Patent ID: 7161778

Claim:
A protective relay for an induction motor, having known operating parameters including known overload thermal current limits, comprising: a circuit for measuring the input current to the motor; a first program function for establishing a first thermal threshold value for a start condition of the motor; means for determining a representation of the thermal condition of the motor during the start condition thereof; a comparator for 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 condition thermal representation; a circuit for establishing a second thermal threshold value for a run condition of the motor, the second thermal threshold value being different from the first thermal threshold value, wherein the circuit for establishing a second thermal threshold value includes a selected time constant which is used to calculate a time-current curve combining the start and run time-current curves resulting in a substantially continuous time-current curve; means for determining a representation of the thermal condition of the motor during the run condition thereof; and a comparator for comparing the run condition thermal representation with said second thermal threshold value and for producing an output signal when said second thermal threshold is exceeded by said run condition thermal representation.