Safety circuit for detecting asymmetry in thyristor load currents

A symmetry sensing safety circuit means controlling electrical current flow through a thyristor to an electrical device which is in parallel with a source of electrical energy, the symmetry sensing safety circuit comprising:

• current sensing means operatively associated in series with the thyristor and the electrical device sensing the electrical current flow;
• amplifier circuit means operatively associated with said current sensing means generating a signal which is indicative of the direction and proportional to the amplitude of said signal generated by said amplifier circuit means integrated over a predetermined time interval;
• comparison circuit means operatively associated with said integrating circuit means generating a signal indicating when said signal generated by said integrating circuit means is outside of a predetermined range of values;
• relay control circuit means operatively associated with said comparison circuit means generating a triggering signal whenever said comparison circuit means generates said signal indicating said signal generated by said integrating circuit means is outside of a predetermined range of values;
• electromechanical relay means operatively associated with said relay control circuit means and with the source of electrical energy switching on and off the electrical current flow to the thyristor in response to said triggering signal.

A method of detecting and acknowledging asymmetry in a load current flow, comprising the steps of:

• sensing said load current flow;
• generating a first signal which is proportional to the amplitude and direction of said sensed load current flow;
• generating a second signal which is proportional to the amplitude and direction of said first signal integrated over a predetermined interval of time;
• comparing said second signal to a predetermined range of values and generating a trigger signal whenever said second signal is outside of said range of values;
• actuating an electromechanical relay means operatively associated with said load current flow by means of said trigger signal to disable said load current flow.

A method of protecting a bidirectional triode thyristor from excessive asymmetrical flow of alternating load current therethrough, comprising the steps of:

• sensing the load current flow;
• generating a first signal indicative of the direction and amplitude of the sensed current flow;
• integrating said first signal over a predetermined interval of time corresponding to a predetermined number of current alterations so as to produce an integrated signal indicative of the asymmetry of the current flow during said predetermined interval of time;
• comparing said integrated signal to a predetermined range of values to produce a comparison signal;
• whenever said comparison signal indicates that said integrated signal is outside said predetermined range of values, producing a trigger signal;
• and,
• in response to the trigger signal, shutting off the flow of load current to the thyristor.