Many readers have commented that while the generating state of an asynchronous motor seems easy to understand superficially—simply reverse the direction of the rotor current—it becomes a generator. However, upon closer examination, it remains somewhat unclear: how does the phase angle of the stator current change when the rotor current direction is reversed? Are there asynchronous generators? The author has experimented with a simple yet in-depth explanation method, achieving good results.
A generator is a device that converts mechanical energy into electrical energy. That is, by using a prime mover (steam turbine, diesel engine, water turbine, etc.) to rotate the rotor of an electric motor, electrical energy can be generated, i.e., electricity generation.
All motors are reversible, meaning they can function as both electric motors and generators. Asynchronous motors are no exception, but they have their own unique characteristics.
Special characteristics of asynchronous generators
First, let's look at how other generators generate electricity.
(1) DC generator
The stator of a DC generator is the magnetic pole, and the rotor is the armature winding, as shown in Figure a.
When the prime mover drives the rotor to rotate, the armature winding generates an induced electromotive force by cutting the stator magnetic field. This electromotive force is then drawn out through the commutator and brushes to obtain a DC voltage.
(2) Synchronous generator
The stator of a synchronous generator is a three-phase winding, and the rotor is a magnetic pole, as shown in Figure b.
When the prime mover drives the rotor to rotate, the three-phase windings of the stator cut the rotor's magnetic field to generate an induced electromotive force, thereby outputting a three-phase alternating voltage.
(3) Common characteristics of DC generators and synchronous generators
1) They all have a fixed magnetic field;
2) Driven by the prime mover, they cause the windings to cut the magnetic lines of force and generate electricity "from nothing to something".
(4) Special characteristics of asynchronous generators
An asynchronous motor has a three-phase stator winding and a short-circuit rotor winding, which itself has no magnetic field. Therefore, an asynchronous motor cannot generate electricity when driven by a prime mover alone. Although, theoretically, electricity can be generated using residual magnetism, it is not practically feasible.
For an asynchronous motor to generate electricity, a magnetic field must first be established. However, as is well known, an asynchronous motor only generates a rotating magnetic field after three-phase alternating current is applied to the three-phase windings of its stator. That is, to obtain a magnetic field, the stator windings of an asynchronous motor must be connected to a three-phase power supply, as shown in Figure c. Undoubtedly, under these conditions, it will operate as an asynchronous motor. However, if driven by a prime mover, causing the rotor speed to exceed the synchronous speed, it becomes an asynchronous generator.
Therefore, asynchronous motors cannot generate electricity independently "from nothing", which is their special characteristic.
