The correct answer is A.
A compensating winding is a winding that is placed in the stator of an AC series motor to reduce the effects of armature reaction. Armature reaction is the magnetic field that is produced by the current flowing through the armature winding. This magnetic field can interact with the main magnetic field of the motor, causing a number of problems, including:
- A decrease in the torque produced by the motor
- An increase in the current drawn by the motor
- A decrease in the efficiency of the motor
A compensating winding can help to reduce these problems by providing a counter-magnetic field that cancels out the effects of armature reaction. This can improve the performance of the motor and make it more efficient.
Option B is incorrect because a compensating winding does not increase the torque produced by the motor. In fact, it can actually decrease the torque in some cases.
Option C is incorrect because a compensating winding does not reduce sparking at the brushes. This is usually done by using a commutator with more segments or by using a brush with a higher resistance.
Option D is incorrect because a compensating winding is used in AC series motors.