VFD stands for variable frequency drive. The speed of the motor depends on the frequency its being powered by. At line frequency (60hz) the motor only goes one speed. If you provide it with a different frequency using the VFD, then the speed changes. The frequency actually is the speed at which the rotating magnetic field of the stator rotates. The rotor follows it because of induced current in the bars of the squirrel cage, and this induces a magnetic field that is attracted to the stator's magnetic field. There will always be some slip, and it's actually required for the motor to operate. Without the slip, the bars of the cage would not be cut by the magnetic field, and no current / field would be induced in the rotor.
In generator mode, the rotor is spun faster than the stators rotating magnetic field. The bars of the squirrel cage in the rotor still cut the magnetic field of the stator, but in the opposite direction. This causes the magnetic field of the rotor to induce current in the stator that ADDS to the current already in the stator. This is the power being produced. Power is being fed into the stator to create the rotating field, but its being added to, and thus, more current out than what went in.
What I want to do is detect the actual speed that the rotor is travelling when attached to, FOR EXAMPLE, an electric vehicle coasting down a hill. Lets say FOR EXAMPLE that this speed is 900 RPM. FOR EXAMPLE. IT COULD BE MORE, OR LESS, 900 is an EXAMPLE for discussions sake. What my VFD would do when the regenerative brake is applied, it would apply power to the stator of the motor at a frequency that is slightly less than 900 RPM. FOR EXAMPLE, it would feed the motor a frequency that creates a rotating magnetic field in the stator at 580 RPM. In actuallity, it may need to feed it a frequency that equals out to 590RMP or 595 RPM, but for discussion sake I am not concerned with that at this time. Since the new synchronous speed of the motor is now 590 RPM, since the motor is actually spinning 600 RPM, in theory, excess power will be produced. As the brake causes the motor to slow down, the speed of the motor will decrease, and the VFD will detect this and adjust the frequency of the power being applied to the stator.
I hope everyone who reads this will take the time to understand the purpose of the questions I'm trying to ask here.