Abstract: A controller for a motor or like electrical equipment. A thyristor in series with a power conductor that feeds the equipment and a control circuit for activating the gate terminal of the thyristor in accordance with the operating conditions of the equipment. The control circuit includes a transformer having a secondary winding connected to the gate terminal and means for varying the current flow in the secondary winding according to load demands. A three phase system having a thyristor in each conductor and a transformer secondary winding for each thyristor, which windings are on a common core.