Functions, monitoring, and protective functions
9.3 Drive functions
Drive converter cabinet units
Operating Instructions, 07/07, A5E00288214A
317
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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.
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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 – 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. Alarm A7402 (drive: DC link voltage minimum controller
active) is issued while the Vdc_min controller is active.
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 is switched off with fault
F30003 (power unit: DC link undervoltage).
If a speed threshold set with parameter p1257 (p1297) for active Vdc_min control (see
"Switching Vdc_min control on/off" <2>) is undershot, the drive is shut down with F7405
(drive: kinetic buffering minimum speed undershot).
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.
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 is switched off with fault F7406 (drive:
kinetic buffering maximum time exceeded). The standard fault reaction 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.
Function diagram
FD 6220 (FP 6320) Vdc_max controller and Vdc_min controller
