6.2.1.3 Rated motor current/setting scale
For motor protection, the motor's rated current must be set on the motor starter protector's
setting scale. The new IE3 motors generally have lower rated currents. This can lead to situations
in which a motor starter protector with a lower rated current has to be chosen for the same motor
rating. Thus, the short-circuit detection response value also drops and can lead to tripping
during motor starting.
See also
Short-circuit detection (Page 28)
6.2.2 Optimization of 3RV2 motor starter protectors for IE3 motors
The motor starter protectors have been revised in relation to the higher locked rotor currents and
inrush currents as follows:
• Raising of the lower short-circuit detection response tolerances without changes to the
maximum values.
→ No change to system dimensioning
• Increase in the making / breaking capacity.
→ Far-reaching avoidance of restrictions due to increased motor starting currents and inrush
currents, see the chapter "Technical modications/restrictions of 3RV2 motor starter
protectors (Page 30)".
• Adaptation of some motor starter protector versions' overload releases and setting scales.
→ Avoidance of using smaller motor starter protectors due to lower rated motor currents (see
chapter "Technical modications/restrictions of 3RV2 motor starter protectors (Page 30)")
In the case of motors with very high locked rotor and inrush currents, problems can arise despite
adaptations, e.g. undesired tripping on starting. It is recommended that motor starter protectors
be selected such that the setting does not need to be made in the upper range of the setting
scale. This reduces power loss in the device (cost saving and reduced temperature rise in the
control cabinet) and increases the distance from the short-circuit releases' response limits.
Selection example
In this example, two motor starter protectors are compared with each other.
Rated motor current: 15 A
Motor starter protector A: Setting scale 10 ... 16 A
Motor starter protector B: Setting scale 14 ... 20 A
Motor starter protector B (14 ... 20 A) is recommended since its power loss is lower and it has a
higher distance to the response limits.
The power loss of motor starter protector B is approximately 35 % lower than that of motor
starter protector A.
Conguration
6.2 Use of 3RA2 load feeders with highly energy-ecient motors (IE3)
SIRIUS 3RA Load Feeders
Equipment Manual, 02/2022, A5E03656507520A/RS-AC/004 29
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