All questions of Three Phase Line Commutated Converter for Electrical Engineering (EE) Exam

The firing angle delay of 45 degrees means that the thyristors in the full converter are triggered to turn on and conduct current after 45 degrees of the input voltage waveform.

To deliver a ripple-free load current of 10 A, the full converter needs to operate in continuous current mode. In this mode, the load current is always positive and never drops to zero.

In a 3-phase full converter, there are 6 thyristors (2 for each phase). Each thyristor conducts for 180 degrees of the input voltage waveform.

With a firing angle delay of 45 degrees, the thyristors will turn on at 45 degrees after the start of each half-cycle of the input voltage waveform. This means that each thyristor conducts for 135 degrees (180 - 45) of each half-cycle.

Therefore, the average load current can be calculated as follows:

Average load current = (conduction time / total time) * peak load current

For a 3-phase full converter, the total time for each half-cycle is 180 degrees.

Conduction time for each thyristor = 135 degrees / 360 degrees (1 cycle) * 180 degrees (1 half-cycle)

Conduction time for all thyristors = 6 * 135 degrees / 360 degrees * 180 degrees

Conduction time for all thyristors = 6 * 0.375 * 180 degrees

Conduction time for all thyristors = 405 degrees

Therefore, the average load current is:

Average load current = (405 degrees / 180 degrees) * peak load current

Average load current = 2.25 * peak load current

To deliver a ripple-free load current of 10 A, the peak load current should be:

Peak load current = 10 A / 2.25

Peak load current = 4.44 A

Therefore, to deliver a ripple-free load current of 10 A with a firing angle delay of 45 degrees, the peak load current should be approximately 4.44 A.

Answer:
A 3-phase semi-converter is the converter that can operate in both 3-pulse and 6-pulse modes. Let's understand why this is the correct answer.

Explanation:

1. 3-phase half wave controller:
A 3-phase half wave controller is a single-phase controlled rectifier used for converting AC voltage to DC voltage. It can only operate in half-wave mode and cannot be used in both 3-pulse and 6-pulse modes. Therefore, option 'a' is incorrect.

2. 3-phase full converter:
A 3-phase full converter is a six-pulse converter that converts AC voltage to DC voltage. It operates by using six thyristors in a bridge configuration. It cannot operate in both 3-pulse and 6-pulse modes simultaneously. Therefore, option 'b' is incorrect.

3. 3-phase semi-converter:
A 3-phase semi-converter is a converter that can operate in both 3-pulse and 6-pulse modes. It consists of three thyristors connected in a bridge configuration. In the 3-pulse mode, only one thyristor conducts during each half-cycle of the input AC waveform, resulting in a 3-pulse output. In the 6-pulse mode, two thyristors conduct during each half-cycle, resulting in a 6-pulse output. The mode of operation can be switched by controlling the firing angles of the thyristors. Therefore, option 'c' is correct.

4. None of the mentioned:
This option is incorrect as a 3-phase semi-converter can operate in both 3-pulse and 6-pulse modes.

Therefore, the correct answer is option 'C' - 3-phase semi-converter.

Single Quadrant Converters

Single quadrant converters are a type of semi-converters that operate in a single quadrant of the input-output plane. They are capable of converting AC voltage or current into DC voltage or current in one direction only. They are also known as half-wave rectifiers.

Working Principle

The working principle of single quadrant converters is based on the fact that a diode conducts current in one direction only. During the positive half-cycle of the input AC voltage, the diode conducts and allows the flow of current through the load. During the negative half-cycle, the diode blocks the flow of current, and no current flows through the load.

Applications

Single quadrant converters are widely used in applications where unidirectional power flow is required, such as battery charging, electroplating, and DC motor control.

Advantages

- Simple and low-cost circuit design
- Easy to implement
- Low power losses

Disadvantages

- Only suitable for unidirectional power flow
- Low efficiency due to the use of a single diode
- Produces harmonic distortion in the output waveform

Conclusion

Single quadrant converters are simple and cost-effective solutions for converting AC voltage or current into DC voltage or current in one direction only. They are widely used in various applications and offer several advantages, including easy implementation and low power losses. However, they also have some limitations, such as low efficiency and harmonic distortion in the output waveform.

Range of Firing Angle for a 3-phase, 3-pulse Converter

To determine the range of firing angles for a 3-phase, 3-pulse converter feeding a resistive load, we need to consider the operation of the converter and the limitations imposed by the system.

Operation of a 3-phase, 3-pulse Converter
- A 3-phase, 3-pulse converter is a type of rectifier that converts AC power into DC power.
- It consists of three diodes connected in a bridge configuration, with each diode connected to one phase of the AC supply.
- The converter operates by turning on the diodes in a specific sequence to rectify the AC voltage into a pulsating DC voltage.
- The firing angle determines the delay between the AC voltage waveform and the turning on of the diodes.

Limitations and Constraints
- For a resistive load, the converter should operate in the continuous conduction mode to ensure a smooth DC output voltage.
- In the continuous conduction mode, the diodes should not turn off during the conduction period of the AC waveforms.
- The firing angle should be chosen such that the diodes remain conducting until the next phase is turned on.
- The maximum firing angle is limited by the requirement that the diodes should not turn off before the next phase is turned on.

Determining the Range of Firing Angle
- In a 3-pulse converter, each phase is turned on for 120 degrees of the AC cycle.
- The firing angle for each phase can be defined as the delay in turning on the diodes with respect to the AC voltage waveform.
- The minimum firing angle is 0 degrees, where the diodes turn on immediately at the start of each phase.
- The maximum firing angle is limited by the condition that the diodes should not turn off before the next phase is turned on.
- Since each phase is turned on for 120 degrees, the diodes should remain conducting for at least 120 degrees to ensure continuous conduction.
- Therefore, the maximum firing angle is 150 degrees, allowing a 30-degree safety margin to ensure continuous conduction.

