Rockwell Automation Low-Voltage Switchgear and Controlgear User Manual

Page 61

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Fig. 2.4-4
The basic design of a power circuit with circuit breaker, contactor and soft starter

Installations that allow heavy-duty starting via a soft starter with a starting time of around 1
minute and longer require besides a specifically selected motor also specifically selected
switching and protective devices.

It is a good idea to protect motors for heavy-duty starting that are activated by soft starters with
electronic motor protective devices. The circuit breaker must be selected and adjusted so that it
does not trip before the motor protective device and that it is thermally capable for the specific
load (harmonics content and starting time). Circuit breakers without thermal releases can be
employed to advantage. The selection and adjustment of the circuit breaker is as for heavy-duty
starting conditions (see Section

4.1.2.2

). In this case, the circuit breaker only has to provide

short-circuit protection and/or line protection.

Short-circuit protection

Short-circuits are critical and endanger the power semiconductors of the soft starters. Circuit
breakers are not sufficiently fast to protect power semiconductors of soft starters against short-
circuits. For short-circuit protection, the specifications provided by the soft starter manufacturer
should therefore be observed. Short-circuit protection for the power semiconductors of soft
starters is often omitted for cost reasons, whereby such coordination only satisfies the require-
ments of coordination type 1.

2.4.3.5

Switchgear for use with frequency converters (inverters)

Overload protection on supply side