Synchronous Motors - The Benefits

By Jim Parrish, Lion Oil

Synchronous Motors
Synchronous motors are suited for precise speed control and power factor management applications.

Industrial production processes often involve induction motors due to their reliability and simplicity. However, induction motors can result in increased lagging power factor burdens in a plant, leading to the need for corrective measures like capacitors. In low-speed applications, synchronous motors equipped with power factor control accessories could be a more efficient choice. 

Synchronous motors can be a valuable alternative to induction motors, as they can control power factors and reduce the plant's reactive loading to the connected system. For example, Lion Oil, a petroleum processing refinery in El Dorado, Arkansas, had to comply with new clean air regulations requiring reduced sulphur emissions for diesel fuel. As a result, Lion Oil needed a new higher-rated motor connected to a hydrogen-reciprocating compressor for their refining process to meet these requirements. The plant previously utilized induction motors ranging from 100 to 3500 horsepower, which caused power factor issues. The decision to switch to a synchronous motor was driven by the need for an optimally efficient plant with lower operating costs.

When choosing synchronous motors, understanding the differences between AC (alternating current) motors, such as induction and synchronous motors, is crucial. An induction motor, known as an asynchronous motor, has fixed stator windings electrically connected to the AC power source. The rotor rotates slower than the stator field for induction, resulting in less than synchronous speed. As a result, induction motors operating at 600 RPM and below have lower efficiency and a lagging power factor, drawing exciting current from the connected source.

On the other hand, synchronous motors also have fixed stator windings connected to the AC supply. However, they use a separate excitation source connected to a field winding on the rotating shaft. Magnetic flux links the rotor and stator windings, causing the motor to operate at synchronous speed. This design allows for adjusting the motor's power factor by varying the excitation supplied to the motor's field, making synchronous motors more suitable for certain applications.

Synchronous motors are suited for precise speed control and power factor management applications. These motors can be single-phase or three-phase synchronous motors, depending on the number of poles in the stator winding. The rotating magnetic field created by the stator interacts with the rotor poles, leading to the rotor speed being locked in step with the stator's field when the appropriate excitation is supplied.

Synchronous machines, such as motors and generators, are versatile and can be designed with various types of the rotor winding. For instance, some synchronous motors use slip rings and brushes to provide excitation to the rotor. In contrast, others have brushless designs, like squirrel cage rotors, simplifying maintenance and improving reliability.

One of the key aspects of synchronous motors is the relationship between the rotor and stator. The stator poles are arranged to interact with the rotor poles, creating a rotating magnetic field that drives the motor at synchronous speed. This synchronization improves efficiency and precise control over the motor's operation.

Synchronous motors offer several advantages over induction motors in specific applications. Their ability to control power factors and provide precise speed control makes them an ideal choice for industries requiring more efficient and reliable motor solutions. By understanding the differences between synchronous and induction motors and their respective operating principles, industries can make better-informed decisions when selecting the appropriate motor type for their needs.


What are the key differences between synchronous motors and induction motors?

Synchronous and induction motors differ primarily in operating principles and speed control. Synchronous motors operate at a constant speed determined by the supply frequency and the number of poles, while induction motors operate at speed slightly below the synchronous speed. In addition, synchronous motors require a separate excitation source for the rotor winding. In contrast, induction motors rely on the induced current from the stator to create a magnetic field in the rotor.


How do synchronous motors provide better control over power factors?

Synchronous motors allow for better control over power factors due to their ability to vary the excitation supplied to the rotor winding. Adjusting the excitation allows the motor's power factor to be controlled, enabling it to operate at a leading, lagging, or unity power factor. This capability can reduce reactive power consumption and improve overall system efficiency.


What are the advantages of using synchronous motors in industrial applications?

Some advantages of using synchronous motors in industrial applications include the following:

  • Precise speed control: Synchronous motors maintain a constant speed, regardless of load changes, which is essential for applications requiring strict speed regulation.
  • Power factor control: The ability to control power factors reduces reactive power consumption and can improve overall system efficiency.
  • High efficiency: Synchronous motors generally have higher efficiency than induction motors, especially at low speeds and high torques.
  • High starting torque: They provide high starting torque, which can benefit applications with heavy loads.


How does the interaction between the rotor and stator affect the performance of synchronous motors?

The interaction between the rotor and stator in a synchronous motor creates a rotating magnetic field that drives the motor at synchronous speed. This synchronization enables better speed control, improved efficiency, and the ability to control power factors.


What types of rotor winding can be found in synchronous motors?

Synchronous motors can have different types of rotor winding, including salient-pole rotors, cylindrical rotors, and squirrel cage rotors. Some designs use slip rings and brushes to provide excitation to the rotor. In contrast, others have brushless designs with the excitation provided by a separate source, such as a permanent magnet or an external exciter.


In what situations are synchronous motors a better choice than induction motors?

Synchronous motors are better than induction motors in situations requiring precise speed control, high starting torque, improved efficiency, and power factor control. These can include low-speed, high-torque applications, such as in mills, crushers, and large fans, and applications requiring constant speed under varying load conditions.


How do single-phase and three-phase synchronous motors differ in their construction and operation?

Single-phase synchronous motors generally have a simplified construction compared to three-phase motors. They often utilize a capacitor or auxiliary winding to generate a second phase, creating a rotating magnetic field. On the other hand, three-phase synchronous motors have three separate stator windings energized by a three-phase power supply, generating a rotating magnetic field without additional components. In terms of operation, single-phase motors are typically used for smaller loads and residential applications. In contrast, three-phase motors are common in industrial settings and large commercial applications due to their higher efficiency and capability to handle more substantial loads.