Patent ID: 7123457

Claim:
Within a protective relay for an induction motor, a method of determining a trip point comprising the steps of: determining I 2 t values in the motor; establishing a first thermal threshold value for a start condition of the motor; determining a first thermal representation of a thermal condition of the motor during a start condition thereof; comparing said first thermal representation with the first thermal threshold value, and producing an output signal when said first thermal threshold value is exceeded by said first thermal representation; establishing a second thermal threshold value for a run condition of the motor, wherein the second thermal threshold value is lower than the first thermal threshold value and wherein the run condition of the motor is substantially cooler than the start condition of the motor; determining a second thermal representation of the thermal condition of the motor during the run condition thereof; comparing the second thermal representation with the second thermal value and for producing an output signal when said second thermal threshold value is exceeded by said second thermal representation; and calculating a transition to the first thermal threshold value, resulting in a calculated transitioning thermal threshold value, wherein the calculated transitioning thermal threshold value is proportional to the motor I 2 t value.