Three-phase power supply phase sequence/missing phase detector

The three-phase power supply phase sequence/phase loss detector introduced in this article is mainly used to detect whether the wiring of the three-phase AC power supply is missing a phase and whether the phase sequence is correct. The circuit principle is shown in the attached figure. In the figure, if phase A (1), phase C (3), and phase B (2) are connected to the A, G, and K poles of the thyristor respectively, the thyristor T will be in single state. When the phase is turned on within half a cycle, the light-emitting diode will emit normal light. When the phase sequence of the three phases connected to A, B, and C is incorrect, the conduction time of the thyristor T will be shortened, and the average current will decrease accordingly. , the LED brightness is greatly reduced. When one or two phases of the three-phase AC power are missing (broken), the thyristor is cut off and the LED goes out. The values ​​of R3, R4 and C in the figure will determine the length of the delay time t.