Star-Delta Starter with Timer Changeover for 3-Phase Induction Motors

A star-delta starter is a reduced-voltage starter that limits the heavy inrush current of a 3-phase induction motor at start-up. Across the supply directly (direct-on-line starting), it can draw five to seven times its rated current, dipping the supply voltage and stressing windings and switchgear; the star-delta method first connects the windings in star, so each sees only 1/√3 of the line voltage (about 58%), and starting line current and torque fall to roughly one-third of their direct-on-line values. Once the motor accelerates, a timer relay automatically reconnects the windings in delta for full-voltage running. This project builds the complete starter as a single demonstrator panel: three contactors (main, star and delta), a dial-adjustable timer relay, a thermal overload relay, an MCB, and a green-start/red-stop push-button station. Star and delta contactors are electrically and mechanically interlocked so both can never close together, and the changeover delay is about 8 seconds for the demo motor.

From supply to running motor, the full sequence takes under ten seconds. With the MCB on, supply reaches the main contactor and the control circuit is live, but nothing moves until the green start button is pressed. Start energizes the main and star contactors — the windings are now in star across the supply, each seeing 1/√3 of the line voltage, so the motor accelerates gently at about one-third of direct-on-line current — a seal-in contact holding both contactors after the button is released. The same start signal triggers the on-delay timer relay, which counts its set delay (about 8 s in this build), its LED indicating timing. When the delay elapses, the timer drops the star contactor and the interlock chain pulls in the delta contactor: the windings reconnect in delta at full line voltage and the motor runs at rated speed. Stop, or an overload trip, drops the main contactor and everything de-energizes.

1. Switch on the 3-pole MCB — the 3-phase supply reaches the main contactor terminals and the control circuit is energized, with all contactors open.
2. Press the green start push button — the main and star contactors energize together, and a seal-in auxiliary contact keeps them energized after the button is released.
3. The motor windings are now connected in star across the supply; each winding sees 1/√3 of the line voltage (about 58%), so starting line current and torque are roughly one-third of direct-on-line values (design target).
4. The timer relay starts counting its set delay from the same start signal — about 8 s in this build, adjustable on the relay dial from 0.1 s to 60 s (datasheet value).
5. When the delay elapses, the timer relay de-energizes the star contactor; the interlock chain then permits the delta contactor to pull in, reconnecting the windings in delta for full-voltage running.
6. The motor runs in delta at rated speed; the timer relay's LEDs and the contactor states on the panel show the running condition.
7. If the motor draws sustained overcurrent, the thermal overload relay trips and drops the main contactor; the red reset button must be pressed before restarting.
8. Pressing the red stop button (or switching off the MCB) de-energizes all contactors and the motor coasts to a stop.

Tech stack:

• 3-pole AC contactors × 3 (main · star · delta)
• Electronic on-delay timer relay (0.1–60 s dial)
• Thermal overload relay with manual reset
• 3-pole MCB incomer
• Green start · red stop push-button station
• 3-phase induction motor (demo load)
• DIN rail · ferruled control and power wiring
• Power + control wiring diagrams

The three capabilities that make this build worth demonstrating are automatic timer changeover, fail-safe interlocking, and genuine overload protection. The electronic timer relay takes the changeover decision out of the operator's hands: the delay is set on a dial (0.1–60 s on the fitted relay, about 8 s in this build), the relay's LEDs show the timing state, and the star-to-delta transition happens the same way every start. Safety comes from two independent interlocks — auxiliary-contact electrical interlocking plus a mechanical interlock between the star and delta contactors — so the one wiring fault that would dead-short the supply is structurally impossible. Protection is completed by a thermal overload relay matched to the motor current, with manual reset after a trip, and an MCB incomer for short-circuit faults. Together with the labelled DIN-rail layout and ferruled wiring, the panel reads as a real industrial starter shrunk to lab scale, which is exactly what examiners want to probe.

• [Automatic timer changeover] The on-delay timer relay counts the set delay from the moment the motor starts in star, then drops the star contactor and pulls in the delta contactor without any manual switching — the dial setting used in this build is about 8 s.
• [Three-contactor power circuit] Main, star and delta contactors form the classic power circuit; each carries a handwritten masking-tape label and the power wiring uses ferruled red, black and yellow-green conductors in standard phase and earth colours.
• [Dual interlocking] The star and delta contactors are interlocked both electrically (through each other's auxiliary contacts) and mechanically, so both can never close together — the single most dangerous fault in this circuit is prevented by construction, not just by wiring care.
• [Thermal overload protection] A thermal overload relay in the main circuit trips on sustained overcurrent from mechanical overload or single-phasing, with a red reset button that must be pressed manually before the motor can be restarted.
• [MCB incomer and push-button station] A 3-pole MCB provides short-circuit protection at the panel incomer, and the green start / red stop station uses a seal-in contact so the motor keeps running after the start button is released until stop is pressed.
• [Live status indication] The timer relay's LEDs show timing and changeover state, and the contactors' mechanical state is visible on the panel, so the start → star → changeover → delta sequence can be watched and explained during the demonstration.
• [Wired demo motor] A small 3-phase induction motor is wired to the panel as the demonstration load, so the reduced-voltage start and the changeover moment are visible and audible rather than theoretical.

• Fully wired star-delta starter panel (3 contactors, timer relay, overload relay, MCB, push-button station)
• Small 3-phase induction motor wired as the demonstration load
• Power and control wiring diagrams
• Project report PDF (theory, working principle, calculations, test procedure)
• PPT presentation for the final review
• Viva Q&A preparation document
• Component list with datasheet references