Operating Instructions ACU
18 Control functions
The frequency inverters provide a selection of established control methods in Configuration 30. The
selected control structure can parameterized as required and optimized for the application by further
functions.
18.1 Intelligent current limits
The current limits to be set according to the application avoid inadmissible loading of the connected
load and prevent a fault switch-off of the frequency inverter. The function extends the current
controller available in the control system. The specified overload reserve of the frequency inverter
can be utilized optimally using the intelligent current limits, particularly in applications with dynamic
load changes. The criterion to be selected via parameter Operation Mode 573 defines the threshold
to the activation of the intelligent current limit. The parameterized rated motor current or the
reference current of the frequency inverter is synchronized as the limit value of the intelligent
current limits.
The function is switched off.
Limitation to the overload of the frequency inverter (Ixt).
Limitation to the maximum heat sink temperature (T
C
)
Operation mode 1 and 10 (Ixt + T
C
)
Limitation to motor temperature (T
Motor
)
Operation mode 20 and 1 (T
Motor
+ Ixt)
Operation mode 10 and 20 (T
C
+ T
Motor
)
Operation mode 10, 20 and 1 (T
C
+ T
Motor
+ Ixt).
The threshold selected via parameter Operation Mode 573 is monitored by the intelligent current
limits. In the operation modes with motor and heat sink temperature monitoring, the reduction of
power selected with the parameter Power Limit 574 is done when the threshold value has been
reached. This is achieved by a reduction of the output current and the speed in motor operation.
The load behavior of the connected machine must be a function of the speed to ensure a sensible
use of the intelligent current limits. The total time of the power reduction as a result of an increased
motor or heat sink temperature contains not only the cooling time, but also the additionally defined
Limitation Time 575.
The definition of the power limit should be selected as small as possible in order to give the drive
sufficient time to cool down. The reference value is the rated output of the frequency inverter or the
set rated power of the motor.
In the operation modes with overload reserve (Ixt), the output current will be reduced if the
threshold is exceeded. A distinction is made in this context between long-time and short-time
overload reserve. After the short-term overload (1s) has been used up, the output current is reduced
to the long-term overload current matching the present switching frequency. After the long-term
overload current has been used up (60s), the output current is reduced to the rated current which
also depends on the switching frequency.
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