High speed ac motor controller

Abstract

An apparatus comprising, in combination, an AC motor, a solid state switch controlling the AC motor, the solid state switch switchable between an on state in which an electric current flows through the solid state switch and an off state in which electric current does not flow through the solid state switch, the solid state switch characterized by a turn off time such that, when the solid state switch is switched between the on state and the off state, at least a portion of the electric current flows through the solid state switch during the turn off time, a first switch circuit for switching off the solid state switch to prevent the electric current from flowing to the AC motor, a diode bridge coupled across the AC motor, a second switch circuit for controllably switching the diode bridge between an active state in which electric current may flow through the diode bridge and an inactive state in which electric current may not flow through the diode bridge, a control circuit for controlling the second switch circuit to switch the diode bridge into an active state a predetermined time period after the solid state switch is turned off, said predetermined time period longer than the turn off time of the solid state switch and shorter than an EMF degradation time, wherein back EMF of the AC motor is rectified into a DC current and injected back to the AC motor to provide high torque braking of the AC motor while avoiding a short circuit condition.