The correct answer is A. Reducing the field current.
A DC shunt motor is a type of DC motor that uses a permanent magnet field. The speed of a DC shunt motor is determined by the field current and the armature current. The field current is the current that flows through the field coils, which create the magnetic field. The armature current is the current that flows through the armature windings, which create the torque that turns the motor.
The speed of a DC shunt motor can be increased by reducing the field current. This is because a reduced field current creates a weaker magnetic field, which allows the motor to turn faster.
The other options are incorrect because they would either decrease the speed of the motor or have no effect on the speed of the motor.
- Option B: Decreasing the armature current would decrease the torque of the motor, which would cause the motor to slow down.
- Option C: Increasing the armature current would increase the torque of the motor, which would cause the motor to speed up.
- Option D: Increasing the excitation current would increase the magnetic field, which would cause the motor to slow down.