10 Motor Start-Up and Alignment

To obtain maximum torque from the PMSM motor, the angle between the rotor and stator fluxes should be 90 degrees. To maintain 90 degrees between the two fluxes, it is important to know the initial position of the rotor. Because the initial position of the motor cannot be controlled, it should be aligned to some known position. This can be achieved by exciting either the d-axis with no excitation to the q-axis, or by exciting the q-axis with no excitation to the d-axis. The later method is preferable during speed control, as the d-axis stator excitation can be continued to be maintained at ‘0.'

After initial alignment by ‘q-axis excitation’, shift the electrical angle by 90 degrees to obtain the maximum torque. Without the offset, the motor will stall as there will not be any angle difference between the stator and rotor fluxes.

The following figures show the rotor flux position at stand still, after the initial excitation to the q-axis and a 90 degree electrical offset.