Utilizing triple frequency voltage speed saturation zero sequence current protection circuit

The circuit as shown in the figure is suitable for three-phase motors with larger capacity △ or Y-shaped connection. Phase failure identification uses a fast saturation current transformer TA, whose primary side is connected in series in the main circuit, and the secondary side is connected in series from beginning to end to form an open triangle. When the three phases of the power supply current are symmetrical (equal), the triple frequency voltage generated by TA is filtered by the rectifier VC and the capacitor C, causing the sensitive relay K to operate and the motor M to operate normally. When one phase is out of phase and the line currents of the remaining two phases are in opposite phases, the resulting current is equal to zero, causing K to be released and the main circuit of the motor to be disconnected.