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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Thyristor firing angle should be between
90° - 145°
90° - 180°
60° - 180°
90° - 150°
90° - 180°
In a basic thyristor-based phase-controlled rectifier (such as a single-phase half-wave circuit), the firing angle (α) is defined as the point on the AC supply voltage cycle where the gate pulse is applied to the thyristor. For full-wave and half-wave control circuits, the theoretical limit for the conduction range extends to 180°, allowing the device to control power across the entire half-cycle of the input waveform.
In a basic thyristor-based phase-controlled rectifier (such as a single-phase half-wave circuit), the firing angle (α) is defined as the point on the AC supply voltage cycle where the gate pulse is applied to the thyristor. For full-wave and half-wave control circuits, the theoretical limit for the conduction range extends to 180°, allowing the device to control power across the entire half-cycle of the input waveform.
Vavg=2πVm(1+cosα) — Average output voltage for a single-phase half-wave rectifier
α∈[0,π] — Standard range for the firing angle in radians
The thyristor acts as a controlled switch. By delaying the firing pulse from the zero-crossing point (0°) to the end of the half-cycle (180°), the output voltage can be varied. An angle of 90° results in conduction during the second half of the positive cycle, while angles approaching 180° result in near-zero average output voltage. At 180°, the device essentially remains in an off-state for the entire duration.
The firing angle α represents the delay in triggering after the natural commutation point.
A range of 90° to 180° allows for deep phase control, moving from positive average output to zero.
Thyristors require a gate current pulse to transition from forward blocking mode to forward conducting mode.
Efficient power regulation
High current and voltage handling capacity
Generation of harmonics
Poor power factor at high firing angles
Controlled rectifiers
AC voltage regulators
Static Var Compensators (SVCs)
In practice, for AC voltage controllers like a TRIAC or anti-parallel SCR pair, the firing angle range is usually 0° to 180°.
The 90° - 180° range specifically highlights the region where output voltage starts to decrease significantly, often used in specific industrial regulation tasks.
B is correct — The standard firing angle control range for thyristors in phase-controlled converters typically spans up to 180° to allow full adjustment of the output power.
Always remember that in power electronics, α=0 represents full conduction and α=180 represents zero conduction in a simple resistive circuit.