Synchronous vs Asynchronous Motors: Full Comparison

 

Electric motors are fundamental components in industrial applications, powering everything from pumps and compressors to conveyor belts and precision machines. Among the most commonly used types are synchronous and asynchronous motors. Understanding the core differences between these two types is critical for engineers, maintenance professionals, and plant managers looking to optimize performance, energy efficiency, and system reliability.

In this guide, we provide an in-depth comparison of synchronous vs asynchronous motors, including how they work, where each is used, their pros and cons, and examples of top manufacturers like ABB, Siemens, and Danfoss.

What Is a Synchronous Motor?

A synchronous motor is an AC motor in which the rotor rotates at the same speed as the stator's rotating magnetic field. This synchronization means the rotor and stator magnetic fields rotate at the same frequency—no slip occurs.

How It Works:

The motor uses a direct current (DC) power source for excitation—either through slip rings and brushes or permanent magnets. The rotor locks in with the rotating magnetic field and maintains a constant speed, regardless of load variations (up to a point).

Advantages:

1. Runs at a constant speed
2. Higher efficiency in some applications
3. Can improve power factor when overexcited
4.  Good for high-inertia loads

Disadvantages:

1. Requires external starting mechanism
2. Higher cost and complexity
3. Requires more maintenance (for brushed types)

What Is an Asynchronous Motor?

An asynchronous motor, also known as an induction motor, is the most commonly used motor type in the world. It works on the principle of electromagnetic induction: the stator generates a rotating magnetic field that induces current in the rotor, causing it to turn.

 How It Works:

The rotor always rotates at a speed slightly less than the synchronous speed of the stator field—this difference is called slip. The slip is essential because it allows for torque generation.

 Advantages:

1. Simple and rugged design
2.  Low cost and low maintenance
3. Self-starting
4.  Widely available in standard sizes and ratings

 Disadvantages:

1. Speed varies slightly with load
2. Power factor typically lower than synchronous motors
3.  Limited speed control without a VFD

Read More: Difference Between IE2 and IE3 Motor Efficiency Explained

Synchronous vs Asynchronous Motors: Side-by-Side Comparison

| Feature                                 | Synchronous Motor                            | Asynchronous Motor (Induction)       
|----------------------------|------------------------------------|----------------------------------------|
| Rotor Speed                        | Equal to stator speed                          | Less than stator speed (slip exists) 
| Starting Mechanism         | Needs external source                        | Self-starting                       
| Power Factor                      | Adjustable (can be leading)              | Lagging                            
| Maintenance                       | Higher (especially with brushes)    | Low                                     
| Cost                                       | Higher upfront                                       | More economical                   
| Applications                       | Precise speed needs, power factor correction | General industrial loads      

Applications of Synchronous Motors

Synchronous motors are best suited for:

• High-precision machinery (e.g., robotics, CNC)

• Large compressors and blowers

• Power factor correction in large plants

• Pulp and paper industry where constant speed is essential

They’re ideal in applications requiring accurate speed control and constant load conditions.

Applications of Asynchronous Motors

Asynchronous motors dominate in general-purpose applications like:

• Pumps and fans

• Compressors

• Blowers

• HVAC systems

• Machine tools

• Conveyors

Because of their rugged design and lower cost, asynchronous motors are the default choice for most standard industrial operations.

Integration with Variable Frequency Drives (VFDs)

Both motor types can benefit from VFDs to enhance efficiency and control.

• For synchronous motors, VFDs allow soft starting and dynamic speed adjustment.

• For asynchronous motors, VFDs enable energy savings and more precise speed control.

Danfoss, ABB, and Siemens manufacture advanced VFDs that support both motor types, ensuring compatibility and ease of system integration.

Major Motor Manufacturers: ABB, Siemens, Danfoss

ABB

ABB is a global leader in electrification and automation, offering a wide range of motors:

• Synchronous motors for industrial and marine applications

• Induction motors for HVAC, mining, water, and general-purpose use

• Known for energy-efficient designs, rugged construction, and digital compatibility (smart monitoring systems)

Siemens

Siemens' SIMOTICS line includes:

• Synchronous motors (SIMOTICS S) for automation and servo applications

• Low-voltage asynchronous motors (SIMOTICS GP and SD) for standard industrial machinery

• Emphasis on high efficiency, compact design, and global availability

Danfoss

While primarily known for its VFDs, Danfoss also partners with motor manufacturers and provides integrated drive systems:

• Offers Permanent Magnet Synchronous Motors (PMSM) as part of energy-saving packages

• Works with OEMs to deliver complete motion systems for HVAC, refrigeration, and water sectors

How to Choose Between Synchronous and Asynchronous Motors

Choosing the right motor depends on your application’s requirements. Here’s a quick breakdown:

Choose Synchronous Motors if:

• Speed must remain constant

• You need to correct the power factor

• Application requires precision (e.g., CNC, robotics)

• Willing to invest in higher upfront cost for long-term efficiency

Choose Asynchronous Motors if:

• You’re handling general-purpose loads

• Cost and simplicity are top priorities

• Speed variation with load is acceptable

• Maintenance needs to be minimal

Frequently Asked Questions (FAQs)

1. What is the main difference between synchronous and asynchronous motors?

The main difference lies in rotor speed. In synchronous motors, the rotor speed equals the stator magnetic field speed. In asynchronous motors, the rotor lags behind (slip exists).

2. Which motor is more efficient?

Synchronous motors are generally more efficient in constant load applications. However, induction motors are efficient and cost-effective for variable loads.

3. Can I use the same VFD for both motor types?

Not always. Many modern VFDs are designed to support both, but the parameters and control modes will differ. Always check compatibility with your manufacturer.

4. Why is the asynchronous motor more popular?

It is cheaper, robust, and easier to maintain—ideal for a wide range of industrial needs.

5. Can synchronous motors operate without a VFD?

Yes, but a VFD adds flexibility, soft-start, and energy efficiency. In large applications, it’s often essential.

6. What does motor slip mean?

Slip is the difference between the synchronous speed and the actual rotor speed in an asynchronous motor. It’s necessary for torque generation.

Conclusion

Understanding the difference between synchronous vs asynchronous motors is key to optimizing system performance, efficiency, and cost. Each motor type has its advantages, and the right choice depends on your specific application.

Top manufacturers like ABB, Siemens, and Danfoss offer both technologies, along with intelligent drive systems that enhance motor performance. By choosing wisely, you can ensure reliability, efficiency, and long-term savings in your operations.