Phase failure protection circuit 2 for detecting line current

The current signals detected by the three current transformers are rectified and filtered and output into
three DC voltages, which are added
between the base and emitter of the transistors VTi, VT2 and VTa respectively. After the signal is amplified by three
transistors, it is connected to
an OR gate circuit composed of diodes VD4, VDs, and VD6. The OR gate circuit controls
the conduction or cutoff of the transistor VT4, thereby controlling
the pull-in or release of the intermediate relay KA. When the power supply loses
phase, KA pulls in, causing the contactor KM to release, and
the motor stops. The disadvantage of
    this protection circuit is that when the stator winding is connected in a △ shape, the current
in the phase-opening line inside the motor winding does not drop to zero, and the protection device will not operate at this time.