A new rotor faults diagnosis method for induction motors based on the double PQ transformation is presented. With the positive sequence fundamental voltage components and its minus Hilbert transformation as elements, the PQ transformation matrix is constructed. The active power P and reactive power Q can be obtained through the PQ transformation of the stator currents. Due to the P and Q are both constant for healthy motor, they correspond to a dot on the PQ reference coordinate. And the P and Q of the rotor broken bar motor will correspond to an ellipse due to they comprise an additional freq...