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# Protection riddle No.73 – Motor protection

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• #532
H.S

I know locked rotor current, of an induction motor, how can be calculated/chosen the suitable setting current of 50 (instantaneous over current ANSI code) of specified motor protective relay?
I know locked rotor current, of an induction motor, how can be calculated/chosen the suitable setting current of 50 (instantaneous over current ANSI code) of specified motor protective relay?

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• #2015
Hamid

The theorem of constant flux linkages applies to each induction motor stator phase separately and so one phase will usually exhibit a higher dc offset current than the other two phases. To see why this is so, assume that the closing times for all three phases of the circuit breaker are identical (pole closure times differing by 1 ms or less) and that a pole closure results only rarely in an initial voltage of zero on any terminal of the motor. If no initial terminal voltage is zero, then the initial voltages on the three terminals will all differ, and the resulting dc offset current in one phase will be higher than in the other two. This dc offset current has a time constant of a few cycles or less. Even so, this dc offset current can add substantially to the maximum rms current. There are other factors to be considered that can cause higher levels of transient inrush current. In above Eq. , Vt is normally assumed to be 100% voltage. It can be substantially less when starting a large induction motor across-the-line on a heavily loaded auxiliary electrical supply network. On the other hand, it can be as high as 110% voltage when starting a small induction motor on a lightly loaded electrical auxiliary network. Therefore: The

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