The estimation of rotor time constant, or rotor resistance, in a vector-controlled induction machine is necessary to achieve high-performance torque control. A new method of estimating the rotor resistance on-line, for use in a vectorcontrolled induction machine, is presented. It uses short duration pulses added to the constant flux reference current i*dse and based on the resultant torque command current produced by a proportional-integral controller i"qse adjusts the rotor resistance estimate. This method of self-tuning the vector controller to the rotor time constant, when operating in a closed-loop speed control loop, does not produce torque pulsations when correctly tuned. In comparison to other on-line methods such as the extended Kaiman filter and the extended Luenberger observer, this method does not require voltage sensors and is computationally simpler. The rotor resistance estimation technique is illustrated through simulation and practical implementation of a vector-controlled induction machine. Induction machines, parameter estimation, vector control. © 1997 IEEE.