Conclusion
The range of firing angle for a 3-phase, 3-pulse converter feeding a resistive load is 0 to 150 degrees. This range ensures continuous conduction and prevents the diodes from turning off before the next phase is turned on.

A) The converter is supplied with three-phase AC voltage and the thyristors are triggered in a specific sequence to convert the input AC voltage into a variable frequency AC output voltage.

Effect of Source Inductance on the Performance of a 3-Phase Controlled Converter



Introduction:


A 3-phase controlled converter is a power electronic device used to convert AC power to DC power. It consists of a rectifier circuit that converts the input AC voltage to a pulsating DC voltage, which is then filtered to obtain a smooth DC output voltage. The performance of a controlled converter is influenced by various factors, one of which is the source inductance.

Explanation:


The source inductance refers to the inductance present in the AC power supply connected to the controlled converter. It can be in the form of line inductance or the inductance of the power transformer. The presence of source inductance affects the operation of the controlled converter in several ways.

1. Reduction in Average Load Voltage:


The presence of source inductance causes a voltage drop across it due to the current flowing through it. This voltage drop reduces the available voltage at the input of the controlled converter, leading to a decrease in the average load voltage. As a result, the output voltage of the converter will be lower than expected.

2. Continuity of Load Current:


The source inductance helps in maintaining the continuity of the load current. The inductance stores energy during the periods of high input voltage and releases it during the periods of low input voltage. This energy storage and release mechanism helps in reducing the fluctuations in the load current, making it more continuous.

3. Ripples in Load Current:


The presence of source inductance in the controlled converter also helps in reducing the ripples in the load current. The inductance smooths out the variations in the input voltage and helps in maintaining a relatively constant load current. This is especially beneficial in applications where a steady and continuous load current is required.

Conclusion:


In conclusion, the presence of source inductance in a 3-phase controlled converter has a significant impact on its performance. It reduces the average load voltage, helps in maintaining the continuity of the load current, and reduces the ripples in the load current. Therefore, the correct answer to the given question is option 'B' - source inductance reduces the average load voltage.

Explanation:

To understand why the correct answer is option C, let's first define some terms and concepts.

PIV (Peak Inverse Voltage):
PIV is the maximum voltage that an SCR (Silicon Controlled Rectifier) can withstand in the reverse bias direction without breakdown. It is an important parameter to consider when designing and operating SCR-based converters.

M-3 Converter:
An M-3 converter is a three-phase converter that uses three SCRs in parallel in each phase to control the flow of current. The SCRs are connected in a manner that allows them to conduct current in alternating half-cycles of the input voltage.

3-Phase Full Converter:
A three-phase full converter is another type of converter that uses six SCRs to control the flow of current in each phase. The SCRs are connected in a bridge configuration, allowing them to conduct current in both half-cycles of the input voltage.

Now let's analyze the given statement: "The PIV experienced by each SCR in an M-3 converter is __________ times that in a 3-phase full converter having the same output voltage."

Key Points:
- M-3 converter uses three SCRs in parallel in each phase.
- 3-phase full converter uses six SCRs in a bridge configuration.
- Both converters have the same output voltage.

Explanation:
1. In an M-3 converter, each SCR in each phase carries one-third of the total current.
2. Therefore, the PIV experienced by each SCR in an M-3 converter is one-third of the total PIV.
3. In a 3-phase full converter, each SCR in each phase carries the total current.
4. Therefore, the PIV experienced by each SCR in a 3-phase full converter is the total PIV.
5. Since the output voltage is the same for both converters, the total PIV is also the same.
6. Therefore, the PIV experienced by each SCR in an M-3 converter is one-third of the PIV experienced by each SCR in a 3-phase full converter.
7. Mathematically, this can be expressed as:
PIV in M-3 converter = 1/3 * PIV in 3-phase full converter
8. Simplifying the equation gives:
PIV in M-3 converter = (1/3) * PIV in 3-phase full converter = 1/3 * 3 = 1
9. Therefore, the correct answer is option C: 2. The PIV experienced by each SCR in an M-3 converter is 2 times that in a 3-phase full converter having the same output voltage.

Conclusion:
The PIV experienced by each SCR in an M-3 converter is twice the PIV experienced by each SCR in a 3-phase full converter having the same output voltage.

Full Converter and Firing Angle

Full converter is a type of AC to DC converter that converts the entire AC input voltage into DC output voltage. It uses thyristor as a switching element to control the voltage output. The firing angle is the delay period between the triggering of the thyristor and the point at which the thyristor conducts the current.

Average Output Voltage of Full Converter

The average output voltage of a full converter with a resistive load can be calculated as follows:

Vdc = (2/π)*Vm*cos(α/2)

where Vdc is the DC output voltage, Vm is the peak value of the AC input voltage, and α is the firing angle.

For zero degree firing, α=0, and the output voltage is:

Vdc = (2/π)*Vm*cos(α/2) = (2/π)*Vm*cos(0/2) = (2/π)*Vm = 0.637*Vm

So, if the average output voltage is 365 V for zero degree firing, then the peak value of the AC input voltage is:

Vm = 365/0.637 = 573.7 V

Output Voltage for 90 Degree Firing

For a firing angle of 90 degree, α=90, and the output voltage is:

Vdc = (2/π)*Vm*cos(α/2) = (2/π)*Vm*cos(90/2) = 0

So, the output voltage for 90 degree firing is zero.

Conclusion

The average output voltage of a full converter with a resistive load can be calculated using the formula Vdc = (2/π)*Vm*cos(α/2). For zero degree firing, the output voltage is 0.637*Vm. For a firing angle of 90 degree, the output voltage is zero.