Configuration
6.15 Configuration information for use downstream of frequency converters
SIRIUS - SIRIUS 3RT Contactors/Contactor assemblies
164 Manual, 09/2017, A5E03656507120A/RS-AE/006
Configuration information for use downstream of frequency
converters
There are important criteria for the selection of switching devices when operating with
converters.
The following general rule applies: Frequency converters comprise DC link capacitors that
cause high inrush current peaks if no attenuation measures have been taken inside the
converter using reactors or precharging resistors. The non-attenuated charging current is
almost the equivalent of a short-circuit and thus a heavy load on the contacts.
Use of 3RT2 contactors on the primary side of a frequency converter
If these current peaks are higher than the making capacity of the contactors, the contactor
contacts can bounce. The arcs created in this way liquefy the contact material with
subsequent contact sticking and welding. Since the level of the actual current peaks at the
installation location of the frequency converter depends essentially on the series impedance
of the power system and on the instant of closing, the following must be noted when
selecting a suitable contactor.
Converter without precharging resistors
If the current peak value is known:
on converters without precharging resistors and where the current peak value is known, the
3RT20 contactor can be used in accordance with its making capacity (10 x I
e
AC-3). This
making capacity must not be lower than the current peaks.
If the current peak value is not known:
on converters without precharging resistors and where the current peak value is unknown,
capacitor contactors are recommended on the primary side.
The use of capacitor contactors attenuates inrush currents to non-critical levels. In addition,
losses must be minimized, particularly in standby periods, but also during operation. In the
case of frequency converters, losses occur up to the DC link due to input circuits with filters
or reactors. These frequency converter standby losses can be reduced to almost zero by
disconnection with a contactor on the main current supply side.
The following 3RT26 capacitor contactors are suitable for use on the primary side of
frequency converters:
● 3RT2617: Suitable for the performance range to 5.5 kW/400 V
● 3RT2627: Suitable for the performance range of 7.5 kW to 15 kW/400 V
● 3RT2637: Suitable for the performance range of 22 kW to 37 kW/400 V
● 3RT2646: Suitable for the performance range from 37 kW to 55 kW/400 V
This depends on the information provided by the frequency converter manufacturer.
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