Operating instructions

Functions
5.8 Protection functions
CU240S and CU240E Control Units, FW 3.2
Operating Instructions, 03/2009, A5E02440075B AA
121
5.8.3 Limiting the maximum DC link voltage
How does the motor generate overvoltage?
An induction motor can operate as a generator if it is driven by the connected load, In this
case, the motor converts mechanical energy into electrical energy. The motor feeds the
regenerative energy back to the inverter.
As a consequence, the DC link voltage is increased. The inverter can only reduce the
increased DC link voltage if it is capable of regenerative feedback into the line supply or is
equipped with a braking resistor.
Without being capable of regenerating into the line supply, only extremely low or brief
regenerative loads – relative to the inverter power – are possible because the inverter may
be damaged if the DC link voltage reaches critical levels. Before the voltage can reach
critical levels, however, the inverter shuts down with the fault message "DC link overvoltage".
Protecting the motor and inverter against overvoltage
The V
DCmax
controller prevents – as far as is technically possible – the DC link voltage from
reaching critical levels.
The V
DCmax
controller is not suitable for applications in which the motor is permanently in the
regenerative mode, e.g. in hoisting gear or when large flywheel masses are subject to
braking. For applications such as these, you must select an inverter that is equipped with a
braking resistor or that can feed energy back into the line supply (e.g. PM250 and PM260)
Table 5- 42 V
DCmax
controller parameters
Parameter Description
P0003 = 3 User access level
3: Expert
P1240 = … Enables the V
DCmax
controller
1: Enables the V
DCmax
controller
3: Enables the V
DCmax
controller with kinetic buffering
r1242 Shows the value of the DC link voltage above which the V
DCmax
controller is active
P1243 = … Multiplier for the output of V
DCmax
controller
(scaling of the V
DCmax
controller output)
P1250 = … Proportional gain of the V
DCmax
controller
P1251 = … Integral time of the V
DCmax
controller
P1252 = … Derivative time of the V
DCmax
controller
P1253 = … Limits the output of the V
DCmax
controller
P1254 = … Activates or deactivates automatic detection of the switch-on levels of the V
DCmax
controller
For more information about this function, see function diagram 4600 in the List Manual.
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