3.1 Start Up Procedure

The sensorless position estimation techniques based on back-EMF require a minimum value of back-EMF to estimate the electrical position of the rotor. The open-loop start-up procedure drives the PMSM till its speed reaches the minimum value where the back-EMF values are large enough to estimate the rotor position.

In the open-loop start-up procedure, the speed loop is deactivated. The current is controlled directly in the dq reference frame by using the assumed rotor position angles for the transformations.

The complete start-up procedure can be divided into the following phases:

  • Initial Field Alignment: In this phase, the PMSM rotor is locked to a specified rotor position by applying a DC current in the stator phase windings. There are two methods by which this can be achieved: d-axis alignment and q-axis alignment. The following figure shows d-axis alignment, where the d-axis current is gradually ramped to a specified value at a fixed rotor position angle.
  • Open-loop Ramp: In this phase, the rotor position is constantly incremented based on the user-defined ramp time at a specified q-axis current to achieve the minimum required speed for the BEMF observer. The minimum ramp time and speed depend on the PMSM drive’s electrical and mechanical parameters. For a reliable start-up, proper tuning of the ramp time and reference current is needed.
  • Stabilization: In this phase, the PMSM is allowed to rotate at a constant open-loop speed. This step ensures a smooth transition to close loop mode.

The complete start-up procedure is shown in the following figure:

Figure 3-1. Open-Loop Start-Up Procedure