What is a Synchronous Motor?

By R.W. Hurst, Editor

What is a Synchronous Motor?

A synchronous motor is an electric motor that synchronizes with the stator's rotating magnetic field. It offers several benefits, such as constant speed operation, high efficiency, and power factor correction capabilities. However, its disadvantages, such as the inability to self-start and complex construction, should also be considered when selecting the appropriate motor for specific applications. One can better appreciate their significance in electrical engineering by understanding the principle of operation, differences from induction motors, and the various applications of synchronous motors.


A synchronous motor is prominent in electric motors due to its unique characteristics and wide range of applications. But what is a synchronous motor, and how does it work? 

A synchronous motor is a type of AC motor that operates at a constant speed, known as synchronous speed, which is directly proportional to the frequency of the AC phase supply and the number of poles in the motor. It derives its name from the fact that the rotor rotates synchronously with the rotating magnetic field produced by the stator. Unlike induction motors, where the salient pole rotor rotates at a slightly different speed than the stator's magnetic field, synchronous motors maintain perfect synchronism between the rotor winding and field of the stator.

To understand the principle of operation of a synchronous motor, one must first know the roles of the rotor and stator. The stator is the stationary part of the motor, consisting of stator poles and windings, which produce a rotating magnetic field when connected to an AC power supply. On the other hand, the rotor is the rotating part of the motor that comprises rotor poles, which can be either wound or made of permanent magnets. When the rotor poles are aligned with the stator's magnetic field, a torque is generated, causing the rotor to rotate and drive the mechanical load.

What is a synchronous motor, and how does it differ from an induction motor? First, the rotor construction in a synchronous motor includes either wound field windings or permanent magnets. In contrast, an induction motor usually has a squirrel cage rotor or a wound rotor with slip rings. Secondly, synchronous motors are not self-starting, whereas induction motors are. Lastly, synchronous motors can operate at unity or leading power factors, making them suitable for power factor correction.

There are several advantages and disadvantages associated with synchronous motors. Among the benefits are their ability to maintain a constant speed, high efficiency, and the capacity to correct power factors, thereby reducing energy costs. However, the disadvantages include their inability to self-start, complexity in construction, and the need for additional starting methods, such as using a separate induction motor or employing a variable frequency drive.

Synchronous motors find applications in various industries due to their inherent characteristics. These include constant-speed drives, such as compressors, pumps, and fans, and high-precision machinery, like machine tools and robotics. Additionally, synchronous motors are often used in power generation as synchronous generators, converting mechanical energy into electrical energy.

Controlling the speed of a synchronous motor can be achieved by altering the frequency of the AC power supply or by varying the number of poles in the motor. In modern applications, variable frequency drives (VFDs) are commonly used to adjust the speed of synchronous motors by modifying the frequency of the input power supply.

What is a synchronous motor and how does it contribute to power factor correction by operating at unity or leading power factors? A leading power factor can help counteract lagging power factors present in other loads on the electrical system, ultimately reducing energy costs and improving system efficiency.