Automatic Phase Selector for Single-Phase Loads (Three-Phase Input)

An automatic phase selector for single-phase loads is a panel that takes a three-phase 400 V input and delivers one healthy 230 V single-phase supply to a load. It exists because single-phase loads wired to one phase of a three-phase supply stop dead when that phase fails or sags — homes, workshops and small offices routinely lose lighting, refrigeration or pump power to exactly this failure. The selector fixes it automatically: isolated sensing circuits watch the R, Y and B phases, pick the first healthy phase in fixed priority, and energize a contactor connecting that phase to the load. If the active phase drops below the undervoltage threshold of about 180 V (adjustable), the panel switches to the next healthy phase after a break-before-make dead time of roughly 100–200 ms, so two phases are never paralleled. It is neither a single-phase preventer (which protects three-phase motors by tripping) nor a mains–generator changeover (which swaps two independent sources).

From sensing to switching, the mechanism runs in a fixed loop. First, three isolated sensing circuits — step-down transformers with rectification and a comparator stage — report whether the R, Y and B phases are present and above the undervoltage threshold of about 180 V. Next, the selection stage (relay logic or a small microcontroller) picks the highest-priority healthy phase in the fixed order R, then Y, then B. Finally, the switching stage energizes the matching power contactor, and a hardware interlock — the normally-closed auxiliary contact of each contactor wired in series with the other two coils — makes it physically impossible for two contactors to pull in together. Changeover is break-before-make: the outgoing coil de-energizes first, and an off-delay of roughly 100–200 ms (design target) guarantees its contacts have opened before the next closes, so no two phases are ever paralleled across the 400 V line-to-line gap. LEDs show which phase is live and which are faulted.

1. Each of the three phases feeds an isolated step-down sensing transformer; the secondary is rectified and divided down to a safe signal voltage of approximately 5 V DC at nominal input (expected).
2. Comparator stages compare each signal against the undervoltage reference set by a trimmer (approximately 180 V line-neutral threshold, adjustable); an optocoupler carries each phase's healthy/fault flag to the logic stage.
3. The selection logic scans the flags in the fixed order R, then Y, then B, and chooses the first healthy phase — no operator action needed.
4. The logic energizes the matching power contactor, which connects that phase (plus neutral) to the load terminals; the LEDs update to show the active and faulted phases.
5. The hardware interlock chain — the NC auxiliary of each contactor in series with the other coils — continuously prevents a second contactor from pulling in, regardless of what the logic outputs.
6. If the active phase's flag drops, the logic de-energizes its contactor and waits the break-before-make dead time of approximately 100–200 ms (design target) before energizing the next healthy phase.
7. If no phase is healthy, all contactors stay open and the fault LEDs show the condition until a phase recovers.

Tech stack:

• 230 V AC / 400 V AC three-phase wiring hardware
• Step-down transformers + bridge rectifiers (sensing)
• LM393-type comparators with trimmer references
• PC817-type optocouplers (signal isolation)
• Power contactors with NC auxiliary interlock chain
• ATmega328-class microcontroller or relay logic (selection control)
• DIN rail, terminal blocks, panel enclosure
• Multimeter + clamp meter (testing and setpoint calibration)

Four capabilities define this build. First, per-phase health sensing: three isolated step-down sensing circuits report presence and undervoltage on each phase, with a comparator stage that flags any phase below about 180 V (adjustable). Second, fixed-priority selection: the control logic always picks the highest-priority healthy phase in R, then Y, then B order, and automatically re-evaluates the moment the active phase degrades. Third, contactor switching with hardware interlocking: three power contactors route the chosen phase to the load, and the normally-closed auxiliary contact of each contactor is wired in series with the other two coils, so two contactors physically cannot pull in together — even a welded main contact holds its own auxiliary open and blocks the rest. Fourth, safe changeover timing: break-before-make sequencing with roughly 100–200 ms of dead time guarantees the outgoing contactor's contacts have opened before the next closes, eliminating any chance of a 400 V phase-to-phase short.

• [Per-phase presence + undervoltage sensing] Three transformer-isolated sensing circuits monitor R, Y and B independently; a comparator with hysteresis flags a phase as unhealthy below approximately 180 V line-neutral, adjustable via an onboard trimmer.
• [Fixed-priority selection logic] The controller selects the highest-priority healthy phase (R > Y > B) and re-runs the selection the instant the active phase fails, with no manual intervention.
• [Hardware-interlocked contactor switching] Three power contactors route the chosen phase to the load; each contactor's NC auxiliary contact is wired in series with the competing coils so simultaneous closure is mechanically impossible.
• [Break-before-make dead time] An off-delay of approximately 100–200 ms between de-energizing the outgoing contactor and energizing the next guarantees contact separation before the new phase connects.
• [LED phase-status indication] Panel-mounted red, yellow and blue LEDs show which phase is feeding the load, with a separate fault indication for each unhealthy phase.
• [Welded-contact interlock reasoning] The interlock is wired in hardware precisely because firmware alone cannot stop a welded contactor: a stuck armature holds its own NC auxiliary open, which keeps the competing coils de-energized regardless of what the code commands.
• [Single-phase 230 V output] The selected phase is delivered with the neutral to a standard 230 V output for lamps, pumps, chargers and similar single-phase loads.

• Working three-phase selector panel prototype (wired and bench-tested)
• Control logic firmware/source or relay ladder documentation
• Complete circuit and wiring diagrams
• Component list with ratings
• Undervoltage setpoint calibration procedure
• Project report PDF (background, design calculations, methodology, results)
• PPT presentation for final review
• Viva Q&A preparation document