Synchronous motors and induction motors are both types of alternating current (AC) motors that convert electrical energy into mechanical energy. But what are the differences between them? Why are some synchronous, while others are asynchronous?
The Key Difference: Synchronization of Rotor and Stator Magnetic Field
“Synchronous” and “asynchronous” describe whether the rotor speed matches the speed of the magnetic field generated by the stator.
To fully understand this, we first need to explore the working principles of these two motors.
Working Principles: Synchronous Motor VS Asynchronous Motor
Synchronous Motor
In a synchronous AC electric motor, the rotor rotates at the same speed as the stator’s magnetic field. There is no slip between them.
Asynchronous Motor
An asynchronous motor is an AC motor that operates differently from a synchronous motor. Because its rotor speed is different from the magnetic field speed generated by the stator current, it is called an asynchronous motor.
The asynchronous stator is fed with three-phase AC to generate a rotating magnetic field, and the rotor is induced to generate a magnetic field. The two magnetic fields act to make the rotor rotate with the rotating magnetic field of the stator. The rotor rotates slower than the stator magnetic field, and there is a slip.
Why is the asynchronous motor also called an induction motor?
Since there is no direct electrical contact between the stator and the rotor, electromagnetic induction can be used to achieve the conversion of mechanical energy and electrical energy, so it is also called an induction motor.
If you want to learn more about induction motors, click “Asynchronous Motor: How Much Do You Know?”
How are synchronous motors and asynchronous motors designed in structure? What are the differences?
Whether it is a synchronous motor or an asynchronous motor, it mainly includes a stator, a rotor, a shaft, a housing, an end cover, etc.
The stator windings of synchronous motors and asynchronous motors are the same, and the main difference lies in the structure of the rotor.
Rotor Structures: Synchronous VS Asynchronous Motor
Synchronous Motor Rotor
The rotor of a synchronous motor is mainly divided into two motor types: permanent magnet and electrically excited.
Permanent Magnet Synchronous Motor Rotor
The permanent magnet synchronous motor rotor primarily consists of a rotor core and magnets. Based on the placement of the magnets on the rotor core, the structure of the rotor can be categorized into surface-mounted, inset, and interior types.
Surface-Mounted Permanent Magnet Synchronous Motor Rotor
The magnetic poles of the rotor are usually tile-shaped and are fixed to the surface of the rotor core using special adhesives.
Inset Permanent Magnet Synchronous Motor Rotor
From the outside, the rotor appears similar to the surface-mounted type, but its performance differs significantly.
Due to the protrusions of the rotor core between the magnetic poles, the inductance of the stator changes with the rotor’s position during rotation. This causes the motor to produce both permanent magnet torque and reluctance torque during operation.
Interior Permanent Magnet Synchronous Motor Rotor
In this structure, the magnetic poles are embedded inside the rotor core, not in direct contact with the air gap. The magnets are protected by the surrounding rotor core, which improves the mechanical strength and reliability of the rotor.
Electrically Excited Synchronous Motor Rotor
The rotor of an electrically excited synchronous motor contains a DC excitation winding. This requires an external excitation power source, with current introduced through slip rings.
Asynchronous Motor Rotor
Induction motor rotors are classified into two types based on the winding structure: squirrel cage and wound rotor.
Wound Rotor
The rotor has a three-phase winding, similar to the stator winding. It is generally connected in a star configuration.
The three-phase leads are connected to three insulated slip rings on the rotor shaft. These slip rings are connected to the external circuit via a brush system.
This setup allows for the addition of resistors or electromotive forces in the rotor circuit to improve the motor’s performance.
Squirrel Cage Rotor
In the squirrel cage rotor, copper bars are inserted into each slot of the rotor core. At both ends of the copper bars, copper rings (called end rings) are used to connect the bars. This is known as a copper bar rotor.
Alternatively, aluminum casting is used to cast the rotor bars and end rings together with fan blades in a single pour. This is called an aluminum-cast rotor. Induction motors under 100kW typically use aluminum-cast rotors.
Applications: Synchronous Motor VS Asynchronous Motor
Synchronous motors are used where constant speed is essential, such as in generators, wind turbines, electric vehicles, and robotics.
Induction motors are widely used in the application of variable speed, low power, or simple operation, including pumps, fans, refrigeration systems, and compressors.
Key Difference Between Synchronous Motor and Induction Motor
| Parameter | Synchronous Motor | Induction Motor |
| Speed | Rotor speed matches stator field, no slip | Rotor speed is always slower than stator field, with slip |
| Starting Method | Synchronous motors require additional starting mechanism to initially rotate the rotor near to the synchronous speed. | Self-starting by the rotor cutting the stator rotating magnetic field |
| Excitation | Double excitation | Single excitation |
| Speed Control | Excellent speed control, follows power frequency | Limited, uses pole or frequency control |
| Power Factor | Adjustable, can reach unity | An induction motor operates only at a lagging power factor. Varies with load, typically 0.7–0.9. |
| Efficiency | It is more efficient than an induction motor of the same output and voltage rating | Its efficiency is lower than that of synchronous motors with the same output power and voltage level. |
| Structure | More complex | Simpler with squirrel cage rotors |
| Cost | Synchronous motors are more expensive than induction motors of the same output and rated voltage | Induction motors are cheaper than synchronous motors of the same output and voltage rating |
Synchronous motors are ideal for high-precision tasks like robotics or specialized equipment. Induction motors are reliable and robust, making them suitable for industrial workhorses. When selecting a motor, opt for synchronous if precision is crucial or induction for cost-effectiveness and simplicity.
Motor Core Lamination Manufacturer – Choose Lamnow
Lamnow is a leading motor lamination manufacturer in China with extensive experience producing cores for both synchronous and induction motors. We specialize in custom permanent magnet stators and rotors for applications like EV drive motors and drones. Induction motor cores are widely used in pumps, blowers, and compressors.
We offer a variety of high-speed molds for induction motor cores, such as 9047 and 11055. Whether you need standard parts or specific services like stator winding, rotor skewing, or rotor casting, we have versatile options to meet your needs. For standard drone motor cores, we also provide high-speed stamping molds like the 2206 model. Feel free to contact us anytime!
