In this video, we slowly increase the amplitude of the stator voltage at a constant frequency of the rotating field. Only after the stator field is strong enough to overcome the frictional forces, the rotor starts to move.
At this operating point, the load angle is exactly 90°. A further increase in the stator voltage increases the stator current and reduces the load angle to almost 0°.
As the rotor is turning at the desired speed, it makes no sense to increase the stator voltage and therefore the stator current any further! This leads to the question of what stator voltage is enough and what value should be used?
Unfortunately, this question is not explained in a few sentences and leads us to the functionality of field-oriented control (FOC).