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Thermal protection is one of the most important aspects of any motor control system. This paper proposes a stator winding temperature estimation method for the thermal protection of inverter-fed electric motors. By modifying the space vector pulse width modulation (SVPWM) in an open-loop motor drive, a dc voltage can be intermittently injected into the motor. The stator temperature can be estimated by measuring only the dc component of the phase current under both constant and variable load conditions. The evaluation of the resultant torque pulsation and the compensation for serial resistances are also discussed. The proposed stator temperature estimation method is validated from experimental results under variable load conditions and both healthy and impaired cooling conditions. The error in the stator temperature estimation is within 8degC under different operating conditions. The significance of this method lies in its non-intrusive nature: only current sensors are required for implementation; the normal operation of the motor is not interrupted.