How an induction motor works as a generator during braking?

Induction motors produce torque based on the DIFFERENCE between the rotor speed and synchronous speed. The applied line voltage produces a rotating magnetic field whose lines of flux are cut by the squirrel cage rotor windings. When those windings cut flux lines, a current is induced that produces a rotating magnetic field. Thus you have 2 magnetic fields attracting each other. If the induction motor is sped up by an overhauling load, the difference can fall to 0 and thus no current in the rotor and thus no magnetic field in the rotor.

Torque can be in either direction depending upon the relative direction of rotation between the rotating magnetic field of the stator and the induced magnetic field of the rotor. If there is no line voltage applied, an induction motor will only generate very small amount of voltage due only to residual magnetism. If the windings are shorted, there will be a small about of braking, but only once it starts turning.

Regenerative braking only works when the line voltage is present on the stator. The line voltage and Hz can be varied by using a VFD, thus the amount of braking is variable. Plugging is just reversing: it too only works if line voltage is present on the stator. If an induction motor is at a standstill, nothing will hold it there. Braking torque is only available if lines of magnetic flux are cut. Either way, the motor needs to be moving and AC voltage is applied to the stator to get any kind of torque.

The term "regenerative braking" has been thrown around for many years regarding squirrel cage induction motors but it was a misnomer until the invention of VFD's that has "regenerative front ends".

The simplest form of braking with an induction motor is "plugging" or "plug reversal". In this case, you simply throw the motor into reverse while it is still running at full speed, usually just for a few seconds. If your power system and mechanical drive can take it, this form of braking will bring things to a screeching halt.

A less abusive form of braking is with "DC injection" where a low voltage DC is applied to the stator of the motor the instant that the supply voltage is removed. This will bring the motor to a gentler stop than does plugging, but it requires a lot more components in a conventional DOL control system to make it work. However, it is now available with some soft starters that have a "soft stop" feature.

Probably the best way to dynamically stop an induction motor is by controlling it with a VFD and then simply keep lowering the frequency and bringing it to a gentle stop. The downside with this system is there must be a way for the VFD to dissipate the braking energy.

On a low inertia load, the deceleration time can be set long enough to limit the rise on the VFD's DC bus voltage. If the load has more inertia or the deceleration time must be reduced, a set of braking resistors can be connected to the VFD to dissipate the energy and limit the rise in the DC bus voltage.

However, if the amount of braking energy required is high, then the VFD must either be equipped with a "regenerative front end" (input rectifier can also act as an inverter to back feed to the utility) or a separate regenerative unit can be connected to the DC bus and back to the 3-phase supply to the VFD.