Short Answer:

A synchronous motor operates without slip because it runs at a speed that matches the synchronous speed of the rotating magnetic field produced by the stator. The rotor in a synchronous motor rotates at the same speed as the stator’s magnetic field, which eliminates slip, unlike induction motors where the rotor speed is always slightly less than the synchronous speed.

The rotor of a synchronous motor is magnetically locked to the rotating magnetic field, ensuring that it always follows the field’s rotation, thus maintaining synchronous operation and preventing slip.

Detailed Explanation:

Synchronous Motor Operation

A synchronous motor operates in sync with the frequency of the power supply. The key feature that differentiates synchronous motors from induction motors is that they do not experience slip, meaning the rotor speed is always the same as the synchronous speed of the stator’s rotating magnetic field. This unique characteristic is essential for applications requiring precise speed control and stability.

In an induction motor, the rotor always rotates slightly slower than the synchronous speed, which is known as slip. This occurs because the rotor needs to be slightly slower than the rotating magnetic field to induce current and generate torque. However, in a synchronous motor, the rotor is designed to lock into the same speed as the magnetic field, ensuring zero slip.

1. Synchronous Speed and Rotor Locking

The synchronous speed (NsN_sNs​) is the speed at which the stator’s magnetic field rotates, and it is determined by the supply frequency and the number of poles in the motor. The formula for calculating synchronous speed is:

Ns=120×fPN_s = \frac{120 \times f}{P}Ns​=P120×f​

Where:

• NsN_sNs​ is the synchronous speed in revolutions per minute (RPM),
• fff is the supply frequency (Hz),
• PPP is the number of poles of the motor.

In a synchronous motor, the rotor is designed to rotate at exactly this synchronous speed, unlike an induction motor where the rotor rotates at a speed slightly less than NsN_sNs​. The rotor in a synchronous motor is typically equipped with permanent magnets or an electromagnet that allows it to lock onto the magnetic field of the stator and follow its rotation without any lag or slip.

2. Excitation and Synchronization

To achieve synchronous operation, the rotor in a synchronous motor requires external excitation. This excitation is provided through a DC supply to the rotor windings, which generates a magnetic field in the rotor. This field interacts with the rotating magnetic field generated by the stator, causing the rotor to rotate at the same speed as the stator’s magnetic field.

Once the rotor reaches synchronous speed, it remains locked to the stator’s magnetic field, and there is no need for slip to induce current in the rotor. In contrast to induction motors, where the rotor current is induced due to slip, the current in the rotor of a synchronous motor is directly generated by the external DC excitation.

3. Zero Slip in Synchronous Motors

Slip (SSS) is the difference between the synchronous speed (NsN_sNs​) and the actual rotor speed (NrN_rNr​):

S=Ns−NrNs×100S = \frac{N_s – N_r}{N_s} \times 100S=Ns​Ns​−Nr​​×100

In a synchronous motor, since the rotor speed (NrN_rNr​) is equal to the synchronous speed (NsN_sNs​), the slip is zero. This means that the rotor rotates at exactly the same speed as the rotating magnetic field in the stator. There is no difference in speed between the two, which eliminates the need for slip.

4. Torque Production in Synchronous Motors

The torque produced in a synchronous motor is a result of the interaction between the stator’s rotating magnetic field and the rotor’s magnetic field. When the rotor is synchronized with the stator field, it is continuously attracted and repelled by the rotating magnetic field, causing the rotor to rotate at synchronous speed. The torque is directly proportional to the strength of the rotor’s magnetic field and the load torque required to maintain the rotor’s speed.

The key point here is that synchronous motors can only operate at synchronous speed and cannot accelerate or decelerate beyond this speed without external mechanical influence. This characteristic makes them suitable for applications requiring constant speed, such as in synchronous clocks, precision machinery, and large pumps or compressors.

5. Advantages and Applications of Synchronous Motors

• Constant Speed: Synchronous motors are preferred in applications where precise and constant speed is crucial, such as in clock mechanisms, timing devices, and certain industrial machinery.
• Power Factor Correction: Synchronous motors can operate at unity power factor or provide leading power factor if they are over-excited. This makes them useful in power factor correction in industrial plants, where they can provide reactive power to the grid, improving the overall power efficiency of the system.
• High Efficiency: Since synchronous motors do not experience slip, they can operate with higher efficiency in constant-speed applications, minimizing energy loss.

6. Limitations of Synchronous Motors

While synchronous motors offer several advantages, they do have some limitations:

• Starting Mechanism: Synchronous motors cannot start on their own, as they require the rotor to match the synchronous speed. This requires an external starting mechanism, such as an induction motor starter or a variable frequency drive (VFD), to bring the motor up to speed before synchronization.
• Complexity: Synchronous motors are more complex than induction motors due to their need for an external excitation system and their reliance on maintaining synchronization. This complexity makes them more expensive to operate and maintain in some applications.

Conclusion

Synchronous motors operate without slip by maintaining a constant speed that matches the synchronous speed of the stator’s magnetic field. The rotor is magnetically locked to the stator’s rotating field, and no slip is required to induce current in the rotor. This results in precise speed control, high efficiency, and the ability to provide reactive power for power factor correction. However, their need for external excitation and a starting mechanism limits their use in some applications. Nonetheless, synchronous motors are ideal for applications requiring constant speed and power factor correction.