Solid state overload relay transformer circuit

A circuit for detecting the magnitude of current in a multiphase alternating current load that is connected to a multiphase alternating current source by a plurality of conductors comprising:

• a plurality of current transformers with each of said transformers having a secondary winding individually energized by current in an individual conductor to provide an alternating polarity output signal having a magnitude indicative of the magnitude of the current in one of the phases of the load across a pair of output terminals with a first of said pair of output terminals of all of the secondary windings connected to a common junction, a first circuit including a impedance connected in a series circuit between the common junction and a second of said pair of output terminals, a second circuit including an impedance connected in a series circuit between the common junction and the second terminal, diodes on the first circuit connected so the second terminal of each secondary winding is connected through a diode to energize the first circuit with rectified common polarity current pulses that are provided by the alternating polarity output signals during intervals when the polarity of the signals causes the second terminals to have one polarity, and diodes in the second circuit connected so the second terminal of each secondary winding is connected through a diode to energize the second circuit with rectified common polarity current pulses that are provided by the alternating polarity output signals during intervals when the polarity of the signals causes the second terminal to have a polarity opposite the said one polarity.

A device for protecting an alternating current motor from abnormal motor currents comprising:

• at least one current transformer having a magnet iron core surrounding at least a portion of a conductor connecting the motor to an alternating current source and a secondary winding wound on the core to provide an alternating polarity output in response to the alternating current in the conductor, a current level sensing circuit including a first capacitor and a switching device having an input connected to the first capacitor for providing an output when the charge across the first capacitor exceeds a preadjusted level, a resettable trip circuit including a bistable element connected to be switched to a tripped state in response to a sensing circuit output, means for causing the motor circuit to be interrupted when the element is in its tripped state, and means including a second capacitor for providing energy for causing the element to switch to its reset state a predetermined time interval after the element is switched to its tripped state and circuit means connected to the secondary winding providing a charging circuit for the first capacitor and a charging circuit for the second capacitor, said charging circuits including diodes poled in their associated charging circuit so the first capacitor and the second capacitor are charged by opposite polarity rectified half wave output current pulses of the secondary winding and an impedance in the charging circuit for the first capacitor for causing the second capacitor to be fully charged before the charge on the first capacitor reaches the preadjusted level.

A device for protecting a three phase alternating current motor from abnormal motor currents comprising:

• three current transformers each having a magnet iron core surrounding at least a portion of a conductor connecting the motor to a three phase alternating current source and a secondary winding wound on the core to provide an alternating polarity output in response to the alternating current in its associated conductor, a current level sensing circuit including a first capacitor and a switching device having an input connected to the capacitor for providing an output when the charge across the first capacitor exceeds a preadjusted level, a resettable trip circuit including a bistable element connected to be switched to a tripped state in response to a sensing circuit output, means for causing the motor circuit to be interrupted when the element is in its tripped state, and means including a second capacitor for providing energy for causing the element to switch to its reset state a predetermined time interval after the element is switched to its tripped state and circuit means connected to the secondary windings providing a charging circuit for the first capacitor and a charging circuit for the second capacitor, said charging circuits including diodes poled in their associated charging circuit so the first capacitor and the second capacitor are charged by opposite polarity half wave output current pulses of the secondary windings and an impedance in the charging circuit for the first capacitor for causing the second capacitor to be fully charged before the charge on the first capacitor reaches the preadjusted level.