Three-phase power supply phase voltage failure automatic switching circuit

In the application of three-phase AC power supply, if one or two phases have low voltage, and you still want your single-phase electrical equipment to work with normal phase voltage, the circuit introduced in this article can solve this problem. However, a fuse must be connected in series to the input end of each phase (A, B, C). In this case, you will obtain normal voltage from other normal phases through relays in the same phase line.

The circuit is composed of transformer, comparator, transistor and relay etc. The figure shows three identical circuits, which are used for phase A, phase B and phase C respectively. The working condition of phase A is analyzed first.

The AC power supply of phase A is stepped down to 12V (300mA) by transformer x1, and then rectified by D1 and filtered by c1 to generate the working power supply of op amp Ic1. The voltage of the inverting pin ② of op amp Ic1 is taken from the voltage divider circuit composed of R1 and VR1. VR1 sets the reference voltage according to the circuit requirements. The reference voltage of the inverting pin ③ is fixed at 5.1v by a voltage regulator diode.

When the A phase voltage is in the normal range of 200V to 230V, the voltage of IC1's inverting pin ② is high, that is, the voltage at this point is greater than the reference voltage of 5.1V, so the output voltage of IC1 is also high. As a result, transistor T1 is not conducting, relay RL1 remains in the released state, and the A phase power is sent to the load L1 through the normally closed contact N/C of RL1.

Once the voltage of phase A is lower than 200V, the voltage of the inverting pin ② is lower than the reference voltage 5.1V, the output of IC1 becomes low, T1 is turned on, RL1 is attracted, the load is disconnected from phase A, and connected to phase B via the contact N/C of R12.

Similarly, the automatic phase switching of the other two phases (B and C) is similar.

The capacity of the relay and fuse should be able to withstand the current impact during phase switching.

Attention should be paid to:

1. Use high-quality multi-strand insulated copper wire to meet the switching current requirements:

2. Use high-quality relays. The contacts and rated current should meet the requirements:

3. Install the transformer and relay in a suitable box and connect the input and output of the power line with the label terminal block (TB).

Note: 1. It is recommended to use I2V, 200Ω, 6A current capacity and 6A fuse:

2. If the voltage of two phases is lower than the specified value, such as L1 and L2, L1 and L2 will be automatically connected to the third phase L3. In this way, the current load of L3 will be doubled. Therefore, the fuse capacity at the input end should also be doubled.