Microprocessor-based protective relay system

In an electric motor system having at least one phase and a main conductor connecting each phase of an electric motor to an ac power source, a protective relay system comprising:

• a plurality of current sensing means for sensing currents through the main conductors, each respective one of said current sensing means providing a separate respective phase current feedback signal representative of current through the corresponding main conductor;
• switching means coupled to each said current sensing means for controlling flow of current through said main conductors to said motor;
• and
• overload sensing means for detecting any overload condition of said motor and turning off said switching means to prevent flow of current through said main conductors if an overload condition occurs, said overload sensing means comprising:
• (a) means for averaging each respective phase current feedback signal over a predetermined time interval to provide separate respective phase current averages corresponding thereto;
• (b) means for computing an average motor current from said phase current averages;
• (c) means for calculating a thermal sum corresponding to said average motor current, said thermal sum being initially set to zero before starting said motor, said thermal sum being increased at a predetermined heating rate as long as said average motor current is greater than a predetermined overload current value, and said thermal sum being decreased at a predetermined cooling rate as long as said average motor current is less than said predetermined overload value;
• and
• (d) means for comparing said thermal sum with a thermal limit determined by a thermal model of said motor and indicating an overload condition if said thermal sum exceeds the corresponding thermal limit for a predetermined time interval.