Functions
2.7 Motor Protection (Motor Starting Protection 48, Motor Restart Inhibit 66, Load Jam Protection)
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
120
Inverse Time Overcurrent Element
The inverse time overcurrent element is designed to operate only when the rotor is not blocked. With a de-
creased startup current resulting from voltage dips when starting the motor, prolonged startup times are eval-
uated correctly and tripping with an appropriate time is enabled. The characteristic (see formula below) can be
ideally adjusted to the condition of the motor by using different startup times depending on the cold or warm
condition of the motor (see Figure 2-33).
The tripping time is calculated based on the following equation:
with
t
TRIP
Actual tripping time for flowing current I
t
max STARTUP
Tripping time for nominal startup current I
STARTUP
(address 4103, STARTUP
TIME or 4105, STARTUP T WARM)
I Current actually flowing (measurement value)
I
STARTUP
Nominal startup current of the motor (address 4102, STARTUP CURRENT)
I
MOTOR START
Pickup value for recognition of motor startup (address 1107, I MOTOR
START),
Figure 2-33 Inverse time tripping curve for motor starting current
Therefore, if the startup current I is smaller (larger) than the nominal current I
STARTUP
(parameter STARTUP
CURRENT) as configured under address 4102, then the actual tripping time t
Trip
is prolonged (or shortened) ac-
cordingly (see Figure 2-33).
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