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The three-phase induction motor, at constant speed, can be represented by five loop voltage equations in terms of variable inductance coefficients and instantaneous winding currents. Voltage control by symmetrical triggering of inverse parallel connected pairs of thyristors in the stator winding branches causes the motor excitation voltages to be segments of sinusoids. Current conduction angle at a given speed depends on the triggering angle. The system currents reduce to a set of five first-order differential equations with periodically varying coefficients. No general solution of these is known but possible approaches are discussed. A numerical or analog approach seems desirable.