regenerative breaking at asynchronous machine

http://en.wikipedia.org/wiki/Regenerative_braking

Unfortunately, the reply by "guest" is not very useful. That Wikipedia article only discusses regenerative braking in very broad terms, mostly as applied to electric vehicles.

Control of induction (asynchronous) machines is complex because both the stator and rotor currents must be controlled and regulated using a single input waveform. The basic principle is:

• The current in the stator windings results in a rotating magnetic flux vector in the airgap of the machine.
• The rotor does not rotate at exactly the same speed as the stator flux vector. The difference, called "slip," results in a current in the rotor cage. This current creates a rotor magnetic flux vector.
  
• The interaction between the rotor flux vector and the stator flux vector creates torque.
• If the rotor is turning slower than the stator flux vector, the torque will in a direction to speed the rotor up - accelerating it.
• If the rotor is turning faster than the stator flux vector, the torque will be in a direction to slow it down - this is regenerative braking.

The actual details of the control scheme for creating regenerative braking are complex, and they vary depending on what type of control you want to achieve. Control schemes are categorized by what modes of operation they allow the motor to perform: motoring, generating, and forward or reverse rotation. The most common control methods are:

Single Quadrant: Forward rotation only, motoring only.

Two Quadrant: Forward or reverse rotation, motoring only.

Four Quadrant: Forward or reverse rotation, motoring or generating (braking).

The particular type of controller you need will depend on how you want to operate the motor, and what type of control precision you need: controllers can regulate torque, speed or position. Precision can vary from coarse speed or torque control (e.g., for a fan or an electric vehicle) to precise high speed position control (e.g., for a machine tool or a robotic actuator).

As you can see, the topic is complex.