Siemens SIRIUS 3RT Manual page 165

Converters with precharging resistors
Siemens converters (SINAMICS / Masterdrives) have precharging resistors. This is why the
charging current is restricted to the rated current of the converter. For this reason, the
contactor itself can be dimensioned in accordance with utilization category AC-1 (resistive
load).
Ultimately, the manufacturer of the frequency converter is responsible for naming the criteria
for contactor selection, because only the manufacturer knows the operating characteristics of
the product precisely enough to be able to make responsible product recommendations or
define product requirements for the contactors.
Use of 3RT2 contactors on the secondary side of a frequency converter
An output contactor is usually not required. Only if specifically requested by the customer, is
an output contactor supplied. An output contactor is required in the following cases:
● Operation of the frequency converter in bypass mode
● Disconnection of the motor in the case of EMERGENCY-STOP
● Use of the frequency converter for several motors that are to be connected optionally
On the output side of frequency converters, contactors are usually dimensioned in
accordance with utilization category AC-3 corresponding to the motor rated current and the
associated voltage (as with direct-on-line starting). Since voltage and frequency are almost
always proportional in frequency converters, shutdown of the load at low frequencies is not
critical for the contactor.
Example
A voltage of only 40 V would apply at 5 Hz with respect to a 400 V / 50 Hz power supply
system. This can be handled by an AC-3-rated contactor without any problem during the
breaking operation.
NOTICE
These statements do not apply to vacuum contactors that are not suitable for low
frequencies.
Procedure when shutting down Siemens converters
1. EMERGENCY-OFF (disconnect drive from power as quickly as possible)
Before the contactor is opened, the converter must shut down via inverter enable / pulse
disable (command OFF 2).
2. EMERGENCY-OFF (controlled ramp-down of the drive)
If a quick stop is to take place, the drive is ramped down (command OFF 3).
The pulse disable must take place before the contactor is opened (command OFF 2).
SIRIUS - SIRIUS 3RT Contactors/Contactor assemblies
Manual, 09/2017, A5E03656507120A/RS-AE/006
6.15 Configuration information for use downstream of frequency converters
Configuration
165