A question about braking and deceleration of an AC motor
Induction motors 101.

In an induction motor, the supplied voltage creates a rotating magnetic field around the rotor. If you consider a two-pole motor running on a 60 Hz system, the rotating field moves at 60 Hz * 60 sec/min = 3600 RPM.

In this case, 3600 RPM would be called the "synchronous speed." Meaning that if the rotor actually was spinning at 3600 RPM, then the rotor and rotating field are in-sync.

But in normal motoring operation, the rotor spins a bit slower than 3600 RPM. This speed difference, called "slip" gives rise to a current in the rotor which then generates torque. The larger the slip, the more torque the motor will produce, to a point. So, basically, as you apply a load to an induction motor, it slows down a bit, and the torque increases, until the motor and load balance each other out (or maybe if the load is too much, the motor will stall).

The way you put an induction motor into regeneration is to simply increase the mechanical speed of rotation higher than synchronous speed. In our example, this means you need to make the rotor spin faster than 3600 RPM. You could do this, for example, by connecting another motor (maybe a gas motor) to your electric motor, then over-drive the electric motor with the gas motor. Now you have an induction generator rather than an induction motor. You will be supplying power to the electrical grid instead of using the power from the grid. You don't need to do anything else. It just happens.

Now lets consider a motor running from a variable frequency drive (VFD). The VFD can control the frequency of the voltage applied to the motor, which means it can control the motor speed. If the motor is turning a giant heavy turntable, for example, and the VFD needs to slow it down quickly, then the VFD will apply a frequency lower than the actual rotation of the motor. Because the electrical frequency is lower than the actual rotor frequency, the motor will be in regen. This will cause mechanical energy in the turntable to be converted into electrical energy. The VFD may even have an over-voltage failure when this happens, unless it has some way to dump the extra energy (for example into a load resistor). Because of the way they are designed, VFD's usually cannot put energy back into the grid.

There is a lot of other stuff that could be written about this topic, but these are the basics. If the rotor is spinning slower than the electrical frequency, then the motor is operating in the usual fashion, as a motor. But if an external force speeds up the rotor faster than the electrical frequency, then the motor will naturally transition into regen as it converts mechanical energy into electrical energy.