Abstract: For the sensorless control of hydrodynamic air bearing permanent magnet synchronous motor (PMSM), the start control strategy is presented, and the three stage method is designed in this paper. In the static state, the rotor position is unknown, but it can rotate to a definite position by pre setting. At the low speed, the speed information is difficult to obtain because the amplitude of back EMF is too little to get the zero crossing pulse. To overcome this problem, a speed open loop acceleration is proposed. A certain speed is obtained with open loop acceleration, and then the speed loop is closed via the zero crossing pulse signal of back EMF. At last, the synchronous speed control is realized. Under the constant current drive, the open loop acceleration curve of PMSM can be approximated to a linear law，and this is verified by simulation and experiment. After the open loop acceleration stage is completed, the closed loop control can be established via the obtained back EMF, and the motor can be accelerated to a given speed. The test results confirm that this strategy is feasible.

Key words: hydrodynamic air bearing PMSM, sensorless control, starting strategy, constant current drive