The correct answer is D. All of the above.
The e.m.f. generated in a D.C. generator is directly proportional to the flux per pole, the speed of the armature, and the number of poles.
The flux per pole is the magnetic field strength at the poles of the generator. The higher the flux per pole, the greater the e.m.f. generated.
The speed of the armature is the speed at which the armature coils rotate in the magnetic field. The faster the armature rotates, the greater the e.m.f. generated.
The number of poles is the number of north and south poles on the stator of the generator. The more poles there are, the greater the e.m.f. generated.
The e.m.f. generated in a D.C. generator can be calculated using the following formula:
$E = \frac{N\phi}{A}B\omega$
where:
- $E$ is the e.m.f. generated in volts
- $N$ is the number of turns in the armature coil
- $\phi$ is the flux per pole in webers
- $A$ is the area of the armature coil in square meters
- $B$ is the magnetic field strength in teslas
- $\omega$ is the angular speed of the armature in radians per second
The e.m.f. generated in a D.C. generator is also affected by the resistance of the armature windings. The higher the resistance of the armature windings, the lower the e.m.f. generated.
The e.m.f. generated in a D.C. generator is also affected by the brush contact resistance. The higher the brush contact resistance, the lower the e.m.f. generated.