10
Project planning
Selecting an inverter
Operating Instructions – MOVIMOT
®
flexible
293
10.7 Selecting an inverter
The selection of the inverter is based on the course of the output current over time.
The required current has to be determined from the required torque characteristic of
the connected motor.
The inverters are dimensioned for a nominal output current I
N
. In many applications,
there is a demand for short-time overload operation. For this purpose, the inverters
can be operated with up to 300% of the nominal output current for a short period of
time.
For overload operation, make sure that the inverter is not thermally overloaded. For
protection of the power components, inverters have various monitoring mechanisms.
The following thermal monitoring functions are available:
• Dynamic utilization
The periodic current load of the switching power semiconductors lets them heat
and cool down cyclically. Due to the different thermal time constants, large temper-
ature differences can occur between power semiconductor and heat sink. Dynamic
utilization monitors the permitted temperature of the barrier layer of the power
semiconductors.
• Thermal capacity utilization
The power semiconductors are limited by the maximally permitted temperature
during operation. Thermal utilization monitors the heat sink temperature of the
power semiconductors.
• Electromechanical utilization (I
2
t utilization)
Electromechanical utilization protects the components that have a large thermal
time constant compared to the power semiconductors.
Due to the complexity of the utilization curves, the calculation can only be performed
using software. The SEW‑Workbench project planning software offers supports for di-
mensioning an inverter.
For a rough selection of the inverter without using the project planning software, char-
acteristic load cycles are given in the following section.
29129451/EN – 12/2019
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