Among the different types of servo motors, asynchronous (induction) and synchronous servo motors stand out as prominent choices for various applications. While both serve the same purpose—to provide precise control of angular position, speed, and acceleration—they have distinct operational characteristics, advantages, and applications.

This article will explore the differences between asynchronous and synchronous servo motors, comparing their design, operating principles, benefits, and ideal use cases.

What is a Servo Motor?

Servo Motor

A servo motor is a rotary actuator that allows precise control of angular position, velocity, and acceleration. It typically includes:

  • A motor mechanism (asynchronous or synchronous).
  • A feedback system (encoder or resolver).
  • A control unit (servo drive).

Servo motors can be broadly classified into two categories: AC (alternating current) servo motors and DC (direct current) servo motors, with AC servo motors being the more popular choice in industrial applications. Among the AC servo motors, we find two main subtypes: asynchronous (induction) and synchronous.

Asynchronous Servo Motors

The most popular kind of motor in industrial settings is the asynchronous servo motor, sometimes referred to as an induction motor. An asynchronous motor’s rotor never precisely matches the speed of the stator’s rotating magnetic field because of the electromagnetic induction theory that underlies its operation. Known as “slip,” the speed differential fluctuates based on the motor’s applied load.


Pros


Cons

  • Simple and robust design

  • Cost-effective 

  • Wide availability and easy maintenance

  • Tolerant to harsh environments

  • Good heat dissipation properties

  • Lower efficiency due to slip

  • Less precise control at low speeds

  • Slower dynamic response

Operating Principle of Asynchronous Servo Motors

An asynchronous servo motor, often known as an induction servo motor, operates on the electromagnetic induction principle.
In contrast to synchronous motors, asynchronous motors have a rotor that slips or lags slightly behind, the stator’s magnetic field.

Key Characteristics of Asynchronous Servo Motors

  • Torque-Speed Curve: Asynchronous motors produce a torque-speed curve that varies depending on the load. There is greater slip and the motor’s speed drops as the load increases.
  • Efficiency: Asynchronous motors are generally less efficient than synchronous motors because the slip causes energy loss in the form of heat.
  • Cost-Effectiveness: Compared to synchronous motors, asynchronous motors are usually less expensive. Since they don’t need brushes or permanent magnets, their design is simpler and they require less upkeep.
  • Starting Performance: These motors tend to have a high starting torque, which is an advantage in applications requiring heavy load starts.
  • Robustness: Asynchronous motors are known for their durability and are capable of handling harsh operating conditions.

Applications of Asynchronous Servo Motors

Asynchronous motors are ideal for applications where exact synchronization is not critical but where robustness and cost-effectiveness are valued. Some typical applications include:

  • Pumps and fans
  • HVAC systems
  • Conveyor systems
  • Basic automation machinery

Synchronous Servo Motors

Asynchronous and synchronous servo motors function in distinct ways. This shows that the rotor synchronizes with the frequency of the supply current to maintain a precise and constant speed under a range of load circumstances.


Pros


Cons

  • High efficiency with minimal energy losses.

  • Precise control over speed and position.

  • High torque density

  • Higher initial cost

  • Requires complex control systems.

  • Sensitive to overheating

Operating Principle of Synchronous Servo Motors

The frequency of the AC supply determines the constant rotation speed required for synchronous motors. A revolving magnetic field is produced by the stator when AC voltage is introduced. In synchronous motors, the rotor contains either permanent magnets or electromagnets and is designed to lock onto the rotating field, making the rotor and the field rotate at the same speed.

Key Characteristics of Synchronous Servo Motors

  • Precise Speed Control: Synchronous motors offer precise speed control because the rotor’s speed and the stator’s magnetic field match. Applications that need to keep up speed under different load conditions might find this particularly helpful.
  • High Efficiency: Synchronous motors are generally more efficient than asynchronous motors because they do not slip and, therefore, less energy is lost in the form of heat.
  • Constant Torque: Synchronous motors provide constant torque across a wide range of speeds, which is beneficial for applications that require consistent performance.
  • Power Factor Correction: Power factor adjustment in industrial systems can be aided by the leading power factor that many synchronous motors can operate at.
  • Complex Construction: Synchronous motors tend to have more complex constructions than asynchronous motors, and they may require additional components like brushes or external excitation systems, depending on the design.

Applications of Synchronous Servo Motors

Due to their precision and high efficiency, synchronous motors are ideal for high-performance applications that demand exact speed and position control. Common applications include:

  • Robotics
  • CNC machines
  • Servo drive systems
  • High-precision manufacturing equipment
  • Motion control systems

Asynchronous vs Synchronous Servo Motors: A Comparison

Characteristic Asynchronous Servo Motor Synchronous Servo Motor
Speed Range (RPM) 0-10,000 0-20,000
Efficiency(%) 85-90 95-98
Cost Index (1-10) 5 8
Construction Simpler construction, fewer components More complex construction may require additional components (e.g., exciter)
Torque Varies with load, less constant torque Constant torque across a range of speeds
Control Less precise control, suitable for general applications More precise control, ideal for high-precision applications

Choosing Between Asynchronous and Synchronous Servo Motors

When choosing between asynchronous and synchronous servo motors, several factors must be considered:

  • Precision Requirements: If your application requires precise speed and position control, a synchronous servo motor is the better choice. Synchronous motors provide exact synchronization with the supply frequency, making them ideal for high-precision tasks.
  • Efficiency Considerations: For applications that require continuous operation and higher energy efficiency, synchronous motors are preferred due to their superior efficiency and constant torque output.
  • Cost and Complexity: Asynchronous motors are generally more cost-effective and easier to maintain, making them suitable for applications that do not require extreme precision or efficiency.
  • Starting Performance: If your system requires high starting torque or is used in heavy-duty applications, asynchronous motors may offer better performance.

Both asynchronous and synchronous servo motors have their unique advantages and are suited to different types of industrial applications. Servo motor manufacturers typically select asynchronous motors for applications where cost, robustness, and simplicity are more important than absolute precision, while synchronous motors are the better choice for high-precision, efficiency-driven tasks.