Directly connecting an AC motor designed for 120 volts to a 240 volt power supply is almost certain to cause rapid damage to the motor, which may burn out within seconds to minutes.
The following chain reactions will occur specifically:
Fatal cause: magnetic saturation and current loss of control
The magnetic flux design of the motor core approaches saturation at 120V.
After the voltage doubles to 240V, the iron core will enter a deep magnetic saturation state.
At this point, the excitation current is no longer increasing linearly, but rather exponentially rising, typically reaching 5-10 times or even higher than the rated current.

Direct consequences: overheating and burning
The temperature of the winding rises sharply within a few tens of seconds, causing the insulation layer to quickly melt and carbonize, resulting in inter turn short circuits.
The ultimate result is the burning of the winding, which may be accompanied by smoke, a burnt smell, and even the burning of the motor.

Mechanical and operational performance (at the moment before burning)
Loud noise: emitting a very dull and intense buzzing sound, due to magnetic saturation causing strong vibration of the iron core.
Abnormal rotational speed:
Asynchronous motor: Due to excessive current and magnetic saturation, the torque characteristics may deteriorate, making it impossible to start normally, or the speed may be unstable and shake violently.
Series/parallel excited motors: The speed will soar significantly (ideally close to doubling), which may cause the rotor to scatter and be damaged due to centrifugal force.
Protection device action: If there is a suitable circuit breaker or fuse in the circuit, it may trip before the winding burns out, but usually the action is not fast enough.

Special risk: single-phase motor with starting capacitor
The withstand voltage of starting and running capacitors is usually around 180V~250V, and directly connecting to 240V may barely withstand it in the short term.
But the bigger problem is that the capacitor current will seriously exceed the standard, causing the capacitor to rapidly expand and burst, which may spray oil or explode the shell.
Situations that require special distinction

Universal (series excited) motor: If you are referring to motors such as hand drills and vacuum cleaners with carbon brushes, although theoretically they can operate with reduced speed, the no-load speed will be too high, and the commutator will produce serious sparks, ultimately leading to damage due to overheating or commutator burnout.
Not safe usage.
Motors with wide voltage design: If the motor nameplate clearly indicates “110V-240V” or “120/240V”, it is necessary to change the wiring method (such as changing the two parallel windings to series).
Cannot directly connect to 240V.

Conclusie
Never attempt to directly plug a 120V AC motor into a 240V socket.
The lightest consequence is burning the motor, and the most serious is causing a fire or electric shock accident.
The correct approach is to use a 240V to 120V step-down transformer (note that the power should be greater than 1.5 times the rated power of the motor).
Alternatively, replace the motor with one designed specifically for 240V voltage.