Siemens SINAMICS DCM Manual page 675

Applications
12.3 Use of the SINAMICS DCM in galvanizing and dip-painting plants
12.3 Use of the SINAMICS DCM in galvanizing and dip-painting plants
2Q devices are used when using the SINAMICS DCM in galvanizing, dip-painting or similar
plants.
2Q devices have a thyristor bridge in B6 circuit. In this power unit topology, the output
voltage of the SINAMICS DCM is partly negative for firing angles between 60° and 120°. In
order to ensure that no negative voltages are present at the load, i.e. at the galvanizing and
paint tanks, an external freewheeling diode is usually installed on the DC side for these
applications. This is not part of the SINAMICS DCM, but rather is installed in the control
cabinet.
Figure 12-3
Make sure that in this case, the current actual value sensing in the converter only measures
part of the load current, i.e. the current through the converter, not the freewheeling current.
This means that the SINAMICS DCM cannot control the load current.
Figure 12-4
In order to be able to control the load current, it is recommended that an external actual
current sensing circuit be used for these applications. Whereby, the entire load current
(including the freewheeling current) is measured via an external shunt.
676
SINAMICS DCM, 2Q device with external freewheeling diode
Voltage and current curves
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SINAMICS DCM DC Converter
Operating Instructions, 02.2015, A5E34763375/RS-AA/001