Short Answer:

A synchronous motor is an AC motor in which the rotor rotates at the same speed as the magnetic field of the stator, meaning there is no slip. On the other hand, an asynchronous motor, also known as an induction motor, has a rotor that rotates at a speed slightly less than the stator’s magnetic field speed, meaning it has slip.

The main difference lies in how the rotor matches the stator’s rotating field. Synchronous motors are used when constant speed is needed, while asynchronous motors are more common because they are simple, rugged, and cheaper, suitable for general-purpose applications.

Detailed Explanation:

Difference between a synchronous and an asynchronous motor

Electric motors are devices that convert electrical energy into mechanical energy. Based on how the rotor behaves with respect to the rotating magnetic field in the stator, motors are classified as synchronous and asynchronous motors. Both are commonly used in industries, but they differ in construction, working principle, speed behavior, and applications.

1. Speed and Slip:

• Synchronous Motor:
  In a synchronous motor, the rotor spins at exactly the same speed as the rotating magnetic field of the stator. This is known as synchronous speed. There is no slip, which means the rotor and stator field rotate in perfect synchronization.
• Asynchronous Motor (Induction Motor):
  In this motor, the rotor always rotates at a speed slightly less than the synchronous speed. This difference in speed is called slip, and it is essential for the operation of the induction motor.

2. Working Principle:

• Synchronous Motor:
  It works based on the interaction between the rotating magnetic field of the stator and a constant magnetic field produced by the rotor (which is excited by DC or permanent magnets). The rotor locks in step with the stator field.
• Asynchronous Motor:
  It operates on electromagnetic induction. The rotating magnetic field in the stator induces current in the rotor, which creates its own magnetic field. The rotor then starts rotating, but it always lags slightly behind the stator field.

3. Starting Mechanism:

• Synchronous Motor:
  It is not self-starting. It needs external means like an auxiliary motor or damper winding to reach near synchronous speed before it can synchronize with the stator field.
• Asynchronous Motor:
  It is self-starting and starts rotating automatically when the power is applied, making it more convenient for general use.

4. Construction and Cost:

• Synchronous Motor:
  More complex in design due to the need for a separate excitation system or permanent magnets. This makes it more expensive.
• Asynchronous Motor:
  Simple and rugged in construction. It does not require a separate excitation system, making it cheaper and easier to maintain.

5. Speed Control:

• Synchronous Motor:
  Maintains constant speed under varying load conditions, which is ideal for applications requiring precise speed.
• Asynchronous Motor:
  Speed varies slightly with the load due to slip. Speed control is more complex and usually requires additional electronics like variable frequency drives (VFDs).

6. Efficiency and Usage:

• Synchronous Motor:
  More efficient at higher loads and used in applications where constant speed is required, such as in power factor correction, conveyors, and synchronous clocks.
• Asynchronous Motor:
  Widely used in pumps, fans, compressors, and household appliances because of its reliability and simplicity.

7. Applications:

• Synchronous Motors:
  Used in industries requiring constant speed like rolling mills, paper mills, and synchronous condensers.
• Asynchronous Motors:
  Common in daily applications such as washing machines, fans, refrigerators, and industrial drives.

Conclusion

The key difference between a synchronous and an asynchronous motor is in the rotor speed. A synchronous motor runs at constant speed equal to the stator’s rotating magnetic field, while an asynchronous motor runs at a slightly slower speed due to slip. Synchronous motors are ideal for precision and constant speed needs, while asynchronous motors are preferred for general, rugged, and economical use. Understanding their differences helps in selecting the right motor for specific applications.