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When Vdc_min control is enabled with p1240 = 2 (p1280), it is activated if the power fails
when the Vdc_min switch-in level (r1246 (r1286)) is undershot. Seen generally, the
regenerative energy (braking energy) of the motor when the speed is reduced is used to
support the converter DC link voltage. This means that when Vdc_min control is active, the
motor speed no longer follows the main setpoint and can be reduced to zero. The drive
continues operating until the shutdown threshold of the DC link voltage is undershot
(see "Switching Vdc_min control on/off" <1>).
Note
Parameter specifications in brackets
All parameter specifications in parentheses refer to V/f control.
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 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 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 a speed threshold set with parameter p1257 (p1297) is undershot when Vdc_min control is
active (see diagram "Switching Vdc_min control on/off" <2>), the drive is shut down with
F7405 (drive: kinetic buffering minimum speed not reached).
If a shutdown with undervoltage in the DC link (F30003) occurs without the drive coming to a
standstill despite the fact that Vdc_min control is active, the controller may have to be
optimized via dynamic factor p1247 (p1287). Increasing the dynamic factor in p1247 (p1287)
causes the controller to intervene more quickly. The default setting for this parameter,
however, should be sufficient for most applications.
Enclosed Drives
Operating Instructions, 12/2018, A5E36652151A
Functions, monitoring and protective functions
9.3 Drive functions
521
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