Final answer:
The thyristor in a single-phase motor speed control circuit adjusts the motor's speed by changing the phase angle of the AC sine wave. This technique, known as phase control, alters the effective voltage to the motor, influencing its speed.
Step-by-step explanation:
In a single-phase motor speed control, the thyristor controls power in the circuit by changing the phase angle of the AC sine wave that drives the motor. To be more specific, the technique is known as phase control or phase angle control. It involves delaying the point at which the thyristor starts to conduct in each cycle, effectively adjusting the voltage that is applied to the motor.
Since the relationship between voltage and speed is direct for many types of single-phase motors, changing the voltage changes the motor speed. The phase control method can be visualized if you consider how a generator works: as shown in Figure 23.21 of the reference material, a sinusoidal emf is induced by rotating a coil in a magnetic field. The same principle applies in reverse for a motor, where the sinusoidal voltage induces rotation.