What could lead an AC electric motor to run too fast?

An AC electric motor can run too fast primarily due to excessive applied voltage or shorted field windings. When the voltage applied to the motor exceeds the rated voltage, it increases the speed at which the motor operates. This happens because the motor's magnetic field strength rises, resulting in a higher rotor speed.

Additionally, if there are shorted field windings, this can also create an imbalance in the magnetic field, which can lead to the rotor spinning at an unexpectedly high rate. Both these conditions compromise the motor's designed operational limits and can be detrimental to its safe and efficient functioning.

Other options, while they may affect the performance in various ways, do not directly cause the motor to run at an increased speed. Insufficient power supply typically results in a reduced operational capacity, worn bearings can lead to increased friction and potential overheating, while overheating is usually a sign of excessive current or a failure in the motor’s performance but not directly linked to speeding up the motor.