of one complete cycle. It is given by the formula:

Duty Cycle = (ON time of thyristor) / (Total time period)

In this case, the total time period is the sum of the ON time and the blocking time (t), which is given as 80 ms.

Total time period = ON time + blocking time = ON time + 80 ms

To find the ON time of the thyristor, we need to use the relationship between the load voltage and the input voltage in a step-up chopper, which is given by the formula:

Load voltage / Input voltage = (ON time) / (ON time + blocking time)

In this case, the load voltage is 500 V and the input voltage is 220 V.

500 V / 220 V = (ON time) / (ON time + 80 ms)

Now, we can solve for the ON time:

ON time = (500 V / 220 V) * (ON time + 80 ms)

Simplifying this equation, we get:

ON time = (500 / 220) * (ON time + 80 ms)

ON time = (500 / 220) * ON time + (500 / 220) * 80 ms

ON time - (500 / 220) * ON time = (500 / 220) * 80 ms

ON time * (1 - 500 / 220) = (500 / 220) * 80 ms

ON time * (220 - 500) / 220 = (500 / 220) * 80 ms

ON time * (-280) / 220 = (500 / 220) * 80 ms

ON time = ((500 / 220) * 80 ms) * (220 / -280)

ON time ≈ -0.1273 * 80 ms

ON time ≈ -10.18 ms

Since the ON time cannot be negative, we take the absolute value:

ON time ≈ 10.18 ms

Now, we can calculate the duty cycle:

Duty Cycle = ON time / (ON time + blocking time)

Duty Cycle = 10.18 ms / (10.18 ms + 80 ms)

Duty Cycle ≈ 10.18 ms / 90.18 ms

Duty Cycle ≈ 0.113 or 11.3%

Therefore, the duty cycle of the chopper is approximately 11.3%.

B)2Vs/π

° electrical angle
b)120° electrical angle
c)180° electrical angle
d)360° electrical angle

Answer: b) 120° electrical angle

Explanation:

The correct answer is option 'B': Both A and R are true but R is not the correct explanation of A.

Assertion (A): The terminal voltage of a voltage source inverter remains substantially constant with variations in load.
Reason (R): Any short-circuit across the terminals of a voltage source inverter causes current to rise very fast.

Explanation:

Terminal Voltage of a Voltage Source Inverter:
- A voltage source inverter (VSI) is an electronic device that converts a DC voltage source into an AC voltage source.
- The terminal voltage of a VSI refers to the voltage across the output terminals of the inverter.
- Ideally, the terminal voltage of a VSI should remain constant with variations in load.

Reason Explanation:
- The reason states that any short-circuit across the terminals of a VSI causes current to rise very fast.
- This statement is true because in a short-circuit condition, the impedance across the terminals becomes very low, resulting in a high current flow.
- However, this reason does not directly explain why the terminal voltage of a VSI remains constant with load variations.

Explanation of Assertion:
- The assertion states that the terminal voltage of a VSI remains substantially constant with variations in load.
- This assertion is true because VSIs are designed to regulate the output voltage regardless of the load variations.
- VSIs achieve this regulation by using control techniques such as pulse width modulation (PWM).
- PWM adjusts the width of the output pulses based on the load requirements, ensuring that the average output voltage remains constant.

Conclusion:
- Both the assertion and reason are true.
- However, the reason does not provide a correct explanation for the assertion.
- The terminal voltage of a VSI remains constant with variations in load due to the control techniques used, not solely because of the potential of a short-circuit.

Single Phase Half Bridge Inverters and Their Requirements

Introduction:
Single-phase half bridge inverters are a type of power electronic device used to convert DC (direct current) power into AC (alternating current) power. They are commonly used in applications such as motor drives, renewable energy systems, and uninterruptible power supplies (UPS). To operate efficiently and effectively, these inverters require specific types of power supplies.

Explanation:

1. Two-Wire AC Supply:
- A two-wire AC supply refers to a single-phase AC source with two conductors: one live (active) wire and one neutral wire.
- While it is possible to operate a single-phase half bridge inverter with a two-wire AC supply, it is not the most common or efficient configuration.
- In this setup, the inverter requires an additional circuitry to create a virtual neutral point, which can increase complexity and cost.

2. Two-Wire DC Supply:
- A two-wire DC supply refers to a DC power source with two terminals: positive and negative.
- Single-phase half bridge inverters cannot be directly connected to a two-wire DC supply since they require a bipolar voltage source.
- Bipolar voltage sources provide both positive and negative voltage levels required for the inverter operation.

3. Three-Wire AC Supply:
- A three-wire AC supply refers to a single-phase AC source with three conductors: one live (active) wire, one neutral wire, and one ground wire.
- This is the most common and efficient configuration for single-phase half bridge inverters.
- The inverter can be directly connected to the three-wire AC supply without the need for additional circuitry.
- The live wire provides the necessary AC voltage, the neutral wire completes the circuit, and the ground wire ensures safety.

4. Three-Wire DC Supply:
- A three-wire DC supply is not a common configuration in power electronics.
- While it is possible to create a three-wire DC supply using additional circuitry, it is not a standard requirement for single-phase half bridge inverters.
- Typically, the DC supply used for these inverters is a two-wire bipolar voltage source.

