Single Phasing Preventer for Three-Phase Induction Motors

Losing one phase of a 3-phase supply is one of the commonest ways small motors die: the motor keeps running on two phases, draws heavy unbalanced current, and burns out its windings — often with no visible warning until it is too late. A single-phasing preventer watches all three phases and disconnects the motor the instant one disappears. This project builds that protector as a bench panel: three phase-sensing inputs feed an Arduino Uno, red-yellow-blue phase lamps show each phase's health, and on any phase failure the controller drops the 3-pole contactor feeding a 0.37 kW test motor. A phase-fail test toggle lets the student simulate the fault safely and watch the trip happen, and a thermal overload relay adds the second classic layer of motor protection. The report explains the negative-sequence heating mechanism — the real reason single phasing kills motors.

1. With all three phases healthy, the phase lamps glow and the controller holds the contactor closed; the motor runs normally.
2. The Arduino samples the three phase-sense inputs every mains cycle.
3. If any phase input goes dead — real fault or the test toggle — the logic flags single phasing.
4. The controller immediately de-energises the contactor coil, opening all three motor phases.
5. The failed phase's lamp goes dark and the trip state latches until reset.
6. Restoring the phase and pressing reset re-closes the contactor for the next demonstration run.

Tech stack:

• Arduino Uno control logic
• 3-phase sensing inputs (isolated)
• 12 A 3-pole contactor
• Thermal overload relay 0.8–1.2 A
• 4-channel 5 V relay module
• R/Y/B phase indicator lamps
• Phase-fail test toggle switch
• 0.37 kW 3-phase test motor

• [Three-phase sensing] All three phases are monitored continuously through isolated sensing inputs to the Arduino.
• [Millisecond trip action] On phase loss the controller de-energises the contactor coil promptly, disconnecting the motor before damaging currents persist.
• [Phase health lamps] Red, yellow and blue lamps show each phase's presence at a glance across the demonstration room.
• [Phase-fail test toggle] A dedicated toggle simulates losing a phase safely, so the trip can be demonstrated on demand.
• [3-pole contactor output] A 12 A 3-pole contactor switches the motor supply under controller command.
• [Thermal overload relay] A 0.8–1.2 A overload relay adds running overcurrent protection in series with the electronic trip.
• [Relay-module interface] A 4-channel 5 V relay module isolates the Arduino logic from the contactor coil circuit.

• Fully wired single-phasing preventer panel with test motor
• Arduino with programmed protection firmware
• Wiring diagram and phase-sensing notes
• Fault-simulation and demonstration procedure
• Project report PDF (background, single-phasing theory, negative-sequence heating, protection coordination, test procedure)
• PPT presentation for final review
• Viva Q&A preparation document (single phasing, contactors, overload relays, protection philosophy)

• This is a demonstration protector for the bench motor; it is not a certified industrial protection relay and must not be presented as one.
• Trip timing is a design target of the firmware; it is not a calibrated protection curve.
• The unit protects against phase loss and adds overload backup — it does not cover earth faults, short circuits or undervoltage beyond phase absence.
• Demonstration needs a 3-phase supply with a safe way to interrupt one phase; the test toggle provides this.
• Phase sensing is designed for 50 Hz mains; other frequencies are outside the scope.
• Like all protection, it must be demonstrated with the operating procedure's safety steps — mains voltages are present on the panel.