In what scenario is the speed of a series motor likely to become dangerously high?

The speed of a series motor is likely to become dangerously high when it is operated without a load. In a series motor, the field windings are connected in series with the armature windings. This configuration means that the field strength is dependent on the armature current. When the motor is under load, there is sufficient current flowing through both the armature and field windings, creating a magnetic field that limits the motor's speed.

However, when the motor is operated without a load, the armature current decreases significantly, leading to a weakened magnetic field. As the magnetic field becomes weaker, the motor tends to draw more speed to maintain the necessary back EMF, potentially reaching a runaway condition. This can result in extremely high speeds, which can be dangerous as it may lead to mechanical failure or damage.

In contrast, other scenarios, such as an incomplete circuit or improper load connection, usually cause the motor to either stall or operate inefficiently rather than reach dangerously high speeds. Operating at high voltages may also strain the motor but does not inherently lead to the same risk of excessive speed as running without a load.