Which transformer configuration is most suitable for a 12-pulse diode rectifier?

• Single-phase transformer with a 90-degree phase shift.

• Two single-phase transformers with no phase shift.

• Three-phase transformer with a 60-degree phase shift.

• Two three-phase transformers with a 30-degree phase shift.

Answer explanation

A 12-pulse rectifier requires a transformer configuration that provides 30-degree phase shifts. Two three-phase transformers achieve this, allowing for better harmonic cancellation and improved performance.

What are the dominant harmonics in a three-phase 6-pulse rectifier diode system?

• 3rd and 9th harmonics

• 5th and 7th harmonics

• 1st and 11th harmonics

• 2nd and 4th harmonics

Answer explanation

In a three-phase 6-pulse rectifier, the dominant harmonics are the 5th and 7th. These harmonics arise due to the non-linear nature of the diode operation, leading to significant distortion in the output waveform.

What is the best way of increasing the current output of multiple rectifier bridges?

• Connect rectifier bridges in series.

• Connect rectifier bridges in parallel.

• Use a single larger rectifier bridge.

• Increase the input voltage to the rectifiers.

Answer explanation

Connecting rectifier bridges in parallel allows for the distribution of current across multiple bridges, effectively increasing the total output current. In contrast, series connections would not increase current output.

Which of the following is a primary advantage of using a 12-pulse rectifier over a 6-pulse rectifier?

• Increased transformer losses

• Simpler transformer configuration

• Lower output voltage

• Reduced harmonic distortion

Answer explanation

The primary advantage of a 12-pulse rectifier over a 6-pulse rectifier is reduced harmonic distortion. This results in improved power quality and less interference in electrical systems, making it more efficient.

In a three-phase rectifier, what effect does connecting the output filters have on the system?

• It increases the ripple in the output voltage.

• It increases the transformer rating required.

• It reduces the output current.

• It smooths the output voltage and reduces harmonics.

Answer explanation

Connecting output filters in a three-phase rectifier smooths the output voltage by reducing ripple and harmonics, leading to a more stable DC output, which is essential for many applications.

What is the main reason for using interphase transformers in multi-bridge rectifier systems?

• To increase the output voltage

• To balance the current sharing between bridges

• To decrease the transformer size

• To reduce the number of diodes required

Answer explanation

Interphase transformers are used in multi-bridge rectifier systems primarily to balance the current sharing between the bridges. This ensures that each bridge operates efficiently and reduces the risk of overload on any single bridge.

What is the effect of significant Total Harmonic Distortion (THD) on a DC motor connected to a three phase rectifier?

• Increased torque and speed of the DC motor.
• Significant THD can cause overheating, reduced efficiency, and potential damage to the DC motor.
• Improved performance and longevity of the motor.
• Enhanced power factor and reduced energy consumption.

In a 24-pulse rectifier system, how many sets of three-phase transformer secondary windings are typically required?

• One

• Two

• Three

• Four

What type of switching semiconductor devices and control techniques are typically used to drive a three phase electromechanical system connected to a rectifier bridge?

• IGBTs or MOSFETs with PWM control techniques
• Thyristors with frequency modulation
• Bipolar junction transistors with phase control
• Diodes with linear control techniques

How can DC link voltage in an uncontrolled diode rectifier be increased from a fixed mains supply?

• Use a step-down transformer and resistors.
• Increase the frequency of the mains supply.
• Connect additional diodes in parallel without capacitors.
• Use a step-up transformer and capacitors.