Three-Phase Motor Starter with Auto-Transformer and Timer

A three-phase motor starter with an auto-transformer and timer is a reduced-voltage starter that starts a three-phase induction motor at a fraction of the line voltage and then switches it to full voltage once it has gathered speed. It solves the inrush problem of direct-on-line starting, where a motor at standstill draws roughly five to seven times its full-load current and can dip the supply voltage and stress the windings. In this build, a tapped auto-transformer supplies the motor at 50%, 65% or 80% of the 415 V line voltage during the start step, and an on-delay timer relay changes over to the run contactor after a set delay. Because the current drawn from the supply falls with the square of the tap ratio, the 50% tap draws only about a quarter of the direct-on-line starting current. Students who build it learn reduced-voltage starting, tap selection, timer changeover sequencing and contactor interlocking on a working panel.

Three-phase 415 V supply enters through the MCCB and feeds two paths: the control circuit and the power circuit through the auto-transformer. When the green START button is pressed, the start contactor closes and connects the motor to the selected tap of the auto-transformer, so the motor receives only 50%, 65% or 80% of the line voltage and begins to accelerate with a much smaller starting current. The same START press energizes the on-delay timer relay, which starts counting the changeover delay. When the timer reaches its set time, its timed contact drops the start contactor and energizes the run contactor, connecting the motor directly across the full line voltage for normal running. Electrical interlocking through normally-closed auxiliary contacts makes it impossible for the start and run contactors to be closed at the same time. The thermal overload relay sits in series with the motor and trips the contactors if the current stays above its setting.

1. Select the starting tap (50%, 65% or 80%) on the auto-transformer terminal links before energizing, matching the tap to the load: lower tap for light loads and the smallest inrush, higher tap when more starting torque is needed.
2. Switch on the 3-pole MCCB to energize the panel; the control circuit is live and the red fault lamp stays off while the overload relay is healthy.
3. Press the green START push button: the start contactor closes, the motor is connected to the selected auto-transformer tap at reduced voltage, the amber starting lamp lights, and the timer relay starts counting.
4. The motor accelerates at reduced voltage while the line current stays near the square of the tap ratio of the direct-on-line value — about 25% at the 50% tap, about 42% at the 65% tap.
5. When the timer reaches its set delay (12 s, design setting), its timed contact drops the start contactor and closes the run contactor, switching the motor to full line voltage; the green running lamp lights.
6. The electrical interlock blocks any overlap: the normally-closed auxiliary contact of the run contactor opens the start contactor coil circuit the instant the run contactor closes, and vice versa.
7. The thermal overload relay continuously monitors motor current; on sustained overload it opens its trip contact, drops both contactors, and lights the red fault lamp until manually reset.
8. Press the red STOP push button at any time to drop both contactors and stop the motor; a fresh start requires pressing START again.

Tech stack:

• Three-phase auto-transformer (50% · 65% · 80% taps)
• 3-pole power contactors (AC-3)
• On-delay timer relay
• Thermal overload relay
• 3-pole MCCB
• Start/stop push-button station
• 230 V AC control circuit
• 1.5 kW three-phase induction motor

The starter's core capabilities are the three-tap auto-transformer, the timer-controlled changeover, the interlocked contactor pair, and the overload protection chain. The auto-transformer offers 50%, 65% and 80% voltage taps so the starting step can be matched to the load: the lowest tap gives the smallest inrush, while the highest tap keeps more starting torque for heavier loads. The on-delay timer relay handles the changeover automatically, dropping the start contactor and closing the run contactor after the set delay without any manual switching. The start and run contactors are electrically interlocked with normally-closed auxiliary contacts, so a welded timer contact or a wiring fault cannot close both at once. A thermal overload relay with an adjustable 2.5–4 A range and a 3-pole MCCB protect the motor and the panel against sustained overload and short circuit.

• [Three-tap auto-transformer] A three-phase auto-transformer with 50%, 65% and 80% voltage taps, selected by terminal links before starting, so the reduced-voltage step can be matched to the load and the available supply capacity.
• [Timer-controlled changeover] An on-delay timer relay (0–30 s range, set to 12 s as a design setting) automatically drops the start contactor and closes the run contactor, so the motor moves from reduced voltage to full voltage without manual switching.
• [Interlocked start and run contactors] Two 3-pole contactors with electrical interlocking: the normally-closed auxiliary contact of each contactor is wired in series with the other contactor's coil, so both contactors can never be closed at the same time.
• [Overload relay and MCCB protection] A thermal overload relay (2.5–4 A adjustable, design value) in series with the motor trips the contactors on sustained overload, and a 3-pole MCCB provides short-circuit protection for the panel.
• [Start/stop control with status lamps] Green START and red mushroom-head STOP push buttons with a seal-in (hold-on) auxiliary contact, plus red, amber and green indicator lamps showing fault, starting and running states.
• [Demonstration motor on the bench] A 1.5 kW three-phase induction motor (demonstration motor, design value) connected to the panel, so the full start sequence can be watched and the changeover heard and seen.
• [Complete wiring documentation] The power and control circuit diagrams, terminal schedule and tap-selection table are supplied with the build, so the student can explain every wire in the panel.

• Working auto-transformer starter panel with demonstration motor
• Power and control circuit diagrams with terminal schedule and tap-selection table
• Timer-setting and overload-setting procedure for the demonstration motor
• Project report PDF (background, reduced-voltage starting theory, tap-ratio calculations, methodology, results)
• PPT presentation for final review
• Viva Q&A preparation document (auto-transformer theory, tap-ratio current and torque relationships, timer changeover, interlocking, protection)