Phase Angle control uses a low frequecy switch to chop an AC sine wave. The firing angle of the switch is varied. The average voltage will be proportional the area under the sine wave. Thus, the average voltage is the integral from the firing angle to the zero crossing, the cosine of the firing angle.

Phase Angle control provides a very inexpensive method to control the average voltage of an AC source. However, the low frequency ac waveform presented to the motor will create some tourque ripple and acoustic noise. All universal motors operated from an AC source will have some torque ripple. Also, the abrupt switching characteristics of the thyristors creates a great deal of elctrical noise.

Permanent magnet DC motors require a rectified dc source. Two SCRs and two diodes are often used to control the voltage of a dc bus. This movie illustrated the principle. If a large capacitor is used the output will be a dc voltage proportional to the sine of the firing angle. This method only requries varing the firing angle by 45°.