METHOD AND APPARATUS FOR CONTROLLING ASCYNCHRONOUS MOTOR

摘要

An induction motor of the squirrel-cage type is driven from a polyphase power line through the intermediary of two cascaded thyristor arrays converting the line current into an adjustable rectified current and reconverting the latter into a stator current of variable frequency. The magnitude of the rectified current is varied via the first thyristor array, under the control of a tachometric signal from a speed sensor coupled with the rotor of the machine, to maintain the rotor speed at a selected level within a predetermined dynamic range in which the stator current varies approximately in proportion to the slip frequency under changing load. The slip frequency is adjusted by the second thyristor array under the control of an arithmetic unit which receives the tachometric signal along with a voltage feedback from the stator input and which maintains the stator voltage substantially proportional to rotor speed in order to stabilize the excitation current of the motor. The maximum value of the tachometric signal fed to the arithmetic unit is limited by a clamping circuit to allow the selection of rotor speeds above the dynamic range without further increases in power.