Use Of 3Ra2 Load Feeders With Highly Energy-Efficient Motors (Ie3); Making And Breaking Capacity Of Motor Starter Protectors; General Information On The Startup Characteristics Of Highly Energy-Efficient Motors (Ie3); Short-Circuit Detection - Siemens SIRIUS Manual

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6.2 Use of 3RA2 load feeders with highly energy-efficient motors (IE3)

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.
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SIRIUS - SIRIUS 3RA load feeders
Manual, 09/2016, A5E03656507520A/RS-AB/003