Why is it said that asynchronous motors have no back electromotive force?

Asynchronous motors only have no back electromotive force when no voltage is applied to the stator winding. If voltage is applied to the stator, an air gap magnetic field will be established. When the rotor rotates, it will induce rotor current, which in turn generates rotor magnetic field. The rotor magnetic field affects the air gap magnetic field. As it is believed that the air gap magnetic field is constant (determined by an external voltage source), the stator current will generate a new current component, which establishes the stator magnetic field and counteracts the rotor magnetic field to ensure that the amplitude and phase of the air gap magnetic field remain unchanged. This balancing process achieves the input of electrical energy from the stator side into the motor, which is then converted into mechanical energy through the rotor.

The above is the basic operating principle of asynchronous motors. So obviously, if a voltage is applied to the stator, regardless of whether the rotor rotates or not, the air gap magnetic field will cut the stator winding when it rotates. Since the stator winding is fixed on the stator core, the stator winding will move relative to the air gap magnetic field, generating a back electromotive force in the stator winding to counteract the external stator voltage. Otherwise, such a large external voltage would be directly applied to the stator coil, resulting in a large current.

But if the stator is not connected to voltage, even if the rotor is rotating, or if the stator voltage is disconnected during the rotor rotation process, the air gap magnetic field disappears, and the stator coil has no back electromotive force. However, for permanent magnet motors, as the rotor is a permanent magnet, regardless of whether there is voltage in the stator, as long as the rotor of the permanent magnet rotates, it will generate a rotating magnetic field, which will also cut through the stator winding, thereby generating a back electromotive force in the stator winding. This is the main difference between asynchronous motors and permanent magnet motors. When rotating a permanent magnet motor, do not touch the motor stator and the cables connected to it.