The back e.m.f. of a synchronous motor depends on

Solution (By Examveda Team)

The correct answer is (C) load angle.
Let's break down why:
Option A: Speed: Synchronous motors operate at a fixed speed determined by the frequency of the power supply and the number of poles. Therefore, the back EMF isn't *directly* dependent on speed in a *variable* way.
Option B: Load: While the motor adjusts to the load, the load itself doesn't *directly* dictate the back EMF's magnitude. The load affects the load angle.
Option C: Load Angle: The back EMF (Eb) and the applied voltage (V) are related by the equation and are also affected by the load angle (δ).
As the load increases, the rotor lags behind the rotating magnetic field, which increases the load angle (δ). This change in load angle directly affects the phase relationship between the applied voltage and the back EMF, thus influencing the magnitude of the back EMF. A larger load angle generally results in a lower effective back EMF.
Option D: All of the above: While load and speed have related effects, the primary and most direct influence on the back EMF's magnitude is the load angle.
Therefore, the back EMF is most directly dependent on the load angle.