Siemens SINAMICS S120 Function Manual page 108

Servo control
3.14 Vdc control
Description of Vdc_max control (p1240 = 1, 3)
Figure 3-22
With Infeed Modules without feedback or in the event of a power failure, the DC link voltage
can increase until it reaches the shutdown threshold when drives in the DC link line-up are
decelerated. To prevent the system from shutting down due to a DC link overvoltage, the
Vdc_max controller can be activated for one or more drives. The Vdc_max controller is
normally activated for drives that have to decelerate/accelerate high levels of kinetic energy
themselves. When the overvoltage threshold in p1244 is reached (recommended setting: 50
V higher than the DC link voltage), the braking torque of the drives with an active Vdc_max
controller is reduced by shifting the torque limit. In this way, the drives feed back the same
amount of energy that is used as a result of losses or consumers in the DC link, thereby
minimizing the braking time. If other drives for which the Vdc_max controller is not active
feed energy back, the drives with an active Vdc_max controller can even be accelerated to
absorb the braking energy and, in turn, relieve the DC link.
Description of Vdc_max control without acceleration (p1240 = 7, 9)
As with p1240 = 1, 3, if the drive must not be accelerated by means of feedback from other
drives in the DC link, acceleration can be prevented by the setting p1240 = 7, 9. The
effective lower torque limit must not be greater than the torque limit offset (p1532).
Description of Vdc controller monitoring functions (p1240 = 4, 5, 6)
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 ensure
that the DC link voltage is not burdened with uncritical drives in the event of a power failure,
these drives can be switched off by a fault (F30003) with a parameterizable voltage
threshold (p1248). This is carried out by activating the Vdc_min monitoring function (p1240 =
5, 6).
In the event of a power failure, the DC link voltage can increase until it reaches the shutdown
threshold when drives are decelerated. To ensure that the DC link voltage is not burdened
with uncritical drives in the event of a power failure, these drives can be switched off by a
108
Switching-in/switching-out the Vdc_max control
Function Manual, (FH1), 10/2008, 6SL3097-2AB00-0BP5
Drive functions