How can a variable field current change the speed of a direct-current shunt motor?

In a direct-current (DC) shunt motor, the speed can be adjusted by varying the field current. Because the motor generates a voltage proportional to both its speed and the magnetic flux, the field current directly influences the magnetic flux produced by the stator poles.1

When the field current is reduced, the magnetic flux decreases. To maintain the same generated voltage (which must balance the supply voltage), the motor must rotate at a higher speed. Consequently, reducing the field current increases the motor's no-load speed.1

While this method allows for higher operating speeds, it results in a reduction of available torque for a given armature current because the magnetic flux is lower. Despite this, the motor remains capable of providing the same mechanical power at the higher speed and lower torque.1