If the field of a shunt motor becomes disconnected during operation, what will happen?

When the field of a shunt motor becomes disconnected during operation, the motor will experience an increase in its speed, leading to a condition known as overspeeding. This occurs because shunt motors are designed to have their field windings connected in parallel with the armature winding, which allows them to maintain a relatively constant speed under varying loads.

When the field winding disconnects, the field current drops to zero. This causes a significant reduction in the magnetic flux within the motor. According to the relationship between voltage, speed, and flux (E = kΦN, where E is the back EMF, k is a constant for the motor, Φ is the magnetic flux, and N is the speed), when the flux (Φ) approaches zero, the motor's speed (N) must increase to maintain the same back EMF to balance the applied voltage.

As a result, the motor accelerates to potentially dangerous speeds that can lead to mechanical failure or damage unless protective measures are in place. This understanding is critical for power engineers to ensure proper operation and safety of electrical machinery.