What Happen When a Synchronous Motor Fall out of Synchronism?

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When a synchronous motor experiences an increased load, several changes occur, particularly concerning the load angle (also known as the torque angle or power angle). If the load continues to increase to the point of overload, it can cause the motor to fall out of synchronism. Let's break down this process in detail:

Load Angle and Synchronization

1. Load Angle (δ):

   - The load angle (delta) is the angle between the rotor's magnetic field and the stator's rotating magnetic field. Under normal operating conditions, the rotor locks in step with the rotating magnetic field of the stator, maintaining a constant load angle.

2. Increasing Load:

   - As the mechanical load on the motor increases, the motor must produce more torque to maintain the same speed. To generate this additional torque, the load angle (delta) increases. This increase in (delta) indicates that the rotor's magnetic field is lagging further behind the stator's magnetic field.

3. Torque and Load Angle Relationship:

   - The electromagnetic torque (T) generated by a synchronous motor is proportional to the sine of the load angle (delta):

Overloading and Falling Out of Synchronism

1. Overload Condition:

   - If the load continues to increase beyond the motor's rated capacity, the load angle (delta) will continue to increase to provide the necessary torque. However, there is a maximum limit to this angle, typically less than 90 degrees.

2. Critical Load Angle:

   - The maximum torque, known as the pull-out torque, occurs at a load angle slightly less than 90 degrees. Beyond this point, the torque starts to decrease because the sine function reaches its peak at 90 degrees.

3. Out of Synchronism:

   - If the load increases to a point where the load angle exceeds this critical value, the rotor can no longer keep up with the stator's rotating magnetic field. This condition is referred to as "falling out of synchronism" or "loss of synchronism."

   - When the motor falls out of synchronism, it loses its ability to maintain a constant speed and starts to slip, resulting in severe oscillations and potential mechanical damage.

4. Consequences of Falling Out of Synchronism:

   - Loss of Torque: The motor loses its ability to produce the required torque, leading to a rapid drop in speed or complete stalling.

   - Excessive Current: The sudden change in rotor position and the loss of synchronism can cause excessive currents in the stator windings, potentially damaging the motor.

   - Instability: The motor becomes unstable, and if not quickly addressed by protective mechanisms, it can lead to overheating and mechanical failure.

Protective Measures

To prevent the motor from falling out of synchronism, several protective measures can be employed:

1. Overload Protection:

   - Devices such as overload relays and circuit breakers can disconnect the motor from the power supply if the current exceeds a predetermined limit.

2. Speed and Position Sensors:

   - Using sensors to monitor the rotor position and speed can help in detecting the onset of synchronization loss. Advanced motor controllers can adjust the excitation and load to keep the motor within safe operating limits.

3. Control Systems:

   - Modern synchronous motors are often equipped with control systems that can adjust the excitation voltage to manage the load angle and maintain synchronism under varying load conditions.

Conclusion

In summary, as the load on a synchronous motor increases, the load angle (delta) increases to generate the necessary torque. If the load continues to increase beyond the motor's capacity, the load angle can exceed its critical value, causing the motor to fall out of synchronism. This results in a loss of torque, excessive current, and potential mechanical instability. Protective measures, such as overload protection and advanced control systems, are essential to prevent and mitigate the consequences of such overload conditions.