Brake Resistor in Regenerative Case for Elevator
How does the Elevator Controller differentiate that the motor is in Braking mode or Driving mode?


In a 4-quadrant drive, motoring vs. braking would be distinguished by determine whether  positive or negative torque is required to maintain the required speed and direction.


  Once the Controller has detected (or decided) that the motor is in Braking mode then what is its switching pattern for the Inverter and the Brake-chopper?


There is no chopper in a 4-quadrant drive. Choppers are only used when braking energy is dissipated in resistors. The inverter switching pattern does not change, between motoring and braking. For motoring, the fundamental waveform frequency is above the mechanical frequency (speed) and the motor slip is positive. For braking, the fundamental waveform frequency is above the mechanical frequency and the slip is negative. The control strategy used to accomplish this varies somewhat from one manufacturer to another.

Added Information re Comments:

Motor current feedback alone is not sufficient to determine the torque direction. Ultimately, the controller needs to "know," at every instant in time, where the operating point is on the motor's torque vs. slip curve. That requires a motor shaft encoder and a means for determining the motor current and voltage phasor values.

It is possible to make a 4-quadrant drive that uses a braking resistor rather than returning braking power to the utility. That is inefficient, but it might be done for an elevator control to allow braking during a power outage. However, that would not eliminate the need of mechanical emergency braking. Elevator control is a very specialized area that uses specialized equipment, not standard industrial motor control equipment.

Braking may not be needed in situations such as counterweighting at maximum load and high ratio worm gear drive. There may be some need for braking if the dc bus is overcharged due to high energy power line transients such as ringing transients caused by power factor correction capacitor switching.