Three-phase power supply automatic transfer circuit principle, backup power supply automatic transfer control circuit

The main distribution cabinet in the power distribution room often trips and loses power due to ground vibration, thunder and line faults in the electrical workshop. As a result, workshop production is affected and even products are damaged. The power distribution cabinet in the power distribution room needs to press and hold the energy storage button with your hand to store energy, and then press the closing button to connect the power to each power distribution cabinet. The following introduces the power circuit that can automatically switch on after power failure.

As shown in the figure, the circuit consists of main power supply, backup power supply, backup power supply automatic switching control circuit and other circuits.

1. Main power circuit

Normally, when the circuit breaker QS is closed, the main power control contactor KM1 is electrically closed. Its main contact KM1-1 is closed to turn on the main power supply. Output the main power from L1, L2 and L3 terminals. At the same time, the auxiliary contact KM1-2 of KM1 is disconnected. After the backup power supply closes the circuit breaker Q52, the backup power input control relay KA1 cannot pull in. As a result, the backup power supply and the automatic switching control circuit power cannot be turned on. The backup power supply cannot be turned on at the same time as the main power supply.

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 supply is also output from the output terminals L1, L2 and L3. Contactors KM1 and KM2 each rely on normally closed contacts KM2-3 and KM1-2 to interlock with each other. This ensures that the backup power supply cannot be turned on at the same time as the main power supply.

3. Standby power supply automatic switching control circuit

This part of the circuit is mainly composed of the backup power input control relay KA1, control transformer T, rectifier filter circuit, delay circuit and backup power input 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 lost. At this time, the C phase has no power. The main power control contactor KM1 coil loses power and is released. Its auxiliary contact KM1-2 is reset and closed, turning on the power supply of the relay KA1 coil. Its auxiliary contact KA1-2 is closed, turning on the 220v power supply of the control transformer T. It is stepped down by T into 15V alternating current. It is rectified by the rectifier bridge UR. It is filtered by the 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 charging through R1 and RP. The voltage at point A is higher than the voltage at point B at the moment capacitor C2 starts charging. The diode VD1 is reverse biased and cut off, and the transistor VT1 is turned on. , VT2 cutoff. As charging proceeds, point A at the lower end of capacitor C2 gradually becomes negative. The potential is lower than the potential at point B, transistor VT1 is cut off, and YT2 is turned on. The coil of delay input control relay KA2 is electrically closed, and its auxiliary contact KA2-1 Close, turn on the backup power control contactor KM2 coil power, and get the power to pull in. Its auxiliary contact KM2-1 is closed and self-locking, and its main contact KM2-2 is closed. The backup power circuit is connected, and the backup power is output from the Ll, L2, and L3 terminals. At the same time, the auxiliary contact KM2-4 is disconnected. The backup power supply is cut off and the control relay KA1 coil power is released and the power is released. Its auxiliary contact KA1-2 is disconnected, cutting off the backup power supply to the input control circuit, causing the delay circuit in the input control circuit and the input control relay KA2 to lose working power. The auxiliary contact KA2-1 of KA2 is reset and disconnected, but at this time the contactor KM2 has self-locked, so that the backup power supply can continue to output, thus completing the entire process of automatic input of the backup power supply.

When installing, 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, they must be reliably interlocked through KM1 and KM2. The main power supply and the backup power supply cannot be turned on at the same time, and must be connected to the same terminal. To prevent phase-to-phase short circuit; the delay time of capacitor C2 in the delay circuit can be adjusted by the potentiometer RP as needed to achieve the required time.