The correct answer is: D. both the speed and rotor flux.
The stator back emf (Eb) is the induced emf in the stator windings of a synchronous motor. It is proportional to the speed of the motor and the rotor flux. The speed of the motor is the rate at which the stator magnetic field rotates relative to the rotor. The rotor flux is the magnetic field produced by the rotor current.
The stator back emf opposes the applied voltage, and the difference between the two is the terminal voltage of the motor. The terminal voltage is the voltage that is available to drive the load.
The stator back emf is also responsible for the torque produced by the motor. The torque is proportional to the square of the stator back emf.
The stator back emf is a very important parameter in the operation of a synchronous motor. It determines the speed of the motor, the terminal voltage, and the torque produced by the motor.
Here is a brief explanation of each option:
- Option A: d.c. excitation only. The d.c. excitation is the current that is supplied to the rotor windings. It is responsible for producing the rotor flux. However, the stator back emf is also affected by the speed of the motor. Therefore, option A is incorrect.
- Option B: speed of the motor. The speed of the motor is the rate at which the stator magnetic field rotates relative to the rotor. The stator back emf is proportional to the speed of the motor. However, the stator back emf is also affected by the rotor flux. Therefore, option B is incorrect.
- Option C: load on the motor. The load on the motor is the resistance that is connected to the motor terminals. The load affects the current that flows through the motor. However, the stator back emf is not affected by the load. Therefore, option C is incorrect.
- Option D: both the speed and rotor flux. The stator back emf is proportional to the speed of the motor and the rotor flux. Therefore, option D is the correct answer.