Alternating current motor control apparatus

An apparatus connected to a source of fixed frequency alternating current (AC) for providing a motor control signal for controlling the speed of an AC motor coupled to drive an elevator cage, comprising:

• a converter connected to the source of AC having rectifier means for converting the AC into direct current (DC);
• an inverter connected through relatively positive and negative conductors to the converter, responsive to the motor control signal, for inverting said DC into AC having adjustable frequency to drive the motor and for providing regenerative power from the motor during regenerative conditions;
• AC power failure detector means, responsive to the fixed frequency AC power source for detecting a power failure and for providing a power failure signal;
• overvoltage detector means, responsive to the voltage between said positive and negative conductors, for providing an overvoltage signal upon detecting said voltage exceeding a selected magnitude;
• said converter further comprising regenerative power switching means responsive to a regenerative power control signal for controlling the flow of said regenerative power from said inverter to the fixed frequency AC source during said regenerative conditions;
• an electric resistor responsive to said regenerative power for dissipation thereof;
• and
• a control circuit for providing the motor control signal for controlling said inverter for driving the motor at a selected speed, said control circuit responsive to said power failure signal for providing said regenerative power control signal for controlling said regeneration means to prevent flow of regenerative power to the power source, said control circuit for providing said motor control signal for reducing the current magnitude of said inverter output power to a predetermined minimum limit enough to maintain a synchronous relationship between said inverter and the motor, said control circuit further comprising means responsive to said power failure signal and said overvoltage signal for connecting said electric resistor between said relatively positive and negative conductors to provide a dynamic braking of the motor in the simultaneous presence of said overvoltage signal and said power failure signal after said power failure signal has persisted for a predetermined time.