Motor control circuits

A squirrel cage motor control circuit for controlling the direction of rotation of a motor having first, second and third stator windings to be fed from a three-phase supply, and having a squirrel cage rotor, said circuit comprising first, second and third saturable reactors having respective main windings to be connected, respectively, between first, second and third phases of the supply and said first, second and third stator windings;

• a fourth saturable reactor having a main winding to be connected between said second supply phase and said third stator winding;
• a fifth saturable reactor having a main winding to be connected between said third supply phase and said second stator winding;
• each said saturable reactor having respective excitation winding means, and each of said second and third saturable reactors having a magnetisation curve relating magnetising force and applied voltage which exhibits a knee point for zero current in the respective excitation winding means and for substantially the phase voltage applied across the main winding;
• the circuit further comprising means to supply direct current selectively to the excitation winding means to cause at least near saturation of the cores of said first, second and third reactors to run the motor in one direction in a high torque condition, whilst said fourth and fifth reactors are in an unexcited/high impedance state;
• and to limit excitation of said first, fourth and fifth reactors to only partial excitation for running the motor in the reverse direction in a lower torque condition, whilst said second and third reactors are in an unexcited state and a magnetising current flows therethrough greater than that required for operation at said knee point but not greater than the full load design current of said main windings of said second and third reactors.