Method and apparatus for providing thermal protection for large motors based on accurate calculations of slip dependent rotor resistance

A system for determining rotor resistance values of a an induction motor from an input voltage and an input current applied to said induction motor, said system comprising:

• first means for repetitively sampling the input voltage and input current of said induction motor to obtain a plurality of voltage and current samples including at least one pair of voltage and current samples taken at start up of said induction motor;
• second means, responsive to said first means, for calculating from said voltage and current samples instantaneous total apparent resistance values including an initial apparent total resistance value obtained at said start up;
• third means, responsive to said second means, for deriving from said initial apparent total resistance value a calculated stator resistance;
• fourth means, responsive to said first second and third means, for repetitively calculating instantaneous slip values from said instantaneous apparent total resistance values and from said calculated stator resistance;
• fifth means, responsive to said second third and fourth means for calculating instantaneous rotor resistance values from said instantaneous slip values
• sixth means for calculating, from said instantaneous rotor resistance values and said current input, the quantity of heat generated and the temperature prevailing in said rotor;
• seventh means for tripping said motor when said temperature reaches a predetermined value, said seventh means including means for repetitively calculating an instantaneous temperature value;
• a microprocessor and an associated control program, said microprocessor and control program comprising said second, third, fourth, fifth and sixth means;
• and
• an interface circuit for interfacing said microprocessor to said motor.