The correct answer is: C. both (A) and (B).
The armature torque of a DC shunt motor is proportional to both the field flux and the armature current. The field flux is the magnetic field produced by the field windings, and the armature current is the current flowing through the armature windings. The torque produced by the motor is proportional to the product of the field flux and the armature current.
The field flux can be increased by increasing the current flowing through the field windings, or by increasing the number of turns in the field windings. The armature current can be increased by increasing the voltage applied to the armature windings, or by decreasing the resistance of the armature windings.
The torque produced by the motor can be increased by increasing either the field flux or the armature current. However, increasing the field flux will also increase the power consumption of the motor, while increasing the armature current will also increase the heating of the motor. Therefore, the optimum operating point for a motor is a compromise between torque and power consumption.
The following is a brief explanation of each option:
- Option A: field flux only. The torque produced by a DC shunt motor is proportional to the field flux. However, the field flux is not the only factor that affects the torque produced by the motor. The armature current also affects the torque produced by the motor.
- Option B: armature current only. The torque produced by a DC shunt motor is proportional to the armature current. However, the armature current is not the only factor that affects the torque produced by the motor. The field flux also affects the torque produced by the motor.
- Option D: none of the above. The torque produced by a DC shunt motor is proportional to both the field flux and the armature current.