The sensorless schemes are not self-starting. In order to sense the back EMF, the
motor must be first started and brought up to a certain speed where the back EMF voltage can be detected. In practice, open-loop starting the motor is accomplished by providing a rotating stator field with a certain increasing frequency profile. Once the rotor field begins to become attracted to the stator field enough to overcome friction and inertia, the rotor will turn. After the speed reaches a threshold voltage value, the back EMF can be detected, providing the position information, the system switches to synchronous commutation mode and the motor acts as a permanent magnet synchronous achine. If there is no specific

requirement for start-up, like fans application, this open loop start-up can be satisfactory. However, for some applications, i.e., automotive fuel pump, the start-up has to be finished with 200ms to build up the pressure. It is very difficult to tune the start-up using the open-loop starting algorithm. On the other hand, if the starting torque is medium or high, usually it is difficult to start the motor with the open-loop algorithm.When the motor stops, the controller doesn’t know the rotor initial position. The first step is to align the motor to a known position by exciting two phases of the motor. For instance, we can choose phase A and phase B to be excited to set the initial position.After the rotor is in the initial position, a preset exciting pattern will be sent out. If three phases are alternately excited, the motor will start to accelerate. Table 4.1 and the exciting pattern for forward /backward rotation. The motor is driven with 6-step mode, and the exciting phase just repeats the same pattern after one cycle When the rotor approaching the alignment position, it will oscillate. The output of the tachometer will tell how long the oscillation lasts.  shows current of phase A and the oscillation waveforms during the pre-positioning period. In order to reduce the oscillation, a progressive ramp-up current can be set to bring the rotor into the desired position. Applying a strong current level directly to the windings will make the rotor move more quickly and in turn this will make it oscillate more severe around the final position. The pre-positioning period has to be long enough that the oscillation stops.
Otherwise, the rotor will be at unknown position if it is still oscillating. In  the time period, from T0 to T1, is the pre-positioning period. The oscillation stops before the end of pre-positioning. The tachometer signal shows the oscillation of the rotor.