Siemens SINAMICS S120 Function Manual page 137

Vdc_min control
Figure 3-16
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 (p1240 = 2, 3).
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
If a voltage recovery is expected, 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 must be supplied, e.g. from an uninterruptible power
supply (UPS).
Vdc_min control without braking
As for Vdc_min control with braking, 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.
Drive functions
Function Manual, (FH1), 01/2013, 6SL3097-4AB00-0BP3
Switching Vdc_min control on/off (kinetic buffering)
Servo control
3.14 Vdc control
135