1MRS751802-MUM
REX 521
109
Protection Relay
Technical Reference Manual, Standard Configurations
7.1.19. 3Ithdev>
The function block protects a motor both from short- and long-duration overloads.
The start situation is a good example of short-duration overload since particularly at
start-up the rotor is in danger to be overloaded by the starting current. The starting
current can be in the range of five to seven times the rated current of the motor.
The motor start-ups also load stator windings. Especially when a warm motor is
restarted, the stator may constitute a limitation regarding thermal protection.
Because the function block calculates the temperature rise both for the stator and the
rotor individually, the motor can be fully protected by proper settings. Furthermore,
the thermal model of two-time constants enables detecting both short-time and long-
time overloadings.
Fig. 7.1.19.-1 Function block symbol of 3Ithdev>
Three-phase thermal overload protection for devices, 3Ithdev
Short time constant for stator 0.1...999.0 min
Long time constant for stator 0.1...999.0 min
Weighting factor of stator’s short time-constant 0.00...1.00
Temperature rise when In for stator 50.0...350.0°C
Maximum temperature for stator 50.0...350.0°C
Short time constant for rotor 0.1...999.0 min
Long time constant for rotor 0.1...999.0 min
Weighting factor of rotor’s short time constant 0.00...1.00
Temperature rise when In for rotor 50.0...350.0°C
Maximum temperature for rotor 50.0...350.0°C
Starting current 0.10...10.00 x I
n
Starting time 0.1...120.0 s
Number of starts 1...3
Device type MOTOR I
MOTOR II
MOTOR III
MOTOR IV
GENERATOR I
GENERATOR II
TRANSFORMER
Trip temperature 80.0...120.0%
Prior alarm 40.0...100.0%
Restart inhibit 40.0...100.0%
Ambient temperature -50.0...100.0°C
Cooling time constant
1.0...10.0 x
Gen&Trafo heating time constant 1...999 min
BLOCK
GROUP
RESET
START
TRIP
CBFP
EN_RESTART
ϑ>
Dev
CBFB
3
3Ithdev>
τ
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