Siemens SINAMICS S120 Function Manual page 150

Operating Modes
Vector V/f control (r0108.2 = 0)
Failure of the supply voltage or supply for the DC link.
-- Remedy
Specify a regenerative torque for the rotating drive to compensate the
existing losses, thereby stabilizing the voltage in the DC link (kinetic
buffering).
Properties
S Vdc control
-- This comprises Vdc_max control and Vdc_min control (kinetic buffering),
which are independent of each other.
-- Joint PI controller. The dynamic factor is used to set Vdc_min and Vdc_max
control to a smoother or harder setting independently of each other.
S Vdc_min control (kinetic buffering)
-- With this function, the kinetic energy of the motor is used for buffering the
DC link voltage in the event of a momentary power failure, thereby delaying
the drive.
S Vdc_max control
-- This function can be used to control momentary regenerative load without
shutdown with "overvoltage in the DC link".
-- Vdc_max control is only recommended with a supply without active
closed-loop control for the DC link and without feedback.
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© Siemens AG, 2005. All rights reserved
SINAMICS S120 Function Manual, 06/05 Edition