Add one more component to improve the phase failure protection function

Usually the starting circuit of a three-phase motor is shown in the figure below. In the figure, K is a three-phase knife switch, R is a fuse, AN1 is a start button, AN2 is a stop button, and C and J are AC contactor coils with a working voltage of 380V. JR is a thermal fuse. D is a three-phase motor. C-1, C-2, C-3, and C-0 are contacts of C, and J-0 is a contact of J.

The working principle of the starting circuit is as follows: After closing the knife switch K, when AN1 is pressed, the coil of contactor C is energized and attracted, and contacts Cl, C-2, C-3, and C-0 are connected, and the three-phase motor D starts to start. Since C-0 is connected in parallel with AN1, even if AN1 is released after the motor starts, CO has been connected, so the motor continues to rotate normally. When AN2 is pressed, the coil of contactor C loses power, contacts C-1, C-2, C-3, and C-0 are disconnected, and the three-phase motor D stops. During the normal rotation of the three-phase motor D, if the A phase of the power grid is powered off, the coil of contactor C cannot be automatically powered off (because the coil of C is connected in series on the B phase and the C phase), and the three-phase motor D cannot be automatically powered off, and is always in a phase-missing operation state. The three-phase motor D is easy to burn out.

In order to prevent the three-phase motor D from running in the absence of a phase, there are many ways in electrical practice. After repeated experiments, the author disconnected point D in the circuit (the cross in the figure) and added an intermediate contactor J in the circuit (the dotted part in the figure), thus improving the phase loss protection function of the three-phase motor.

The working process of the improved circuit is as follows: after closing K, when AN1 is pressed, the coil of contactor J is electrically attracted, and after contact JO is connected, the coil of contactor C is electrically attracted, contacts Cl, C-2, C-3, and C-0 are connected, and the three-phase motor D starts to work. When AN2 is pressed, the coil of contactor C loses power, contacts c-1, C-2, C-3, and C-0 are disconnected, and the three-phase motor D stops.

After the three-phase motor D adopts the improved starting circuit, if during normal rotation, phase A of the power grid is powered off, the coil of contactor J is powered off, the J-0 contact is disconnected, the coil of contactor C is also powered off, contacts Cl, C-2, C-3, and C-0 are disconnected, and the three-phase motor D stops; if phase B of the power grid is powered off, the coil of contactor C is powered off, contacts Cl, C-2, C-3, and C-0 are disconnected, and the three-phase motor D stops; if phase C of the power grid is powered off, the coils of contactor C are powered off, contacts Cl, C-2, C-3, and C-0 are disconnected, and the three-phase motor D stops; if phase C of the power grid is powered off, the coils of contactors C and J are connected in series to a 380V power supply and cannot be normally attracted, contacts C-1, C-2, C-3, and C-0 are disconnected, and the three-phase motor D stops. Therefore, no matter which phase of the power grid is powered off, the three-phase motor D can stop immediately, and the motor D will never burn out due to phase loss again.