Determining reliability of an electrical machine

A computer-implemented method of determining a value of a reliability parameter of an electrical machine, the method comprising:

• estimating an operating temperature of at least a part of the electrical machine based on a measure of a current drawn or supplied by the electrical machine, wherein the operating temperature is estimated iteratively every predetermined time period;
• determining the value of the reliability parameter using the estimated operating temperature as an input to a temperature-based Arrhenius model of the electrical machine which defines the reliability parameter as a function of the estimated operating temperature;
• determining a first value of the reliability parameter using the estimated operating temperature as an input to the Arrhenius model with the electrical machine operating at a given operating power level;
• employing the first value of the reliability parameter, the given operating power level, and a power-based model of the electrical machine, which defines the reliability parameter as a function of the operating power level and a constant, to derive a value of the constant;
• and
• employing the power-based model, the derived value of the constant and a different operating power level to iteratively derive a second value of the reliability parameter corresponding to the different operating power level.