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.