The fundamental difference between Induction Motor and Synchronous Motor is that speed of an induction motor is less than its synchronous speed, while the speed of the synchronous motor is equal to its synchronous speed.
An electric motor is defined as an electrical machine that transforms input electric energy into output mechanical energy. Based on the kind of input power supplied to the electric motor, it is classified into two types.
- DC Motor
- AC Motor
Furthermore, based on the operation, AC electric motors are further classified into the following two types.
- Induction Motor
- Synchronous Motor
In this article, we will discuss the differences between the two types of AC motors namely – Induction motor and Synchronous motor.
What is an Induction Motor?
An induction motor is a kind of asynchronous motor that operates at a speed less than that of the synchronous or constant motor speed. The induction motor operates on the principle of electromagnetic induction, in which a motor rotates due to the EMF induced on the conducting coil by the magnetic field generated by the static stator.
An Induction Motor is comprised mainly of two components namely – the stator and rotor. The stator is constructed by winding a three-phase coil around it. While the rotor is made by winding a short-circuited coil around it. On supplying the stator with three phase AC supply the stator winding produces a rotating magnetic field. This rotatory magnetic field in turn induces the EMF on the rotor that makes it rotate, due to the difference in speed of rotation of the static rotor and the rotatory magnetic field.
Due to the short circuit in the winding of the rotor, the electric current flows in the rotor winding. Thus, when the rotor is cut by the rotatory magnetic field of the stator, it initiates rotation in the same direction as that of the rotating magnetic field due to the production of torque. Therefore, the rotation of an induction motor is possible only due to the difference in the speeds of the rotor and the stator magnetic field, which in turn results in the generation of torque, EMF, and electric current flowing through the rotor windings.
What is a Synchronous Motor?
A Synchronous Motor is described as an electrical machine whose rotor runs at the same speed as that of the speed of the rotatory magnetic field of the stator. The speed of the stator’s rotating magnetic field is called the synchronous speed. The synchronous motor is made up of a stator and rotor as its primary parts, along with slip rings and winding. Stator is supplied with AC power, while the rotor winding is supplied with DC power.
Hence, based on the type of excitement, synchronous motors can be subdivided into the following two categories:
- DC current excited motor
- Non-excited motor
The synchronous motor operating principle is as follows.
In a synchronous motor, the AC power supplied to the stator winding of the motor produces a rotational magnetic field surrounding it. DC power is supplied to the rotor winding of the synchronous motor through the slip rings attached to the rotor or a permanent magnet is kept near the rotor, to create a magnetic field around the rotor. A synchronous motor is also sometimes referred to as a ‘dual-excited machine’ as it is supplied with both AC and DC currents to the stator and rotor winding of the motor respectively.
Difference between Induction Motor and Synchronous Motor
The comparison table given below highlights all the key differences between an Induction Motor and a Synchronous Motor –
| Criteria of Difference | Synchronous Motor | Induction Motor |
| Description | A synchronous motor is an AC machine whose rotor speed is synchronized with the rotatory magnetic field of the stator winding. | An induction motor is an asynchronous motor whose rotor speed is not synchronized with the rotatory magnetic field of the stator winding. |
| Rotor speed | The rotor speed of the synchronous motor is the same as that of the rotational speed of the stator magnetic field. | The rotor speed of the induction motor lags the speed of the stator rotating magnetic field and thus the rotor speed is less than the synchronous speed of the induction motor. |
| Working Principle | The synchronous motor works on the principle of overlapping of rotor and stator magnetic fields. | The induction motor operates on the electromagnetic induction mechanism existing between the magnetic fields of the stator and rotor. |
| Expression for rotor speed | The speed of the synchronous motor is given by,  Where f is the supply frequency and P is the number of poles in the motor. | The rotor speed of the induction motor is given by,  Where NR is the speed of the rotor, NS is the synchronous speed, and s is the slip. |
| Factors affecting motor speed | The number of stator poles and the supplied input AC power frequency are the deciding factors of the synchronous motor speed. | The motor slip, mechanical load and the resistance of the rotor circuit, the number of poles, and supply frequency are the deciding factors of the induction motor speed. |
| Unit of measurement | The SI unit of synchronous speed measurement is radian per second (Rad/s) or revolution per minute (rpm). | The SI unit of asynchronous speed measurement is also radian per second (Rad/s) or revolution per minute (rpm). |
| Motor slip | The slip of the synchronous motor is zero, i.e., the speed of rotation of both rotor and stator are equal. | The slip of the Induction motor is ranging between 0 and 1. |
| Load dependence of speed | Change in mechanical load does not affect the speed of the synchronous motor. | Change in mechanical load results in variations in the speed of the induction motor. |
| Power supply to rotor | To generate a rotor magnetic field, DC power is supplied to the rotor | The Induction motors do not need any form of excitation and can start rotating on their own. |
| Motor speed control | To control the speed of the synchronous motor, the variable frequency drive (VFD) is used to vary the frequency. | To control the speed of the induction motor, the variable frequency drive (VFD) and variable rotor resistance (VRR/GRR-Grid Rotor Resistance) are used. |
| Initial torque | The initialization torque of the synchronous motor is comparatively high. | The initialization torque of the induction motor is comparatively low. |
| Capital Expenditure | The expense of managing a synchronous motor is higher. | The expense of managing an induction motor is comparatively lower. |
| Performance | The efficiency of the operation of a synchronous motor is comparatively higher. | The efficiency of the operation of the induction motor is comparatively lower. |
| Need for Excitation | Excitation provided by direct current supply is required at the rotor. | Excitation is not required at the rotor input. |
| Magnitude of motor speed | The average synchronous motor speed is 300 RPM. | The average induction motor speed is slightly above 600 RPM. |
| Power factor | The power factor of a synchronous motor is either unity, leading, or lagging. | The power factor of the induction motor is always lagging. |
| Applications | Synchronous motor finds its application in power factor correction and operation of mechanical loads. | The induction motor is used only to operate mechanical loads. |
Conclusion
In conclusion, synchronous and induction motors play a vital role in industrial applications for driving heavy mechanical loads. Although synchronous motors are comparatively more efficient in terms of performance, they have a high cost of maintenance and installation.
The key difference between them is that synchronous motors have their rotor and stator magnetic fields rotating at the same speed, while induction motors run at different speeds, commonly referred to as slip. Moreover, the ease of operation of induction motors in comparison to synchronous motors is an added advantage.
