Siemens Sinamics S110 Function Manual page 169

In the event of a power failure, the Power Module will no longer be able to maintain the
supply voltage, particularly if the motor is drawing active power. To maintain the supply
voltage in the event of a power failure (e.g. for a controlled emergency retraction), the
Vdc_min controller can be activated for the drive. If the voltage threshold set in p1248 is
undershot, this drive will be decelerated so that its kinetic energy can be used to maintain
the supply voltage. The threshold should be considerably higher than the trip threshold of the
motor (recommendation: 50 V below the supply voltage). When the power supply is restored,
the Vdc controller is automatically deactivated and the drive will ramp back up to the speed
setpoint. If the supply voltage cannot be restored, the power supply will fail once the kinetic
energy of the drive has been 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 would
need to 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 a reduction in the
supply 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.
Function Manual
Function Manual, 01/2011, 6SL3097-4AB10-0BP3
Drive functions
7.1 Servo control
167