The correct answer is: A. High starting torque.
Differentially compound D.C. motors have a series field winding and a shunt field winding. The series field winding is connected in series with the armature, while the shunt field winding is connected in parallel with the armature. The series field winding has a higher resistance than the shunt field winding, so it produces a stronger magnetic field when current flows through it. This stronger magnetic field produces a higher starting torque.
Low starting torque is required for applications where the motor is started from a standstill and needs to accelerate quickly. Variable speed is required for applications where the motor needs to operate at different speeds. Frequent on-off cycles are required for applications where the motor is turned on and off frequently.
Here are some additional details about each option:
- High starting torque: Differentially compound D.C. motors have a high starting torque, which makes them suitable for applications where the motor needs to start from a standstill and accelerate quickly. For example, they are often used in cranes, hoists, and elevators.
- Low starting torque: Low starting torque is required for applications where the motor is started from a standstill and needs to accelerate slowly. For example, they are often used in conveyor belts and fans.
- Variable speed: Variable speed is required for applications where the motor needs to operate at different speeds. For example, they are often used in machine tools and textile machines.
- Frequent on-off cycles: Frequent on-off cycles are required for applications where the motor is turned on and off frequently. For example, they are often used in power tools and appliances.