Patent Document ID: 5436784
Application ID: 08105009
Patent Flag: 1

Claim One:
1. A protective relay for an induction motor, having known operating parameters, comprising: means for measuring the input current to the motor; means for establishing a first thermal threshold value for a first, start condition of the motor, based on selected operating parameters; means for determining a representation of the thermal condition of the motor during the start condition thereof, including means for determining positive and negative sequence power to the motor during the start condition, using positive and negative sequence current values and resistance-dependent values determined from selected operating parameters; means comparing the start condition thermal representation with said first thermal threshold value and for producing an output signal when said first thermal threshold value is exceeded by said start thermal representation; means for establishing a second thermal threshold value for a second, run condition of the motor, the second thermal threshold value being different than the first thermal threshold value; means for determining a representation of the thermal condition of the motor during the run condition thereof, including means for determining positive and negative sequence power to the motor during the run condition, using positive and negative sequence current values and resistance-dependent values determined from selected operating parameters; and means comparing the run condition thermal representation with said second thermal threshold value and for producing an output signal when said second thermal threshold value is exceeded by said run thermal representation.