Synchronous motor needs an excitation power supply, so that the rotor always has a magnetic field, which can be driven by the rotating magnetic field provided by the stator. As long as the braking torque is reasonable, the speed of rotor can always reach the speed of rotating magnetic field in stator, that is, synchronous speed.
Asynchronous motors are relatively simple. The magnetic field of the rotor comes from the current generated by the rotating magnetic field provided by the stator winding cutting the conductor in the rotor. The greater the speed difference between the rotor and the rotating magnetic field, the greater the rotor current and the stronger the effect of the two magnetic fields. With the increase of rotating speed, the rotor current is getting smaller and smaller, but it is definitely not there. Therefore, the rotor speed must be different from the synchronous speed to maintain the rotating magnetic field to cut the rotor conductor. So as to keep the rotor rotating continuously. The difference between this speed and the synchronous speed is the slip rate. Asynchronous motor speed can never reach synchronous speed, so it is called asynchronous motor.