KFU 2-/4-
186
16 Control
Functions
The frequency inverters provide a selection of established control methods in
Con-
figuration
30
. The selected control structure can be parameterized as required and
optimized for the application by further functions.
16.1 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 func-
tion extends the current controller available in the control system. The overload re-
serve of the frequency inverter can be used optimally by means of the intelligent cur-
rent limits, in particular in applications with dynamic load alternations. The criterion to
be selected via the parameter
Operation Mode
573
defines the threshold to the acti-
vation 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.
Operation Mode
573
Function
0
-
Off
The function is switched off.
1
-
Ixt
Limitation to the overload of the frequency inverter (Ixt).
10
-
Tc
Limitation to the maximum heat sink temperature (T
C
).
11
-
Ixt + Tc
Operation mode 1 and 10 (Ixt + T
C
).
20
-
Motor temp.
Limitation to the motor temperature (T
Motor
).
21
-
Motor temp.+ Ixt
Operation mode 20 and 1 (T
Motor
+ Ixt).
30
-
Tc + Motor temp.
Operation mode 10 and 20 (T
C
+ T
Motor
).
31
-
Tc + Motor temp.
+ Ixt
Operation mode 10, 20 and (T
C
+ T
Motor
+ Ixt).
The threshold value selected via the parameter
Operation Mode
573
is monitored by
the intelligent current limits. In the operation modes with motor and heat sink tem-
perature 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.
Parameter
Settings
No.
Description
Min.
Max.
Fact. sett.
574 Power Limit
40.00 %
95.00 %
80.00 %
575 Limitation time
5 min
300 min
15 min
In the operation modes with overload reserve (Ixt) there is a reduction of the output
current when the threshold value is exceeded, with a distinction being made between
long and short-term overload reserve. After the short-term overload (1 s) 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
(60 s), the output current is reduced to the rated current which also depends on the
switching frequency.