Conclusion:
The correct answer is option D, which states that single-phase half bridge inverters require a three-wire DC supply. This is because a three-wire DC supply is not a standard requirement for these inverters. Instead, they are commonly connected to a three-wire AC supply, which provides the necessary voltage and circuit completion.

In the single-pulse width modulation method, the Fourier coefficient bn is given by:

bn = (2Vs/T) * ∫[t_on, t_off] sin(nωt)dt

where bn is the nth Fourier coefficient, Vs is the amplitude of the sine wave that is being modulated, T is the period of the modulation signal, t_on is the starting time of the pulse, t_off is the ending time of the pulse, ω is the angular frequency (2πf) of the sine wave, and n is the order of the Fourier coefficient.

To find the inductor value (L) for the boost converter, we can use the following equation:

L = (Vo * (Vin - Vo)) / (Vin * F * Io)

where:
Vo = average output voltage = 18 V
Vin = input voltage = 6 V
F = switching frequency = 20 kHz
Io = average load current = 0.4 A

Plugging in the given values:

L = (18 * (6 - 18)) / (6 * 20,000 * 0.4)
L = (-180) / (4,800)
L = -0.0375 H

However, it is not physically possible to have a negative inductance value. Therefore, there may be an error in the given values or calculation. Please double-check the values and provide the correct information for a valid calculation.

Introduction:
A three-phase bridge inverter is a type of power electronic device used to convert DC power into AC power. It is widely used in various applications such as motor drives, renewable energy systems, and grid-tied inverters. The bridge inverter consists of switching devices that are used to control the flow of current in the inverter circuit.

Explanation:
To understand why a minimum of six switching devices are required for a three-phase bridge inverter, let's first take a look at the basic structure of the inverter.

Basic Structure:
A three-phase bridge inverter consists of three legs, with each leg consisting of two switching devices. Each leg is connected to one phase of the three-phase AC output. The switching devices in each leg are typically semiconductor devices such as power transistors or insulated gate bipolar transistors (IGBTs). The switching devices are controlled by a pulse width modulation (PWM) technique to generate the desired AC output waveform.

Working Principle:
During the positive half-cycle of the AC output waveform, the switching devices in one leg are turned on, while the switching devices in the other two legs are turned off. This allows the current to flow through the load in the intended direction. During the negative half-cycle, the switching devices in the other two legs are turned on, and the switching devices in the first leg are turned off. This reverses the direction of current flow through the load. By controlling the switching devices in each leg, the desired AC output waveform can be generated.

Switching Devices:
In a three-phase bridge inverter, each leg requires two switching devices to control the flow of current. Since there are three legs in total, we need a minimum of six switching devices. These switching devices can be arranged in different configurations, such as a half-bridge or a full-bridge configuration, depending on the specific requirements of the application.

Conclusion:
In conclusion, a three-phase bridge inverter requires a minimum of six switching devices. These switching devices are used to control the flow of current in each leg of the inverter circuit. By properly controlling the switching devices, the desired AC output waveform can be generated.

The Amplitude of the Output Voltage in Voltage Source Inverters (VSIs)

Introduction:
Voltage source inverters (VSIs) are electronic devices used in power electronic systems to convert DC (direct current) power into AC (alternating current) power. They are commonly used in applications such as motor drives, renewable energy systems, and power grid interfaces. One of the key characteristics of VSIs is the amplitude of the output voltage.

Explanation:
The amplitude of the output voltage in voltage source inverters (VSIs) is independent of the load. This means that it remains constant regardless of the type or magnitude of the load connected to the inverter. This is a desirable feature in many applications where a stable AC voltage is required.

Reasons:
There are a few reasons why the amplitude of the output voltage in VSIs is independent of the load:

- PWM Technique: VSIs utilize Pulse Width Modulation (PWM) techniques to control the output voltage. PWM involves switching the DC input voltage on and off at a high frequency to create an AC waveform. The amplitude of the output voltage is determined by the duty cycle of the PWM signal, which remains constant regardless of the load.

- Feedback Control: VSIs often incorporate feedback control mechanisms to regulate the output voltage. These control systems continuously monitor the output voltage and adjust the PWM signal to maintain a constant amplitude. This feedback control compensates for any variations in the load and ensures a stable output voltage.

- Constant Voltage Source: The name "voltage source inverter" itself implies that the inverter is designed to act as a constant voltage source. This means that it strives to maintain a fixed output voltage regardless of the load conditions. The internal circuitry of the inverter is designed to achieve this constant voltage output.

Advantages:
The independence of the output voltage amplitude from the load in VSIs offers several advantages:

- Stable Operation: Regardless of the connected load, the VSI provides a stable and consistent output voltage, which is essential for many applications.

- Compatibility: The load independence allows VSIs to be used with a wide range of electrical loads without the need for additional adjustments or modifications.

- Flexibility: The load independence simplifies the design and operation of power electronic systems that incorporate VSIs, as the focus can be placed on other aspects such as efficiency and control rather than worrying about load variations.

Conclusion:
In summary, the amplitude of the output voltage in voltage source inverters (VSIs) is independent of the load. This characteristic is achieved through the use of PWM techniques, feedback control, and the inherent design of VSIs as constant voltage sources. Understanding this behavior is crucial for the successful implementation of VSIs in various power electronic applications.