Synchronous motor is a type of AC motor that is widely used in applications that demand high precision. At steady state, the rotation shaft of the AC motor synchronizes with the frequency of the supply current. This means that the rotation period is equal to an integral number of AC cycles. The AC synchronous motor features multiphase AC electromagnets on the stator, which create a magnetic field that rotates in the same time with the oscillations of the line current. The rotor is featured with electromagnets and it turns at the same rate with the stator magnetic field. This creates the second synchronized rotating magnetic field of the AC synchronous motor.
The two most popular types of AC motor are induction and synchronous motor. The only major difference between these two AC motors is that the AC synchronous motor rotates in synchronized with the line frequency. Also, it does not depend on if there is current induction or not to produce the magnetic field of the rotor. On the other hand, the induction motor heavily relies on current induction, because its rotor must rotate a little slower than the AC to induce current in the rotor.
Small AC synchronous motors are widely used in timing applications, including timers, synchronous clocks, timers in appliances, tape recorders and other applications where the motor is required to operate at an accurate speed. To suit different applications, AC synchronous motors are available from compact self-excited sizes to high horsepower industrial sizes. The smaller models are used where precise and constant speed is needed, while high horsepower industrial models offer two advantages: highly efficient ability of converting AC energy and operation at leading power factor.
The speed of the AC synchronous motor is directly proportional to the frequency of the electricity. This means that if the operator has control over the frequency of the electricity, the speed of the AC synchronous motor can be accurately controlled. With this in mind, there should be no more doubts why synchronous motors are perfect for high precision applications.
If the rotor of the AC synchronous motor is not self-starting (has no initial rotation), the situation is different. The north pole of the rotor will get attracted by the south pole of the rotor’s magnetic field and will eventually start to move in the same direction. But because of the inertia, the starting speed will be very low. The south pole will be replaced by the north pole, which will provide a repulsive force. And since the rotor will have a tendency to move backwards, it won’t be able to start.