Siemens Sinamics S120 Function Manual page 118

Servo control
3.14 Vdc control
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. When the set
voltage threshold of p1248 is fallen below, these drives are switched into the generator mode
so that they can buffer the DC link voltage with their kinetic energy. The threshold should be
set considerably higher than the shutdown threshold of the Motor Modules
(recommendation: 50 V below the DC link voltage). When the line supply returns, the Vdc
controller is automatically inactive. The drives approach the speed setpoint again. If the line
supply does not return, then the DC link voltage collapses as soon as the kinetic energy of
the drives is exhausted with an active Vdc_min controller.
Note
You must make sure that the drive line-up is not disconnected from the line supply. It could
become disconnected, for example, if the line contactor drops out. The line contactor should
e.g. be supplied from an uninterruptible power supply (UPS).
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 go into the generator mode and does not draw any active
power from the DC link.
Description of Vdc_max control (p1240 = 1, 3)
Figure 3-24
116
Switching-in/switching-out the Vdc_max control
Function Manual, (FH1), 01/2012, 6SL3097-4AB00-0BP2
Drive functions