The electromagnetic structure of the phase-shift autotransformer (PSAT) is introduced. The operation of the PSAT in connection with a double 3-steps rectifier is discussed. The algorithm for deduction of the main design quantities of such machine is described. The circuit model of the PSAT is arrived at; the circuit analysis allows deducing the quantities from which the machine design can be performed and that are the inputs for the thermal analysis. The results of as thermal analysis on a machine designed and built by the Authors are shown.