Abstract: A method for determining electrical and mechanical performance of a polyphase electric motor system comprises sensing the instantaneous current signal and the instantaneous voltage signal as a function of time for at least one electrical phase of the motor. The sensed current signals and the sensed voltage signals are both processed to provide an instantaneous power factor of at least one electrical phase of the motor as a function of time. The resulting power factor is used to determine the tonal real and the total reactive power of the motor as a function of time. The total real power and the total reactive power signals are bandwidth limited and time-limited. The filtered and time-limited, total real power is then cross-plotted as a function of the filtered and time limited, total reactive power signal, generating a performance diagram. The mechanical performance characteristics of the motor are then extrapolated from the performance diagram.