Siemens SINAMICS S120 Function Manual page 104

Servo control
3.14 Vdc control
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 reestablished, the Vdc controller is automatically
deactivated and the drives approach the speed setpoint again. If the power supply cannot be
reestablished, 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.
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
104
Switching-in/switching-out the Vdc_max control
Function Manual, (FH1), 07/2007 Edition, 6SL3097-2AB00-0BP4
Drive Functions