Star Delta Starter Wiring Diagram

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Star delta starter wiring reduces a 3-phase motor's starting current to approximately one-third of direct-on-line by first connecting motor windings in star, then switching to delta once the motor approaches running speed — protecting supply networks and driven equipment from starting surge.

When a 3-phase induction motor starts direct-on-line, it draws 5–8 times its full-load current for several seconds. On large motors (typically above 5–7 kW, though the threshold varies by supply authority and installation), this inrush can cause voltage dips that affect other equipment on the same network. The star-delta starter is the most widely used reduced-voltage starting method in industrial settings.

How it works: Motor windings are designed with six terminals (U1/V1/W1 and U2/V2/W2 per IEC). In star configuration, the three U2/V2/W2 ends are shorted together at a neutral point, and the U1/V1/W1 ends receive the three-phase supply. Each winding sees only 58% (1/√3) of line voltage, so starting current is reduced to approximately 33% of DOL. Once the motor accelerates to roughly 75–85% of rated speed, the star contactor opens, a brief interlock pause occurs, then the delta contactor closes, connecting the windings end-to-end in a closed triangle at full line voltage.

The three contactors: A main contactor (KM1) connects the supply to U1/V1/W1 at all times during starting and running. A star contactor (KM3) shorts U2/V2/W2 to neutral during starting. A delta contactor (KM2) connects U2 to V1, V2 to W1, and W2 to U1 (the delta cross-connection) during running. KM2 and KM3 are mechanically and electrically interlocked — both must never close simultaneously or a phase-to-phase short circuit will result.

The control circuit uses a timer relay (KT) to control the star-to-delta transition, a thermal overload relay for motor protection, and Start/Stop push buttons with a hold-in (latch) contact across the Start button.

Star-delta starting is not suitable for high-inertia loads that need full torque during acceleration — the reduced starting torque (also one-third of DOL) may not accelerate the load sufficiently before transition.

How to wire star delta starter wiring

Isolate the supply and apply lockout-tagout Switch off and lock out the main circuit breaker feeding the starter panel. Verify all three phases are dead at the panel busbars using a calibrated three-phase voltage tester before opening the enclosure.
Confirm the motor has six accessible terminals Open the motor terminal box and verify six winding terminals are present (U1, V1, W1, U2, V2, W2 per IEC, or T1–T6 per NEMA). Remove any existing star or delta links. Record the original terminal arrangement before disconnecting anything.
Wire the main contactor (KM1) to the supply and motor Connect three-phase supply (L1/L2/L3) to the input of KM1. Connect the output of KM1 to motor terminals U1, V1, W1. KM1 closes on Start and remains closed throughout starting and running.
Wire the star contactor (KM3) to short the winding ends Connect all three output terminals of KM3 together and then to motor terminals U2, V2, W2. When KM3 closes, the three winding ends are shorted, forming the star neutral point. KM3 must be interlocked so it cannot close when KM2 is closed.
Wire the delta contactor (KM2) for running Connect KM2 to cross-connect the winding ends to the supply ends in delta configuration: U2 connects to V1 (L2 supply), V2 connects to W1 (L3 supply), W2 connects to U1 (L1 supply). KM2 must be interlocked so it cannot close when KM3 is closed.
Wire the control circuit Connect the Stop NC push button, Start NO push button, and KM1 hold-in auxiliary contact in series. Wire the timer relay (KT) coil to energise with KM1. Connect KT timed-close contact to energise KM2 coil and a KT auxiliary to de-energise KM3 coil at transition. Include the overload relay NC contact in series with all coils.
Test the sequence before connecting the motor With the motor disconnected, energise the starter and verify the contactor sequence: KM1 and KM3 close on Start; after the timer period, KM3 opens and KM2 closes. Use a multimeter to confirm interlocks prevent simultaneous closure. Then connect the motor and carry out a loaded test run, monitoring current during the transition.

Specifications

Supply voltage (typical)	380–415 V AC three-phase, 50 Hz (IEC); 460 V, 60 Hz (NEMA)
Starting current (star)	Approximately 33% of DOL locked-rotor current (1/3 of full DOL inrush)
Starting torque (star)	Approximately 33% of DOL starting torque — not suitable for high-inertia or heavy-start loads
Star run period (typical)	3–15 seconds — set by timer relay based on load inertia and motor size
Transition voltage step	Motor sees 58% of line voltage in star (line voltage ÷ √3); 100% in delta
Minimum motor terminal count	Six separate terminals (U1, V1, W1, U2, V2, W2) — cannot use internally-connected motors
Overload relay position	Must be in motor winding leads (delta circuit), not incoming supply lines
Applicable standards	IEC 60947-4-1 (contactors), IEC 60034 (motors); local wiring regs apply

