The correct answer is: A. no-load and greatly over-excited fields.
A synchronous motor is an AC electric motor that uses a rotating magnetic field to induce current in the rotor windings, which in turn creates a magnetic field that interacts with the stator field to produce torque. The rotor of a synchronous motor can be either wound or solid. In a wound rotor, the rotor windings are connected to slip rings, which allow an external voltage to be applied to the rotor. In a solid rotor, the rotor is made of a ferromagnetic material, such as steel, and the rotor current is induced by the stator field.
Synchronous motors are used in a variety of applications, including power generation, industrial drives, and traction motors. In power generation, synchronous motors are used to drive generators that produce AC electricity. In industrial drives, synchronous motors are used to drive a variety of machinery, such as pumps, fans, and compressors. In traction motors, synchronous motors are used to power electric locomotives and other rail vehicles.
Power factor correction is a technique used to improve the power factor of an AC electrical system. The power factor of an AC electrical system is the ratio of the real power to the apparent power. The real power is the power that is actually used by the load, while the apparent power is the product of the voltage and current. A low power factor indicates that the load is drawing reactive power from the system, which can cause problems such as voltage drop and increased losses in the system.
Synchronous motors can be used to improve the power factor of an AC electrical system by operating at a leading power factor. A leading power factor occurs when the current leads the voltage in an AC electrical system. This can be achieved by over-exciting the field of the synchronous motor. When the field is over-excited, the magnetic field produced by the stator is stronger than the magnetic field produced by the rotor. This causes the rotor to lag behind the stator, which results in a leading power factor.
The following are the brief explanations of each option:
- Option A: no-load and greatly over-excited fields. This is the correct answer. When the synchronous motor is operated at no-load and greatly over-excited fields, the rotor will lag behind the stator, which results in a leading power factor.
- Option B: no-load and under-excited fields. This is not the correct answer. When the synchronous motor is operated at no-load and under-excited fields, the rotor will lead the stator, which results in a lagging power factor.
- Option C: normal load with minimum excitation. This is not the correct answer. When the synchronous motor is operated at normal load with minimum excitation, the rotor will lag behind the stator, which results in a leading power factor. However, the power factor will not be as high as when the motor is operated at no-load and greatly over-excited fields.
- Option D: normal load with zero excitation. This is not the correct answer. When the synchronous motor is operated at normal load with zero excitation, the rotor will lock to the stator field and the motor will not operate.