Induction Motor vs. Synchronous Motor

Main Difference

The main difference between Induction Motor and Synchronous Motor is that the speed of the Induction Motor keeps on changing according to the load, whereas Synchronous Motor always works at synchronous speed.

Difference Between Induction Motor and Synchronous Motor

Comparison Chart

Induction Motor vs. Synchronous Motor

The induction motor also recognized as the synchronous motor. On the flip side, there is no other name for synchronous motor. Induction motor is the type of AC motor whose working speed depends upon the load, i.e., speed of the motor decreases with the increase in load. On the other side, the speed of the synchronous motor does not depend upon load. It keeps on working with synchronous speed.

The induction motor is a type of single excited machine, whereas, the synchronous motor is a type of doubly excited machine. The induction motor has a simple design, while the synchronous motor has a complex design. In the supply system of the induction motor, stator winding is linked by an AC source. On the other side, in a synchronous motor, the armature winding is energized by an AC source while a DC source energizes the field winding.

The induction motor is a self-starting motor in which torque created due to the reaction between a changing magnetic field that produced in the stator and the current that induced in the coils of the rotor. On the other hand, the synchronous motor cannot be self-started. It needs a source to be run up to synchronous speed before its synchronization to AC supply.

The induction motor works at a lagging power factor only because the power factor becomes very poor at high loads. On the flip side, the synchronous motor designed in such a way that it can use with both leading and lagging power by changing its excitation. An induction motor has less efficiency as compared to the synchronous motor of the same voltage rating and the output.

The induction motor has a low price as compared to the synchronous motor of the same voltage rating and the output. The induction motor is utilized to operate mechanical loads only. On the other hand, the synchronous motor is utilized to supply torque to drive mechanical loads as well as for power factor correction.

What is Induction Motor?

The induction motor is a type of AC motor whose working speed depends upon the load. It also recognized as an asynchronous motor. It is one of the first invented electric motors and has a simple design. Nowadays, an induction motor is the most prolific electric motor that is used in industries.

The induction motor has a simple structure that is made up of an outer stator and an inner rotor. These parts interact through the electromagnetic induction effect to produce mechanical rotation. Different types of induction motors attain this rotation in different ways. In the supply system of the induction motor, stator winding is linked by an AC source.

The induction motor can self-start. Such motor plays its role by passing an alternating current through coils in the stator that will create a magnetic field, and the oscillating frequency of the alternating current supply will rotate this magnetic field. This RMF or rotating magnetic field will create opposing magnetic fields in the rotor and cause useful rotation.

The working speed of the induction motor is not synchronous and depends upon the load, i.e., the speed of the motor decreases with the increase in load. So, the frequency of its alternating current does not match the number of rotations of the output shaft. This process is called “slip” because it takes place due to the magnetic game of “catch-up” played by the rotor with the RMF. So, due to the presence of slip, accurate timing is hard with induction motors.

It works just at a lagging power factor because the power factor gets very low at high loads. The induction motor has less efficiency as compared to the synchronous motor of the same properties. As the induction motor is without a commutator, brushes, and other such moving parts, so it is easy to manufacture and is less expensive as compared to the synchronous motor.

The induction motor is utilized to operate mechanical loads only. So, these motors are used in household appliances, large mechanized industrial equipment, and electric vehicles. Moreover, the induction motors are available in hundreds of torques, voltages, speeds, sizes, and forms.

What is Synchronous Motor?

It is the type of AC motor in which speed is directly proportional to the frequency of the operating current. It covers the bases that cannot be covered by an induction motor. It has a complex design. In its construction, the stator or rotator has axial slits that include rotator twisting wound for a particular number of poles. Moreover, the rotor winding is mounted on a salient pole rotor, which is served by a direct current supply with the help of slip rings. We can also use a rotor with permanent magnets.

The synchronous motor keeps on working with synchronous speed as its output rotational frequency is equal to the input AC frequency so that these motors can be used by designers in precisely timed applications, for example, rolling mills, record players, and clocks, etc. This purpose is obtained by linking the magnetic poles of the stator and rotor. Due to this, the stator RMF turns the rotor at synchronous speeds.

For a steady-state operation of such motors, the constant relationship between the rotating speed of the rotor and the grid frequency is given as, N = Ns = 120f / p here f is the grid frequency, p is the motor pole number, and Ns is the synchronous speed.

It is not able to self-start. It needs a source to be run up to synchronous speed before its synchronization to AC supply. Moreover, as they have synchronous speed, so it is difficult to change their speed. That is why the designers used the AC motor controller to control the speed of such motors. Such a motor is a type of doubly excited machine. It is designed in such a way that it can be used with both leading and lagging power by altering its excitation. It has high efficiency and high price as compared to the induction motor of the same voltage rating and the output.

The synchronous motors are divided into different types on the basis of their ways of creating a field magnet. The external source is used to energize magnetic poles of the separately excited motors. On the other side, in the self-excited, non-excited, or directly excited machines, the magnetic poles have stimulated the motor itself. The examples of non-excited types are reluctance motors, permanent-magnet motors, and hysteresis motors. The synchronous motor is utilized to supply torque to drive mechanical loads as well as for power factor correction. It has great importance in crushers, grinders, mills, and other low speed and high-power applications.

Conclusion

The above discussion summarizes that the induction motor is a type of AC motor with a simple design and low efficiency whose working speed depends upon the load. On the other side, the synchronous motor is a type of AC motor with complex design, high efficiency, and synchronous speed.