Safety warnings

Always isolate and lock out all three phases of the supply before working inside the starter enclosure. Three-phase systems are lethal — verify all phases are dead with a calibrated three-phase voltage tester. Do not rely on the main isolator being off without confirming with a tester.
The interlock between KM2 (delta) and KM3 (star) is a critical safety requirement — simultaneous closure causes a direct phase-to-phase short circuit. Both mechanical (auxiliary contact cross-interlock) and electrical interlocks must be fitted and tested before commissioning.
Star-delta starters do not reduce starting torque risk to the driven machine — they reduce electrical current but the torque step at delta transition can cause mechanical shock on couplings, belts, and driven equipment. Ensure mechanical connections are correctly specified for the application.
The thermal overload relay must be wired to measure current in the delta (running) configuration — in the motor winding leads, not the incoming supply lines. If wired in the wrong position, it will read only 58% of actual winding current in delta and will not provide correct overload protection.
All star-delta starter installations must be designed, installed, and commissioned by a qualified electrician in accordance with the applicable standard for your jurisdiction (IEC 60947, NEC/NFPA 70, BS 7671, SANS 10142, or AS/NZS 3000). This diagram is for reference and educational purposes only.

Tools needed

Calibrated three-phase voltage tester
Lockout-tagout kit (three-phase rated)
Digital multimeter with continuity and AC voltage functions
Clamp meter (AC current, 0–600 A range for commissioning)
Insulated screwdrivers (flathead and Phillips)
Crimping tool and ferrule kit for control wiring
Wire strippers
Cable labels and marker

Common mistakes

Omitting or bypassing the electrical interlock between KM2 and KM3 — this is a critical safety requirement, not optional.
Setting the transition timer too short, causing the motor to be still at low speed when it transitions to delta, resulting in a large current spike and possible overload trip or motor stall.
Wiring the overload relay in the supply lines rather than in the winding connections — this causes incorrect trip settings and unreliable motor protection.
Using the star-delta starter on a motor with only three terminals (internally connected) — it will not function and may damage the motor.
Failing to test the contactor interlock sequence before connecting the motor — always simulate the start cycle and verify that KM2 and KM3 cannot close simultaneously.
Using contactors all rated identically when KM3 (star) can be a smaller, lower-rated device — oversizing is not dangerous but undersizing KM1 or KM2 is.

Troubleshooting

Motor trips on overload during or just after star-to-delta transition: Cause: Transition timer set too short — motor has not reached sufficient speed in star, so delta transition causes excessive current surge Fix: Increase the star run time on the timer relay. Monitor starting current with a clamp meter during starting. Allow the current in star to drop near to FLA before transitioning. If the motor cannot accelerate adequately in star, the load may be too high for this starting method.
Contactor KM2 and KM3 both close and blow fuses: Cause: Electrical interlock missing, failed, or wired incorrectly — both contactors energise simultaneously causing a phase short Fix: Isolate and lock out supply immediately. Check interlock wiring: KM2's NC auxiliary must be in series with KM3's coil circuit, and KM3's NC auxiliary must be in series with KM2's coil circuit. Also verify any mechanical interlocks on the contactor block are correctly fitted.
Motor only runs in star — does not transition to delta: Cause: Timer relay not energising, timer contact not closing, or KM3 auxiliary contact not releasing to allow KM2 to pick up Fix: With care, observe the timer relay indicator during a start. Check that KT coil energises with KM1. Check the KT timed-close contact with a multimeter after the set time elapses. Verify KM3 drops out before KM2 picks up — an interlock delay or sticky contactor can prevent transition.
Motor hums in star position but does not accelerate: Cause: One phase missing at the motor — single phasing during start Fix: Isolate and lock out. Check all three supply fuses. Check all three poles of KM1 and KM3 for burnt contacts. Single-phasing during star start produces low torque and humming — the motor will overheat rapidly if left in this condition.

Frequently asked questions

Why does my motor stall or slow down during the star-to-delta transition?

There is a brief torque interruption during changeover. If the timer is set too short, the motor has not reached sufficient speed by the time it transitions to delta, causing a current spike and possible trip. Extend the star run time. If the load inertia is very high, star-delta may be unsuitable — consider a soft starter instead.

What happens if the star and delta contactors close at the same time?

A direct phase-to-phase short circuit occurs, causing a fault current that will blow fuses or trip the circuit breaker and can destroy contactor contacts and motor windings. This is why mechanical and electrical interlocking between KM2 and KM3 is mandatory — never omit the interlock contacts.

Does the motor need six terminals for star-delta starting?

Yes, all six motor winding terminals must be accessible. A motor with only three terminals has the windings internally connected (already in star or delta) and cannot be started with an external star-delta starter. Check the motor terminal box for six separate terminals before specifying this starting method.

How do I set the transition timer on a star-delta starter?

There is no universal setting — it depends on motor size, load inertia, and voltage. As a starting point, set the timer to allow the motor to reach approximately 80% of full speed in star before transitioning. Monitor starting current with a clamp meter and adjust the timer until the transition current spike is acceptable.

What is the wiring difference between star and delta connections?

In star: the three winding ends (U2/V2/W2) are shorted to a common neutral point; supply is applied to U1/V1/W1. In delta: each winding end connects to the start of the next winding in a triangle — U2 joins V1, V2 joins W1, W2 joins U1 — creating a closed loop with supply connected at the three junction points.

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