Both synchronous and induction machines work on AC supply, ie; they are AC machines. For AC machines, the basic requirement is to create a rotational magnetic field so that the rotor can rotate, and energy can be obtained. (mechanical for motor, electrical for alternator) Now, the difference between a synchronous machine and an induction machine is based on how the rotational magnetic field is utilized.
The three phase supply, when given to the stator windings, creates a rotating magnetic field. Now, the rotor of an induction machine is a group of short circuited windings where the rotating magnetic field is to act. The underlying principle is electromagnetic induction, similar to transformers – the rotor gets an induced current, that means – a current carrying loop inside the magnetic field. This makes the rotor rotate. But, if both rotating magnetic field and rotor rotate in the same synchronous speed, there will be no induced EMF inside the rotor, because the field will be then constant w.r.t the rotor, and for rotation, change in flux should be there.
Thus, the rotation happens with a “slip” between the rotor and the stator. The slip refers to the inability of the rotor to catch up with the (speed of) rotating stator magnetic field. Thus rotor will rotate, and output is obtained, if there is slip between the rotor and stator magnetic field. This is the principle of working of an induction machine.
Now, consider am utilizing the rotating magnetic field by making an electromagnet rotate as a rotor, which is fed with a DC supply. In such a case, the machine will run in synchronous speed, and this machine is called a synchronous machine. However, it is to be noted that due to inertia of the rotor, the rotor electromagnet cannot immediately respond to the rotating magnetic field. This means that the synchronous machine does not start by itself. To provide the starting torque, additional winding called “damper winding” is provided in rotor, which make the synchronous machine similar to an induction machine during starting. As the machine starts, the rotor naturally locks up with the rotating magnetic field and rotates in synchronous speed. This means that during normal running conditions, the damper winding will carry no current. (Similar to the rotor winding in an induction machine at synchronous speed)
Here are the few Differences between Synchronous and Induction Motor