A three-phase synchronous motor is a double-excited machine, whereas the induction motor is single excited machines.

The major difference between synchronous and asynchronous motor is the speed of the rotor that is aligned with the direction of the rotating magnetic field in the stator. In case of the same rotation speed and stator field speed, it will be synchronous motor otherwise will be considered as an asynchronous motor.

The speed of the induction motor is always lower than the synchronous motor speed, and it usually relies on the load, also the speed decreases when the load increases. On the other hand, the synchronous motor can be operating at synchronous speed i-e Ns = 120f/P. It does not matter how much the loads would be because these kinds of motor do not depend on the loads.

Furthermore, there are other broad variations between the two based on the output criteria and applications.

Construction of the motor: 

• Synchronous motor: In a synchronous motor, the stator has axial slots which are consists of stator winding wound for a particular amount of poles. A salient pole rotor is usually mounted on the rotor winding, and with the help of slip rings, the rotor winding is supplied with DC source. A rotor with permanent magnet can also be applicable.
• Induction motor: In an induction motor, the stator winding is similar to a synchronous motor which is also wound for a definite number of poles. In case of induction motor, the squirrel cage rotor or a wound rotor can be used. The rotor bars are permanently short-circuited in the squirrel cage rotor with end rings. But on the other side, the windings are also permanently short-circuited in wound rotor; hence no slip rings are necessary.

Starting torque: 

• Synchronous motor: The synchronous motor is not self-starting compared to an induction motor.
• Induction motor: The induction motor has its kind of self-starting torque. It must be operating at synchronous speed in some way until it can be synchronized with AC supply. Induction motor requires auxiliary for starting the synchronous machine.

Armature winding: 

• Synchronous motor: The synchronous motor armature winding is energized by AC source and its field winding by DC source.
• Induction motor: In an induction motor, the stator winding is energized from an AC source.

Efficiency:  

• Synchronous motor: The efficiency of a synchronous motor is much more efficient than the induction motor of the same output voltage rating.

•  Induction motor: It is comparatively less efficient than the synchronous motor of the same output and voltage rating.

Cost:

• Synchronous motor: The cost of a synchronous motor is much than the cost of the same output voltage rating induction motor.
• Induction motor: An induction motor is cheaper in contrast to the synchronous motor of the same rating.

Applications: 

• Synchronous motor: The application of synchronous motor is applicable in those areas where precise and constant speed required. They are mostly found in large generators, and their low power applications include positioning of machines. In addition, these motor are also sometimes used as a servo motor and timing machine.
• Induction motor: The applications of induction motors are almost used as the motors to drive machines. The single-phase induction motors are mostly found in washing machines, blowers, Vacuum cleaners and centrifugal machines. While the use of three-phase induction motor in rolling mills, chimney in power plants, printing machines, paper and textile mills, conveyors, elevators, cranes, compressor and a lot of other applications.