Functions
2.8 Motor Protection
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
163
The setting for address STARTUP CURRENT (I
STARTUP
) as a secondary value is calculated as follows:
For reduced voltage, the startup current is also reduced almost linearly. At 80 % nominal voltage, the startup
current in this example is reduced to 0.8 · I
STARTUP
= 2.5.
The setting for detection of a motor startup must lie above the maximum load current and below the minimum
start-up current. If no other influencing factors are present (peak loads), the value for motor startup I MOTOR
START set at address 1107 may be an average value:
Based on the Long-Term Current Rating:
For ratios deviating from nominal conditions, the motor tripping time changes:
At 80% of nominal voltage (which corresponds to 80% of nominal starting current), the tripping time is:
After the time delay (4104 LOCK ROTOR TIME) has elapsed, the binary input becomes effective and generates
a trip signal. If the locked rotor time is set just long enough that during normal startup the binary input „>Rotor
locked“ (FNo. 6805) is reliably reset during the delay time LOCK ROTOR TIME, faster tripping will be available
during motor starting under locked rotor conditions.
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