Multi-function control system for an induction motor drive

Claim 1

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.