Power Electronics Circuits
In phase control with an AC supply and thyristors, which of the following correctly describes how the average output voltage changes when the firing angle (control angle) α is increased?
- AThe average output voltage stays constant regardless of the firing angle
- BThe larger the firing angle, the higher the average output voltage
- CThe larger the firing angle, the lower the average output voltage
Show answer and explanation
Answer: C. The larger the firing angle, the lower the average output voltage
A thyristor starts conducting when a signal is applied to its gate while it is forward biased. Phase control adjusts the part of each half cycle during which current flows by changing the angle α from the zero crossing of the supply voltage to the moment of firing. Increasing α shortens the conduction interval and lowers the average output voltage. The method has long been used in lamp dimmers and in speed control of DC motors.
- A. Changing the firing angle is precisely how the output is adjusted.
- B. The later the firing, the shorter the interval during which voltage is applied, so the average falls.
- C. Correct. For single-phase full-wave control with a resistive load, the average is proportional to (1 + cos α)/2.