The correct answer is: D. Synchronous motor.
A synchronous motor is an AC electric motor that rotates at a speed that is synchronized with the frequency of the AC power supply. The speed of a synchronous motor is determined by the frequency of the AC power supply and the number of poles in the stator and rotor. The length of the air gap between the stator and rotor has a significant effect on the power factor of a synchronous motor. A shorter air gap results in a higher power factor, while a longer air gap results in a lower power factor.
In a synchronous motor, the stator windings are supplied with AC power, which creates a rotating magnetic field. The rotor windings are connected to a DC power source, which creates a magnetic field that is always aligned with the stator field. The rotor is pulled into alignment with the stator field, and the motor rotates at the same speed as the stator field.
The power factor of a synchronous motor is the ratio of the real power to the apparent power. The real power is the power that is actually used by the motor, while the apparent power is the product of the voltage and current. The power factor is important because it affects the amount of current that is drawn from the power supply. A low power factor results in a higher current draw, which can cause problems with the power supply.
The length of the air gap between the stator and rotor has a significant effect on the power factor of a synchronous motor. A shorter air gap results in a higher power factor, while a longer air gap results in a lower power factor. This is because a shorter air gap results in a stronger magnetic field, which requires less current to produce the same amount of torque.
In conclusion, the length of the air gap to be provided in a synchronous motor is primarily determined by power factor consideration. A shorter air gap results in a higher power factor, while a longer air gap results in a lower power factor.