Star Delta Starter Circuit Diagram: KM1, KM2, KM3 Contactor Wiring, Timer and Interlocks

Star Delta Starter Circuit Diagram: KM1, KM2, KM3 Contactor Wiring, Timer and Interlocks — interactive diagram. Open it in the editor to customise components and wiring.

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A star delta starter circuit uses three contactors and a timer to reduce inrush current during motor starting, transitioning from a star connection through to full delta running.

The star-delta (Y-Δ) motor starter is one of the most widely used reduced-voltage starting methods for three-phase squirrel-cage induction motors. Its purpose is to limit the high inrush current that occurs when a large motor is connected directly to full supply voltage — a direct-on-line (DOL) start can draw 6–8 times the motor's full-load current for several seconds, stressing cables, switchgear, and the supply network. The star-delta starter reduces starting inrush to approximately 33 % of the DOL value by initially connecting the motor windings in star (Y) configuration, then automatically transitioning to delta (Δ) when the motor has accelerated to near-running speed.

The circuit uses three main contactors, universally designated KM1, KM2, and KM3 in IEC-compliant diagrams:

KM1 is the main (line) contactor. It connects the motor terminals U1, V1, W1 to the supply phases L1, L2, L3. KM1 closes on start and remains closed throughout both star and delta running.

KM2 is the star contactor. It short-circuits the three motor winding ends U2, V2, W2 together to form the star point. KM2 closes simultaneously with KM1 on start, connecting the motor in star configuration for the starting period.

KM3 is the delta contactor. It interconnects the motor winding ends U2, V2, W2 back to the supply (in the cross-connection pattern that forms a delta). KM3 closes after the timer operates, transitioning the motor to delta — full-voltage running configuration.

The critical interlock between KM2 and KM3 prevents both contactors from being closed simultaneously, which would create a three-phase short circuit. The interlock is implemented using normally closed (NC) auxiliary contacts: a NC auxiliary contact of KM3 is wired in series with the KM2 coil circuit, and a NC auxiliary contact of KM2 is wired in series with the KM3 coil circuit. Additionally, a mechanical interlock is strongly recommended — an electrical-only interlock is not sufficient for equipment where failure could cause injury or equipment damage.

The transition from star to delta is timed by a timer relay (KT), typically an ON-delay type, set to the motor's accelerating time — usually 5–15 seconds depending on the driven load inertia. During the transition instant, both KM2 and KM3 are briefly open. Some designs insert a brief dead time or use a closed-transition method to reduce the voltage surge during changeover.

The star-delta starter is one of the most widely used reduced-voltage starting methods for three-phase induction motors, and its circuit diagram is a staple of electrical engineering courses. The circuit of a star-delta starter comprises a main contactor, a star contactor, a delta contactor, an overload relay, and a timing relay that controls the transition from star to delta after a set period. You can diagram a complete star-delta starter — both power and control circuits — free in the browser at circuitdiagrammaker.com.

How to wire star delta starter circuit diagram

Verify motor suitability for star-delta starting Confirm the motor has six accessible terminal leads (U1, V1, W1, U2, V2, W2). Verify the motor's delta voltage rating matches the supply voltage — on a 400 V three-phase supply, the motor must be rated for 400 V in delta configuration (often shown as 230/400 V, star/delta). A 400/690 V motor on a 400 V supply must not be connected in delta.
Select contactors, overload relay, and timer KM1 must be rated for the motor's full-load current in delta. KM2 and KM3 carry line and phase currents that differ during star and delta operation — select per manufacturer's star-delta contactor selection guide. The overload relay connects in the motor supply lines and is set to the motor's full-load current. The timer (KT) is an ON-delay type, adjustable range covering the expected acceleration time.
Wire the main power circuit Supply phases L1, L2, L3 connect to the input terminals of KM1. KM1 output connects to motor terminals U1, V1, W1 and to the input terminals of KM3. KM2 star contactor output terminals short U2, V2, W2 together. KM3 delta contactor connects U2 to L2 (or the equivalent phase cross-connection) — the exact cross-wiring creates the delta — verify from the wiring diagram for the contactor arrangement and motor rotation direction required.
Wire the NC electrical interlock between KM2 and KM3 In the control circuit, wire a normally closed auxiliary contact of KM3 in series with the KM2 coil supply. Wire a normally closed auxiliary contact of KM2 in series with the KM3 coil supply. This ensures that if one contactor is energised, it breaks the coil supply path to the other, preventing simultaneous closure. Also fit mechanical interlocks on the contactor pair if the manufacturer provides them.
Wire the control circuit for start, stop, and timer START push-button (normally open) energises KM1 and KM2 coils simultaneously and starts timer KT. KM1 auxiliary contact (NO) latches the START circuit. Timer KT's timed-close contact is wired to energise KM3 after the set delay. KM3 energising simultaneously causes the wired NC interlock to open the KM2 coil circuit, de-energising KM2 (star contacts open before KM3 closes in properly sequenced designs). STOP push-button (NC) de-energises KM1 and the entire circuit.
Set the timer delay With the motor and load disconnected (safe conditions), calculate the expected run-up time from knowledge of the load's inertia or from a test observation with a stopwatch. Set the timer to that duration. Initial commissioning should be done with the load disconnected or at minimum load, observing motor speed (using a tachometer) and current (using a clamp meter) during the star-to-delta transition.
Commission, test, and set overload relay With all safety checks complete, perform a controlled start. Observe current during star starting and during the star-to-delta transition. The transition should not produce a current spike greater than approximately 3–4 times full-load current. Verify motor runs up to speed and reaches stable full-load current in delta. Set the overload relay to the motor's rated full-load current per the nameplate.

