PPT Slide
These change AC input voltage to a controllable DC voltage which governs the motor’s speed. They take advantage of natural (line) commutation (by way of sinusoidal zeros) so no complex commutation circuitry is needed. See text pp. 519-522 for details including performance figures of merit (i.e., input power, displacement and harmonic factors)
Section 13.10: Speed Control of DC Motors cont.