Which type of motor is most likely to reach a dangerous speed when operated at normal voltage and no load?

The series motor is specifically designed such that the field winding is connected in series with the armature winding. This configuration results in the field strength being directly proportional to the armature current. When a series motor operates under no load conditions, the current through the armature decreases significantly. Consequently, the magnetic field weakens, which leads to an increase in speed, as there is less back EMF generated to oppose the armature current.

At normal voltage, with minimal resistance or load, the series motor can accelerate uncontrollably, reaching dangerous speeds very quickly. The lack of a sufficient magnetic field combined with the high speed can result in mechanical failure or damage to the motor. Thus, among the various types of motors, a series motor poses the most risk of reaching dangerously high speeds when operated at normal voltage and no load.