Decelerating the motor quickly requires motor mechanical energy to be extracted and disposed - input DC voltage increases if this energy is returned to the DC input supply. When active braking is enabled, energy taken from DC power supply is used to brake the motor - this prevents DC voltage spike during fast deceleration. The mechanical energy of the motor and energy taken from DC source, both are dissipated within the motor itself. ACTIVE_BRAKE_EN should be set to 1b to enable active braking and avoid DC bus voltage spike during fast motor deceleration. Active braking can also be used during reverse drive (see Reverse Drive) or motor stop (see Active Spin-Down) to reduce the motor speed quickly without DC voltage spike.

The maximum limit on the current sourced from the DC bus (i_{dc_ref}) during active braking can be configured using ACTIVE_BRAKE_CURRENT_LIMIT. The power flow control during active braking is achieved by using both Q-axis (i_q) and D-axis (i_d) components of current. The D-axis current reference (i_{d_ref}) is generated from the error between DC bus current limit (i_{dc_ref}) and the estimated DC bus current (i_dc) using a PI controller. The i_dc value is estimated from the measured phase currents, phase voltage and DC bus voltage, using power balance equation (equating the instantaneous DC bus power to sum of all three instantaneous phase power assuming 100% efficiency). During active braking, the DC bus current limit (i_{dc_ref}) starts from zero and linearly increases to ACTIVE_BRAKE_CURRENT_LIMIT with current slew rate as defined by ACTIVE_BRAKE_BUS_CURRENT_SLEW_RATE. The gain constants of PI controller can be configured using ACTIVE_BRAKE_KP and ACTIVE_BRAKE_KI. Figure 7-52 shows the active braking id current control loop.

Figure 7-42 Active Braking Current Control Loop for i_{d_ref}