3.39.2 Protection Principle
If the circuit breaker of the protected equipment is closed and the current is less than the
predefined setting, the undercurrent protection will operate.
The following figure shows the logic diagram:
Figure 3.39-1 Functional module diagram for 37C
Where:
[Ena] is the enable/disable setting
[Cur_Str] is the current starting setting
[Op_T] is the time delay setting
[BI52a] is the binary input from the auxiliary normally opened contact of circuit breaker.
[Blk] is binary input for blocking all the binary output signals of the function
[BlkStr] is binary input for blocking all the start outputs of the function
[BlkOp] is binary input for blocking all the trip signals of the function
3.39.3 Application Scope
The speed of dc motor is inversely proportional to the excitation current. When the magnetic field
weakens, the speed of motor increases. Therefore, the excitation current of dc motor must be
greater than a certain value, otherwise there will be under excitation or under current situation, the
motor speed will exceed the rated speed, causing a flying accident (Power machinery over speed
accident). In case of undercurrent fault, if the motor has not been started, it is not allowed to start.
If the motor has been running, cut off the main circuit and stop the motor running immediately.
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