Siemens Sinamics G120P Operating Instructions Manual page 372

Functions
7.6 Protection functions
Distinction between V/f control and speed control:
● V/f control
The Vdc_min controller acts on the speed setpoint channel. When Vdc_min control is
active, the drive setpoint speed is reduced so that the drive becomes regenerative.
● Speed control
The Vdc_min controller acts on the speed closed-loop controller output and affects the
torque-generating current setpoint. When Vdc_min control is active, the torque-generating
current setpoint is reduced so that the drive becomes regenerative.
If the power fails, the DC-link voltage decreases due to the lack of power from the supply
system. When the DC-link voltage threshold set via parameter p1245 (p1285) is reached, the
Vdc_min controller is activated. Due to the PID properties of the closed-loop controller, the
motor speed is reduced to the extent that the regenerative drive energy maintains the DC-
link voltage at the level set in p1245 (p1285). The kinetic energy of the drive governs the
dropout characteristic of the motor speed and, in turn, the buffering duration. In centrifugal
mass drives (e.g. fans), buffering can last a few seconds. In drives with a low centrifugal
mass (e.g. pumps), however, buffering can last just 100 to 200 ms. When the power is
restored, the Vdc_min controller is deactivated and the drive is ramped up to its setpoint
speed at the ramp-function generator ramp. As long as the Vdc_min controller is active, an
alarm A7402 (drive: DC-link voltage minimum controller active) will be issued.
If the drive can no longer generate any regenerative energy (because, for example, it is
almost at a standstill), the DC-link voltage continues to drop. If the minimum DC-link voltage
is undershot (see "Switching Vdc_min control on/off" <1>), the drive will shut down with fault
F30003 (power unit: DC-link undervoltage).
If, during active Vdc_min control, a speed threshold set with parameter p1257 (p1297) (see
"Switching Vdc_min control on/off" <2>) is undershot, the drive will shut down with F7405
(kinetic buffering minimum speed not reached).
If a shutdown with undervoltage in the DC link (F30003) occurs without the drive first coming
to a standstill despite the fact that Vdc_min control is active, the closed-loop controller may
have to be optimized via dynamic factor p1247 (p1287). Increasing the dynamic factor in
p1247 (p1287) causes the closed-loop controller to intervene more quickly. The default
setting for this parameter, however, should be sufficient for most applications.
Parameter p1256 = 1 (p1296) can be used to activate time monitoring for kinetic buffering.
The monitoring time can be set in parameter p1255 (p1295). If buffering (i.e., the power
failure) lasts longer than the time set here, the drive will shut down with fault F7406 (drive:
kinetic buffering maximum time exceeded). The standard fault response for this fault is
OFF3, which means that this function can be used for controlled drive deceleration in the
event of a power failure. In this case, excess regenerative energy can only be dissipated via
an additional braking resistor.
372
Inverter cabinet units
Operating Instructions, 12/2014, A5E32923362A