Functions
8.19 Closed-loop Vdc control
Control Units CU240S
8-138 Operating Instructions, 11/2006, A5E00766042B AA
8.19 Closed-loop Vdc control
Overview
In addition to DC, compound and dynamic braking, for the SINAMICS G120 it is possible to
prevent a DC link overvoltage condition using the closed-loop Vdc controller. With this
technique, the output frequency is automatically modified during operation using the closed-
loop Vdc controller so that the motor doesn't go too far into the regenerative mode.
Using the Vdc controller, it is also possible to prevent the inverter from being shut down
(tripped) during brief line supply dips – which cause a DC link undervoltage condition. Also in
this case, the output frequency is automatically modified by the Vdc controller during
operation. Contrary to an overvoltage condition, in this case the motor is operated with
increased regenerative operation in order to support and buffer the DC link voltage.
DC link overvoltage
● Cause:
The motor regenerates and feeds too much energy back into the DC link.
● Remedy:
The DC link voltage is further reduced using the Vdc_max controller by reducing the
regenerative torque down to zero.
DC link undervoltage
● Cause:
Line supply voltage failure or dip (blackout or brownout)
● Remedy:
A regenerative torque is entered for the operational motor which compensates the
existing losses and therefore stabilizes the voltage in the DC link. This technique is
carried-out using the Vdc_min controller and is known as kinetic buffering.
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