An initial description of power electronic devices and the use of them as switches leads to their application in three-phase rectifiers and three-phase inverters. 3, 6 and 12 pulse rectifiers are examined, including full harmonic analysis of the waveforms and the calculation of power factor. Rectifier overlap and output voltage regulation are also analysed. Quasi-squarewave inverters are examined in detail and the principles of sinusoidal PWM are introduced. A section is devoted to the creation of rotating flux in AC machines; the steady-state equivalent circuit and characteristics of the induction motor, and AC variable speed drive systems. The final section covers filtering, high-power-factor rectification, power quality issues and legislation.