Principle of three-phase power supply automatic switching circuit

The main distribution cabinet in the power distribution room often trips and loses power due to ground vibration, thunder and power line failure in the power-consuming workshop. This affects the production of the workshop and even causes product damage. The main distribution cabinet in the power distribution room needs to press the energy storage button by hand to store energy, and then press the closing button to connect the power to each distribution cabinet. The following introduces the power circuit that can be automatically switched on after power failure.

As shown in the figure, the circuit consists of several circuits including the main power supply, backup power supply, backup power supply automatic control circuit, etc.

1. Main power circuit

Normally, when the circuit breaker QS is closed, the main power control contactor KM1 is energized and attracted. Its main contact KM1-1 is closed to connect the main power. The main power is output from the L1, L2, and L3 terminals. At the same time, the auxiliary contact KM1-2 of KM1 is disconnected. After the backup power is closed, the backup power input control relay KA1 cannot be attracted. Therefore, the backup power automatic input control circuit power cannot be connected. The backup power cannot be connected at the same time as the main power.

2. Backup power circuit

The backup power circuit consists of circuit breaker Qs1, fuse FU2, backup power control contactor KM2 and main contact KM2-2. The power is also output by output terminals L1, L2 and L3. Contactors KM1 and KM2 are interlocked with each other by normally closed contacts KM2-3 and KM1-2 respectively. This ensures that the backup power cannot be connected at the same time as the main power.

3. Backup power supply automatic control circuit

This part of the circuit is mainly composed of the backup power supply control relay KA1, the control transformer T, the rectifier filter circuit, the delay circuit and the backup power supply control relay KA2.

When the main power supply is cut off due to ground vibration, lightning strike or other reasons, the circuit breaker QS1 trips and the power is cut off. At this time, since there is no power in phase C, the main power control contactor KM1 coil loses power and releases. Its auxiliary contact KM1-2 resets and closes, connecting the power supply of the relay KA1 coil. Its auxiliary contact KA1-2 closes, connecting the 220v power supply of the control transformer T. It is stepped down to 15V AC by T. It is rectified by the rectifier bridge UR. It is filtered by capacitor C1. It is sent to the delay circuit and the delay input control relay KA2 for use. At the same time, the auxiliary contact KA1-3 of KA1 is disconnected, and C2 in the delay circuit starts to charge through R1 and RP. At the moment when capacitor C2 starts to charge, the voltage at point A is higher than the voltage at point B. The diode VD1 is reverse biased and cut off, the transistor VT1 is turned on, and VT2 is turned off. As the charging progresses, the lower end of capacitor C2 at point A gradually becomes negative. The potential is lower than the potential at point B, and transistor VT1 is turned off. YT2 is turned on. The coil of the time-delayed input control relay KA2 is energized and attracted, and its auxiliary contact KA2-1 is closed, connecting the power supply of the coil of the backup power control contactor KM2, and is energized and attracted. Its auxiliary contact KM2-1 is closed and self-locked, and its main contact KM2-2 is closed. The backup power circuit is connected, and the backup power is output from the L1, L2, and L3 terminals. At the same time, the auxiliary contact KM2-4 is disconnected. The power supply of the coil of the backup power input control relay KA1 is cut off and released when power is lost. Its auxiliary contact KA1-2 is disconnected. The power supply of the backup power input control circuit is cut off. The delay circuit in the input control circuit and the input control relay KA2 both lose their working power. The auxiliary contact KA2-1 of KA2 is reset and disconnected, but at this time the contactor KM2 has been self-locked, so that the backup power can continue to output, thus completing the entire process of automatic input of the backup power.

During installation, it should be noted that the main power supply A, B, C and the backup power supply A'B'C' are connected to the same terminal. Therefore, it must be reliably interlocked through KM1 and KM2. The main power supply and the backup power supply cannot be connected at the same time, and short circuit between phases must be prevented; the delay time of the capacitor C2 in the delay circuit can be adjusted by the potentiometer RP according to the needs to achieve the required time.