To provide a reliable and economical method of estimating a phase winding resistance of an electrical machine which can operate over all conditions of load and a wide range of ambient temperatures.

In the electrical machine having unidirectional excitation applied to a winding 16, mean values of voltage and current can be computed from instantaneous phase current 38 and instantaneous phase voltage 37 by the use of, for example, low pass filters 39, 40 (in either the analogue or digital domain). A value of winding resistance 42 can then be calculated by dividing the mean voltage by the mean current. This avoids the cost, fragility and potential inaccuracy of conventional temperature sensors, and provides a controller with an ongoing estimate of average winding temperature.