The synchronous electric motor needs to have its speed of rotation constant and similar to the speed of the rotating field, which we will call synchronous speed. The only way to do this is to ensure that the rotor is a fixed polarity electromagnet with constant field so that it does not follow the variations of the stator field. For this, we will use a rotor similar to the rotor of the DC motor.
Therefore, it is necessary to accelerate the rotor up to about 95% of the synchronous speed, and only then apply the direct current that will cause it to rotate the synchronous speed, only with a constant small difference angle, the so-called torque angle. If a higher load is applied than the synchronous motor is capable of triggering and the torque angle is too high, the motor is turned off.
The rotor is winded, rather than caged, and this format shortens if there are induced currents. The rotor is made in this way so that we can inject into it direct current, and magnetize it in order to create an electromagnet of fixed poles. The next step is to make it spin, which by itself does not occur because of the fast rotating spinning speed field.