Multi-function control system for an induction motor drive

In an induction motor drive supplied with AC voltage for the motor through SCR devices connected in a bridge across an AC power supply and controlled by gate pulses derived from a gating circuit to establish said motor voltage, including:

• a plurality of control channels for deriving respective analog control signals for motor voltage control in accordance with a corresponding operative mode of the motor drive each control channel being responsive to a corresponding command for establishing such operative mode of the motor by applying a corresponding analog control signal for controlling said gating circuit to apply a motor voltage in accordance with such analog control signal;
• analog-to-digital conversion means having a single analog input;
• said analog control signals being applied to said single analog input as a common input;
• said analog-to-digital conversion means providing a digital control signal for said gating circuit to generate gate pulses upon said SCR devices in relation to the supplied AC power to provide said motor voltage;
• the combination of:
• switching means for grounding said common input to override said analog control signals and to reduce the motor voltage to zero by control of said gating circuit,
• an additional control channel responsive to a motor overcurrent exceeding a predetermined critical level for generating a tripping control signal for controlling said switching means;
• delay means being interposed between said tripping signal and said switching means to delay grounding of said common input by said switching means;
• comparator means responsive to said common input and to a predetermined low level motor voltage reference signal for controlling said gating circuit directly to effect pulse suppression thereupon.