Speed control of wound rotor induction motors by AC regulator based optimum voltage control

Wound-rotor induction motors are generally used for high torque and adjustable speed conditions. At lower speeds, either power is wasted in rotor resistances or power is pumped back from the rotor circuit to grid using different standard energy recovery methods. At low torque and low speed conditions, the power wastage in the rotor circuit becomes enormously high. Furthermore, if stator voltage control method is used it is effective for fan type loads only. For constant torque loads, the speed control range thus achieved is very limited. An efficient, low cost, simple speed control scheme is presented for wound rotor induction motors. Both stator voltage control and rotor resistance control methods are combined together and an optimum condition is found for voltage control. A thyristor based AC regulator used for stator voltage control, works efficiently at this condition. Then the rotor resistance is controlled for the adjustable speed requirements. A full speed control range has achieved, i.e. about zero to the rated speed. The motor operates satisfactorily even at very low torque and low speed conditions.