The magnitude of armature current in synchronous motor operation

In synchronous motor operation, the armature current plays a crucial role in determining the motor's performance and behavior. The magnitude of the armature current is influenced by various factors, including the level of excitation. The correct answer to the question is option 'B', which states that the armature current has a large value and lags for low excitation.

Explanation:

1. Armature current in synchronous motor:
The armature current in a synchronous motor is the current flowing through the motor's armature winding. It is responsible for producing the magnetic field necessary for the motor's operation. The armature current is influenced by the excitation level and other factors, such as the load on the motor.

2. Excitation level:
The excitation level refers to the strength of the magnetic field produced by the rotor's field winding. It is determined by the excitation current, which is supplied to the rotor winding. The excitation current creates a magnetic field that interacts with the stator's magnetic field, resulting in the motor's operation.

3. Armature current magnitude and excitation:
The magnitude of the armature current in synchronous motor operation is influenced by the excitation level. When the excitation level is low, the armature current tends to have a large value. This is because a lower excitation level leads to a weaker magnetic field produced by the rotor winding. To compensate for the weaker magnetic field, a larger armature current is required to generate the necessary torque.

4. Armature current phase angle:
The phase angle of the armature current in a synchronous motor is also influenced by the excitation level. When the excitation level is low, the armature current tends to lag behind the stator's magnetic field. This lagging of the armature current is due to the fact that the weaker magnetic field produced by the rotor winding is not fully aligned with the stator's magnetic field.

Conclusion:

In conclusion, the magnitude of the armature current in synchronous motor operation has a large value and lags for low excitation. This is because a lower excitation level leads to a weaker magnetic field, requiring a larger armature current to generate the necessary torque. The phase angle of the armature current also lags behind the stator's magnetic field in this scenario.