Both induction and synchronous motors are AC machines. The basic difference between these two machines is that synchronous motor as the name suggest always runs at constant (synchronous) speed. Induction motors are asynchronous motors and they runs at speed less than synchronous speed. Now, let us discuss the difference between Synchronous Motor vs Induction Motor in various factors.
Synchronous Motor vs Induction Motor :
Stator of induction motor consists of a steel frame which encloses hollow cylindrical core constructed from stacked laminations. These laminations have number of evenly spaced slots at specific positions in order to provide space for the stator winding. 3 phase stator winding is wound for specific number of poles as per the requirement of speed. The constructional diagram of induction motor is as follows;
Rotor of an induction motor can be of squirrel cage↗ or wound rotor↗ type. In squirrel cage rotor, rotor bars are permanently short circuited by end rings. For wound rotor, winding ends are connected to slip rings in order to allow the addition of external resistance in the rotor circuit.
Construction of synchronous motor stator is almost similar to that of induction motor. Synchronous motor rotor consists of salient poles↗ which are made up of steel laminations. Field winding is provided on the pole shoe of rotor. Rotor coils are connected in series with slip rings. Dc current is fed to the rotor windings with the help of slip rings in order to produce electromagnet. Constructional diagram of synchronous motor is as follows;
Induction motor works on the principle of electromagnetic induction. A three-phase AC supply is given to the stator which produces rotating magnetic field. Since magnetic field is rotating, emf is induced in the rotor conductors and current begins to flow. Current carrying rotor conductor placed in a rotating magnetic field experiences force which produces a rotating effect and rotor will start to rotate.
For detail working principle of induction motor visit:
Synchronous motor is doubly excited motor since its stator is fed by three phase AC supply and its rotor is excited by separate dc source. The rotor is made to rotate at synchronous speed by some external prime mover to get magnetically locked with the rotating stator field poles. Once its done, motor starts to rotate at synchronous speed.
For detail working principle of Synchronous motor visit:
Synchronous motor is doubly excited machine since its stator winding is excited by three phase AC supply and its rotor is excited by separate DC source. On the other hand, Induction motor is singly excited machine since only its stator winding is excited by AC source.
Synchronous motor doesn’t have any self-starting torque. It requires additional starting mechanism to initially rotate the rotor up to the synchronous speed so that it can be synchronized with rotating stator field.
For more, kindly read the following article: Why Synchronous motor is not self-starting?↗
Induction motor has a self-starting torque due to which no starting mechanism is required in case of induction motors.
Synchronous motor can be operated at any power factor(leading, lagging or unity) as per requirement by changing its field excitation. An induction motor always operates at lagging power factor.
Synchronous motor is not a self-starting motor since it doesn’t have starting torque. Hence it requires some external starting mechanism to rotate the rotor up to synchronous speed. On the other hand, three phase induction motor is self starting machine.
Cost of synchronous motor is high as compared to the induction motor with having same output and voltage rating.
Synchronous motors are generally more efficient than induction motor of the same output and voltage rating.
Besides driving mechanical loads, over excited synchronous motor can be used for power factor improvement. Induction motor is only use for driving mechanical loads.
Synchronous motor is constant speed machine (i.e. it always runs at synchronous speed Ns= 120f/p). The speed of induction motor is always less than synchronous speed since it is not constant speed machine.
Effect of Load:
The speed of synchronous motor is not dependent on load. In simple words, synchronous motor always operates at constant speed irrespective of the applied load (whether it is full load or no load). On the other hand, speed of induction motor is dependent on the load. If the load on motor increases, speed of the motor decreases (i.e. speed of induction motor decreases with the increase in load and vice versa).