Siemens SIRIUS 3RA, Equipment Manual

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6.2 Use of 3RA2 load feeders with highly energy-efficient motors (IE3)
6.2.1 General information on the startup characteristics of highly energy-efficient
motors (IE3)
Motor starter protectors have been conceived to protect and switch motors. They assume the
task of line protection in the event of an overload or short-circuit.
To this end, motor starter protectors are equipped with sensors for overload and short-circuit
detection and possess an interruption point for switching motor and short-circuit currents.
Without technical adaptation to the new highly energy-efficient motors (IE3), the problems
described below can arise.
6.2.1.1 Short-circuit detection
In the event of unusually high currents in the electrical installation, short-circuit detection serves
to keep the thermal and dynamic load low and ensure safe shutdown. The response threshold
should lie above the currents that a starting motor causes. The dimensioning of a system is
influenced by the response value. The higher the response value, the higher the chosen cable
cross-sections to be protected must be. This increases costs in the system and for the switching
devices. This is why the response values have been adapted to the previously typical motor
starting currents.
The drawback of the new, more efficient motors (IE3) is that, on average, the locked rotor and
magnetization currents (inrush currents) arising at the moment of switching on are
considerably higher than in the case of the previous generations of motors. The spread of locked
rotor and inrush currents is very wide. Motors with high values can therefore cause the motor
starter protector's short-circuit detection to respond. This leads to unintentional shutdown
("early tripping") during motor starting. Early tripping can occur whenever the motor current lies
in the top range of the motor starter protector's setting scale and a motor with a high inrush
current is used.
6.2.1.2 Making and breaking capacity of motor starter protectors
Making and breaking capacity tests are conducted in compliance with the standard under three-
phase AC loading with 10 to 8 times the rated current. In some cases, the locked rotor and inrush
currents of IE3 motors are clearly above these values. In isolated cases, current spikes that arise
during switching on can cause brief lifting of the contacts without triggering a breaking
operation by short-circuit detection.
Where the motor is switched on by a contactor, for example, the motor starter protector's
making/breaking capacity is irrelevant. In this case, the motor current is only carried. The current
that can be carried without any problems is normally higher than the switching device's making/
breaking capacity.
Configuration
6.2 Use of 3RA2 load feeders with highly energy-efficient motors (IE3)
SIRIUS 3RA Load Feeders
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Equipment Manual, 02/2022, A5E03656507520A/RS-AC/004