Abstract: As the sliding mode observer of the interior permanent magnet synchronous motor (IPMSM) is easily affected by the d/q-axis inductance difference，this paper proposes an L q model of IPMSM，constructs a synthetic back EMF of IPMSM under the L q model，and designs a sliding mode state observer based on the synthetic back EMF． Meanwhile，the back EMF information is extracted through the two-stage filter method，the constant phase compensation is realized by means of the variable cutoff frequency method，and the high-speed running of system is achieved by using a phase locked loop technique． Then，corresponding control strategies are adopted to meet various requirements for actual applications when the motor starts． Moreover，a new switching strategy is presented for realizing smoothly and steady switching of system at low-to-medium speed switching． Experimental results on an IPMSM indicate that the proposed sliding-mode sensorless control algorithm，the starting control selection and the initiative switching strategy can realize the sensorless high-performance control of IPMSM within a wide speed range． Thus，the effectiveness of the proposed control algorithm is verified．

Key words: sliding mode control, sensorless control, phase locked loops, switching strategy, interior permanent magnet synchronous motor