In a DC series motor, if the field winding gets open-circuited while running under no load, the motor will:

Explanation

In a DC series motor, torque is proportional to field flux and speed is inversely proportional to flux (N ∝ E_b / Φ). If the field winding opens or operates under no load, flux approaches zero, causing speed to rapidly rise to dangerously high levels that can destroy the rotor.