During the fault diagnosis of asynchronous motor based on stator current signals,the fault feature frequency component of broken bar always hides behind the fundamental component in the spectrum of the stator current. In order to solve this problem,a novel fault diagnosis method for the broken bar of speed sensorless induction motor is proposed based on the vector control. In this method,the reactive power is constructed by using the cross production of the rotor back electromotive force and the current vector,and then the corresponding reactive power spectrum is analyzed,with the characteristic frequency 2sf as the fault diagnosis criterion for the broken bar. Meanwhile,the reactive power is used to identify the speed and correct the magnetic field orientation,thus eliminating the impacts of the stator resistance and integral term of the traditional model reference adaptive system speed identification equation of stator flux. Experimental results demonstrate that the proposed method is effective in the fault diagnosis of broken bars,and that it can improve the speed identification accuracy as well as the vector control performance.