AC motors can generate electricity, but their ability to generate electricity depends on the operating mode – AC motors are essentially a unity of “motor” and “generator”, and their core principles are based on the law of electromagnetic induction. The switching between “electric” or “power generation” functions is achieved only by changing the energy input method (electrical energy → mechanical energy or mechanical energy → electrical energy).

1、 Core principle: “Bidirectionality” of electromagnetic induction
Both the “electric mode” and “power generation mode” of AC motors revolve around electromagnetic induction, with the only difference being the “energy flow direction”:

Electric mode (power consumption): external input AC power → stator winding generates rotating magnetic field → magnetic field drives rotor (conductor or winding) to cut magnetic induction lines → rotor generates induced current → current is subjected to ampere force in the magnetic field → drives rotor rotation (electrical energy → mechanical energy).

Power generation mode (electricity generation): External mechanical force (such as engine, windmill, water turbine) drives the rotor to rotate → the magnetic field generated by the rotor (permanent magnet or excitation winding) rotates with the rotor → the magnetic induction line cut and rotated by the stator winding → the stator winding induces alternating electromotive force → after connecting to the load, outputs alternating current (mechanical energy → electrical energy).

2、 Three key conditions for AC motor power generation
Not all AC motors can generate electricity just by turning on. The following three core conditions must be met to output effective electrical energy:

1. There must be a “rotating magnetic field” (magnetic source)
To induce electromotive force in the stator winding, a “variable magnetic field” is first required (rotating magnetic field is the most typical form), and the sources of magnetic sources are divided into two categories:

Permanent magnet type: The rotor itself is a permanent magnet (such as a permanent magnet synchronous motor), which does not require additional power supply and directly generates a rotating magnetic field during rotation. It has a simple structure and is commonly used in small generators (such as household wind turbines and portable power generation equipment).
Excitation type: The rotor is a common winding (such as asynchronous motors and synchronous generators), which requires an external “excitation current” (DC or AC) to generate a magnetic field in the rotor (similar to an electromagnet).

2. There must be a “mechanical driving force” (overcoming reverse torque)
The essence of power generation is to convert mechanical energy into electrical energy, so there must be external mechanical force to drive the rotor to rotate, and the speed must meet two requirements:
Higher than “synchronous speed” (for synchronous motors): The power generation frequency of synchronous motors (such as 50Hz) is strictly related to the speed (formula: speed n=60f/p, f is frequency, p is pole number), and the speed needs to be precisely controlled by mechanical force to ensure stable output frequency (such as steam turbine generators in power plants).
Above the critical speed corresponding to the slip rate (for asynchronous motors): When an asynchronous motor generates electricity, the rotor speed needs to be slightly higher than the speed of the stator rotating magnetic field (“super synchronous speed”) in order for the stator winding to cut the magnetic field and generate electrical energy (otherwise it is only “electric operation”).

3. There must be a “closed loop” (load or energy storage)
The stator winding induces an “alternating electromotive force”, which needs to be connected to a closed circuit (such as a resistive load, battery, or power grid) to form an “alternating current” – if there is an open circuit (no load), only electromotive force exists but no electrical energy output, and the winding insulation may be damaged due to high voltage.