What type of excitation is given to the rotor winding of a synchronous motor?

Rotor Winding Excitation in a Synchronous Motor

Definition: The excitation of the rotor winding in a synchronous motor involves the process of supplying current to the rotor winding to produce a magnetic field. This magnetic field interacts with the rotating magnetic field of the stator to produce torque and maintain synchronous speed. Proper excitation is critical for the efficient and stable operation of a synchronous motor.

Correct Option Analysis:

Option 1: DC supply at 100-250 V

This is the correct option. The rotor winding of a synchronous motor is excited using a direct current (DC) supply. This DC excitation creates a constant magnetic field in the rotor. When the stator produces a rotating magnetic field due to the supply of an alternating current (AC), the interaction between these fields ensures that the rotor locks into synchronism with the stator's magnetic field, maintaining a constant speed. The voltage range of 100-250 V is typical for rotor excitation systems, depending on the motor's design and size.

DC excitation is provided by an external source such as a DC generator (called an exciter) or a rectifier connected to an AC supply. The primary purpose of this excitation is to create the necessary magnetic flux in the rotor, which is essential for the motor's operation at synchronous speed.

Advantages of DC Excitation:

• Precise control of the rotor magnetic field, enabling better voltage regulation and stability.
• Efficient torque production, as the interaction between the stator and rotor magnetic fields is optimized.
• Easy control of reactive power by varying the field excitation, which is beneficial in power systems.

Working Principle:

In a synchronous motor:

• The stator winding is supplied with a three-phase AC current, creating a rotating magnetic field.
• The rotor winding is excited with a DC supply, generating a steady magnetic field in the rotor.
• The interaction between the rotating magnetic field of the stator and the constant magnetic field of the rotor causes the rotor to lock in synchronism with the stator field.

This synchronous operation ensures that the rotor rotates at the same speed as the stator's magnetic field, maintaining a constant speed regardless of load variations (up to a certain limit).

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: AC supply at 400 V

This option is incorrect. The rotor winding of a synchronous motor is not directly excited with an AC supply. If an AC supply were applied to the rotor winding, it would produce an alternating magnetic field rather than the constant magnetic field required for synchronous operation. AC supply is used for the stator winding, not the rotor.

Option 3: Revolving field

This option is misleading. The term "revolving field" refers to the rotating magnetic field produced in the stator by the three-phase AC supply. It does not pertain to the excitation of the rotor winding. The rotor winding requires a steady DC supply to produce a constant magnetic field, which interacts with the revolving magnetic field of the stator.

Option 4: Induction from stator currents

This option is incorrect. While induction is the principle behind the operation of induction motors, a synchronous motor operates differently. The rotor winding of a synchronous motor cannot rely solely on induction from stator currents to create the necessary magnetic field for synchronous operation. Instead, it requires a dedicated DC excitation for the rotor winding.

Conclusion:

The excitation of the rotor winding in a synchronous motor is a crucial aspect of its operation. The correct method involves supplying a DC current to the rotor winding to produce a constant magnetic field. This ensures efficient torque production and stable synchronous operation. Evaluating the incorrect options highlights the unique requirements of synchronous motors and the importance of proper rotor excitation using a DC supply.