A sensorless rotor temperature estimator for induction machines based on a current harmonic spectral estimation scheme

This paper proposes a sensorless rotor temperature estimator for small-to medium-sized mains-fed induction machines. With measurements obtained only from voltage and current sensors, the proposed estimator can capture the rotor temperature online. The rotor speed is first extracted from the stator current harmonic spectrum based on the estimated rotor slot and eccentricity harmonic frequencies. Then the inductances are estimated according to the induction machine equivalent circuit developed from the rotor flux field orientation. The stator winding resistance at ambient temperature is the only motor parameter needed as input in this stage. Once the inductances are obtained, they are fed into the rotor resistance estimation algorithm to yield an estimate of the rotor resistance. Finally, the rotor temperature is calculated from the linear relationship between the temperature and rotor resistance. The experimental results from different motors are shown to validate the proposed algorithms. The whole scheme is efficient and reliable and is therefore suitable for implementation in a motor overload protection relay to provide thermal protection against rotor overheating.