The specification discloses a system for preventing current overload in a multi-phase alternating current motor. Current transformers are coupled to each of the phases of the motor and voltage outputs of each of the transformers and are connected to a summing point. A differential amplifier is connected to the summing point for detecting the occurrence of a current overload on any of the phases. A digital circuit is connected to the differential amplifier for generating a logic level in dependence upon the output of the amplifier. A timing circuit including a normally conductive field effect transistor is coupled to the digital circuit. Upon the occurrence of an overload signal, the timing circuit initiates a timing cycle during which current may not be reapplied to the motor. A transistor switch normally maintains a triac in conduction and the triac operates the contactor relay to normally supply current to the motor phases. Upon the occurrence of an overload current, the transistor switch turns the triac off and terminates current flow to the phases of the motor. Light emitting diodes are provided to indicate when the timing cycle is in effect and when a current overload condition exists.

Disclosed is an exciter circuit for an electrical load such as the field winding of a direct current (DC) motor. The exciter circuit regulates the DC current applied to the load from an alternating current (AC) source in response to an isolated feedback signal developed by means of a saturating square loop current transformer. The current signal developed is compared against a signal corresponding to the desired current value and an error signal is generated which is fed to photo-optical firing circuit means which are coupled to and operative to control at least one thyristor (SCR) which, when rendered conductive, is adapted to provide a freewheeling current path for the load current during the half cycle of the AC line voltage when it is disconnected from the load by at least a half wave rectifier. The present invention includes embodiments for both single and bidirectional current flow through the load which, as noted above, preferably comprises the field winding of a DC motor.

The specification discloses a system for preventing current overload in a three phase dual speed A. C. motor in both high and low speed modes. The system includes current transformers for sensing the current level in each of the three phases applied to the motor. Circuitry is provided to detect the occurrence of a current level above each of a plurality of levels depending upon the speed of the motor. Circuitry is responsive to the detection of an overload current level in order to operate a switching transistor to terminate conduction of a triac in order to terminate the current flow through the phases. A light emitting device is connected to the switching transistor to emit light upon the occurrence of the overload condition. A light sensor senses operation of the light emitting device and initiates a timing cycle by a timing circuit during which the triac may not be rendered conductive.

A circuit for controlling the duty cycle of a two-stage electrostatic precipitator power supply. The circuit includes a switching device connected in series with the primary winding of the power supply transformer and a circuit operable for controlling the switching device. A capacitive network, adapted to monitor the current in the primary winding of the power supply transformer, is provided for operating the control circuit. Under normal operating conditions, i.e., when the current in the primary winding of the power supply transformer is within normal limits, the capacitive network operates the control circuit to allow current flow through the power supply transformer primary winding in a normal manner. However, upon sensing the increased primary current level associated with a high voltage transient generated by arcing between components of the precipitator and reflected from the secondary winding of the power supply transformer to the primary winding thereof, the capacitive network operates the control circuit for causing the switching device to inhibit current flow through the primary winding substantially until the arcing condition associated with the high voltage transient is suppressed. Following an interval after termination of the high voltage transient, the switching device is automatically caused to re-establish primary winding current flow thereby re-establishing normal operation of the electrostatic precipitator power supply.

Circuitry for rapidly switching off the power to a universal motor upon sensing of overloading of the motor. Sudden motor overloading results in rapid speed reduction that is detected by measuring the voltage drop across the motor armature. The circuitry detects this drop and removes a gating signal to a triac switch in series with the motor to remove the power to the motor within one cycle of the applied AC.