Difference between Synchronous Motor and Induction Motor
AC motors are divided into two types, synchronous motors and asynchronous motors which are also called induction motors. The biggest difference between synchronous motors and asynchronous motors (induction motors) is whether the speed of rotor is consistent with the speed of the rotating magnetic field in the stator. If the rotor’s rotation speed and the stator’s field speed are the same, it is called synchronous motor; if not, it is asynchronous motor. Furthermore, there are big differences specific to the performance parameters and applications between the two.
Difference in Construction
The stator windings of synchronous and induction motors are alike, and the main difference lies in the structure of the rotor. There are DC field windings in the rotor of synchronous motor, which need to be provided with the external excitation power introduced through the slip ring. However, the rotor windings of induction motor is short-circuit, which produce current by electromagnetic induction. In contrast, synchronous motors are more complex and costly.
Stator components of synchronous motor are basically the same as ones of induction motors, playing a role of receiving, outputting the electric energy and producing rotating magnetic field. There is not much difference in the form of the result. The stators of both synchronous motor and induction motor are made of magnetic stator core, conductive three-phase AC windings, the base to fixing core, the terminal cover etc.
Synchronous motor: the pole core of rotor is laminated by the steel sheets which are punched into by steel plates. The pole core is put on by excitation windings which are wound with insulated copper wires. For permanent magnet synchronous motor, the permanent magnet on the rotor is the key factor to distinguish it from other motors.
Induction motor: the rotor is composed of iron core and windings, and it is made of steel sheets laminated, and it is installed on the rotating shaft. There are two types of the rotor: squirrel-cage type and wound type. The wound-type induction motor is also equipped with a slip ring and a brush mechanism.
Difference in Working
1. Synchronous motor
The synchronous motor rotates for the interaction between the rotating magnetic field produced by the powering-on stator windings and the magnetic field the rotor generated. For PM synchronous motor, it rotates by the driving torque generated by the interaction between the rotating magnetic field of the stator and the secondary magnetic field of the rotor. As for the rotor winding, it does not induce current during the normal rotation of the motor and also does not participate in the work. It only serves to start the motor.
During the steady-state operation of synchronous motor, there exists a constant relationship between the rotor rotating speed and the grid frequency:
N = Ns = 120f / p
f - the grid frequency, p - the motor pole number, Ns - synchronous speed.
2. Induction Motor
Three-phase induction motor’s stator core is embedded with three-phase symmetrical windings. After the power on, between the stator and rotor produce a rotating magnetic field which rotates at a synchronous speed. The rotor bar is cut by the rotating magnetic field in which produce the induced current. The powering-on rotor bar is subjected to electromagnetic force in the rotating magnetic field, thus the rotor overcomes the rotation of load torque and accelerates its rotation. When the electromagnetic torque is equal to the load torque, the motor rotates at a constant speed.
Induction motor’s rotating speed (stator speed) is slower than the rotating magnetic field speed, and this difference is called "slip", expressed by the percentage of synchronous speed:
S = (Ns-N) / Ns.
S - slip, Ns - the magnetic field speed, N - the rotor speed.
Difference in Applications
Synchronous motors are mostly used in large generators, while induction motors are almost used as the motors to drive machines.
For synchronous motor, its power factor can be flexibly adjusted by excitation. However, induction motor power factor is not adjustable, so in some large factories, for the more applied induction motors, a synchronous motor can be added as a phase modifier, to adjust the power factors of the factory and grid interface. However, due to the high cost of synchronous motors and much maintenance, capacitors are now commonly used to compensate for power factor.
Synchronous motor’s operation is not easy as induction motor because the synchronous motor has the excitation winding and slip ring in need of high-level controlling the excitation. In addition, compared with the maintenance-free of induction motor, the work for maintaining the synchronous motor is large. Therefore, as a motor, the induction motor is the choice at the most time.