Siemens SINAMICS S120 Function Manual page 107

Description of Vdc_min control (p1240 = 2, 3)
Figure 3-21
In the event of a power failure, the Line Module can no longer supply the DC link voltage,
particularly if the Motor Modules in the DC link line-up are drawing active power. To maintain
the DC link voltage in the event of a power failure (e.g. for a controlled emergency
retraction), the Vdc_min controller can be activated for one or more drives. If the voltage
threshold set in p1248 is undershot, these drives are decelerated so that their kinetic energy
can be used to maintain the DC link voltage. The threshold should be considerably higher
than the shutdown threshold of the Motor Modules (recommendation: 50 V below the DC link
voltage). When the power supply is re-established, the Vdc controller is automatically
deactivated and the drives approach the speed setpoint again. If the power supply cannot be
re-established, the DC link voltage collapses if the kinetic energy of the drives is exhausted
with an active Vdc_min controller.
Note
You must make sure that the converter is not disconnected from the power supply. It could
become disconnected, for example, if the line contactor drops out. The line contactor should
have an uninterruptible power supply (UPS), for example.
Description of Vdc_min control without braking (p1240 = 8, 9)
As with p1240 = 2, 3, however, active motor braking can be prevented by reducing the DC
link voltage. The effective upper torque limit must not be less than the torque limit offset
(p1532). The motor does not switch to regenerative mode and no longer draws any active
power from the DC link.
Drive functions
Function Manual, (FH1), 10/2008, 6SL3097-2AB00-0BP5
Switching Vdc_min control on/off (kinetic buffering)
Servo control
3.14 Vdc control
107