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
136
Figure 2-40 Example of the time characteristic for mechanical rotor blocking
Logic
A continuous comparison of the motor current with the configured threshold values of the protection function
takes place for the purpose of detecting a locked rotor. Figure 2-41 shows the logic diagram. The threshold-
value comparison is blocked during the motor startup phase, as the startup currents usually move in a size
similar to the occurring currents when a rotor is locked.
The algorithm verifies the motor standstill according to currents and (if available) the message „>52-a“. As
soon as a current increase is applied after detection of the motor standstill, the load jam protection is tempo-
rarily blocked in order to avoid motor shutdown during the motor startup phase.
The motor is detected as being in standstill when none of the three phase currents exceeds the threshold set
via address 212 BkrClosed I MIN and the binary signal „>52-a“ is inactive. The „>52-a“ signal is only
taken into account if the binary input is allocated accordingly.
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