For asynchronous motors, slip is a necessary condition for motor operation, that is, the rotor speed is always smaller than the speed of the rotating magnetic field. For synchronous motors, the magnetic fields of the stator and rotor always maintain the same steps, that is, the rotation speed of the motor is consistent with the speed of the magnetic field. When the steps of the two are inconsistent or different, it is out of step.

From a structural analysis, the stator structure of a synchronous motor is no different from that of an asynchronous machine. When a three-phase current is passed through, a synchronous rotating magnetic field will be generated; the rotor part of the motor also has a DC-excited sinusoidal distributed magnetic field, and this excitation magnetic field can also be generated by permanent magnets.

When the motor is running normally, the rotation speed of the rotor magnetic field is consistent with the rotation speed of the stator magnetic field, that is, the stator and rotor magnetic fields are relatively fixed in spatial position. This is the synchronization essence of the synchronous motor. Once the two are inconsistent, the It is thought that the motor is out of step.

Taking the rotation direction of the rotor as a reference, when the rotor magnetic field leads the stator magnetic field, it can be understood that the rotor magnetic field is the z conductor, that is, transformed by the n quantity under the power effect, the synchronous motor is in the generator state; on the contrary, the motor is still The direction of rotation of the rotor is used as a reference. When the rotor magnetic field lags behind the stator magnetic field, we can understand that the stator magnetic field pulls the rotor to move and the motor is in a motor state. During the operation of the motor, when the load dragged by the rotor increases, the lagging degree of the rotor magnetic field relative to the stator magnetic field will increase. The relationship between the two is represented by the circumferential angle, which is also commonly called the power angle. The size can reflect the power of the motor, that is, when the rated voltage and rated current are the same, the greater the power, the greater the corresponding power angle.

  Regardless of whether it is a motor or a generator, when the motor is no-load, the theoretical power angle is zero, that is, the two magnetic fields completely overlap. However, the actual situation is that due to some losses of the motor, there is still a gap between the two. The Gong Angle exists, it's just smaller.

When the rotor and stator magnetic fields are out of sync, the motor power angle will change. When the rotor lags behind the stator magnetic field, the stator magnetic field exerts a driving force on the rotor; when the rotor magnetic field leads the stator magnetic field, the stator magnetic field exerts resistance on the rotor, so the average torque is zero. Because the rotor does not get torque and power, it gradually stops.

When a synchronous motor is running, the stator magnetic field drives the rotor magnetic field to rotate. There is a fixed torque between the two magnetic fields, and the rotational speeds of the two must be equal. Once the speeds of the two are not equal, the synchronous torque does not exist and the motor will gradually stop. This phenomenon in which the rotor speed is out of sync with the stator magnetic field, causing the synchronous torque to disappear and the rotor to gradually stop, is called "out-of-synchronization phenomenon". When out-of-step occurs, the stator current rises rapidly, which is very unfavorable. The power supply should be cut off as soon as possible to avoid damage to the motor.