Specifications

Starting current reduction (star vs DOL)	Approximately 1/3 of DOL starting current (33 %)
Starting torque reduction (star vs DOL)	Approximately 1/3 of DOL starting torque (33 %)
KM1 contactor utilisation category	AC-3 (starting and switching squirrel-cage motors)
KM2 / KM3 current rating relative to KM1	≥ 58 % of KM1 rating (phase current in delta = line current / √3)
Timer type for standard open-transition star-delta	ON-delay timer, timing range typically 5–15 s (adjust per load inertia)
Motor terminal configuration required	Six accessible terminals: U1, V1, W1 (winding start) and U2, V2, W2 (winding end)
Motor voltage requirement (400 V supply example)	Motor must be rated 400 V in delta (not 690 V delta / 400 V star)
IEC reference standard	IEC 60947-4-1 (Low-voltage switchgear: contactors and motor starters)

Safety warnings

Star-delta starters operate at full three-phase line voltage (typically 400 V AC or 690 V AC). All work on star-delta starter panels must be performed by qualified electrical engineers or electricians in compliance with IEC 60364, BS 7671, AS/NZS 3000, NFPA 70, or the applicable national standard. Lock out and isolate the main circuit breaker before working inside any panel — verify dead on all three phases with an approved voltage tester before touching any conductors.
The electrical interlock alone between KM2 and KM3 is necessary but not sufficient. A welded or stuck-closed contact could fail the electrical interlock without warning. Always install mechanical interlocks between the star and delta contactors as well. Never depend solely on software or electrical logic to prevent a catastrophic simultaneous closure.
Verify the motor's terminal voltage rating before connecting in star-delta configuration. A motor rated 400/690 V (delta/star) connected in delta on a 690 V supply will be at correct voltage. The same motor connected in delta on a 400 V supply is also correct. A motor rated 690 V delta only must not be connected in star on a 400 V supply — it will run at approximately 230 V in star and will be severely under-voltaged under load.
During the transition from star to delta, a voltage transient occurs as the switching takes place under load. On high-inertia loads, this transient can produce surge currents approaching DOL levels if the transition occurs before the motor has sufficiently accelerated. Do not reduce the timer delay below the minimum required acceleration time.
Fit the overload relay in the supply lines (not the star-point wiring) and set it to the motor's full-load current nameplate rating. An incorrectly set or bypassed overload relay removes thermal protection from the motor and can cause winding insulation failure or fire.

Tools needed

True-RMS digital multimeter (AC voltage, resistance, continuity)
Clamp meter (for measuring line and phase current during commissioning)
Lockout/tagout device and padlock
Approved voltage tester (non-contact and probe type) for three-phase verification
Torque screwdriver (for terminal torquing to manufacturer specification)
Oscilloscope or current logger (for observing transition current behaviour during commissioning)

Common mistakes

Omitting the mechanical interlock between KM2 and KM3 and relying only on the electrical auxiliary contact interlock — a contact weld in one contactor can fail the electrical interlock undetected, leading to a simultaneous star-delta closure and a fault.
Setting the timer delay too short — the motor has not sufficiently accelerated and the transition to delta produces a large current surge that may trip the overload relay or damage the motor windings.
Wiring KM3 delta cross-connections incorrectly — the delta connection requires specific cross-wiring of the winding ends U2, V2, W2 back to the opposite supply phases. An incorrect cross-connection produces a phase-to-phase short in the delta or incorrect motor rotation.
Setting the thermal overload relay to line current rather than the motor's rated current — in a star-delta starter, the overload relay must be set to the motor's full-load current as stated on the nameplate, not to a derated value.
Using single-phase rated auxiliary contacts in the interlock circuit — the auxiliary contact must be rated for the control circuit voltage and current; using contacts rated below the coil drive voltage causes premature contact wear and interlock unreliability.

Troubleshooting

Motor starts in star but overload relay trips during or immediately after transition to delta: Cause: Timer delay is too short — motor had not reached sufficient speed before delta transition, causing excessive transition current that trips the overload relay Fix: Increase the timer delay in increments of 2–3 seconds and retest. Monitor current with a clamp meter during the star-to-delta transition — the transition current spike should not significantly exceed the motor's full-load current. If persistent, consider a closed-transition starter or a soft starter.
Motor will not start — contactors are energised but no motor movement: Cause: Power circuit fault: phase missing, overload relay tripped, motor winding fault, or delta contactor cross-wiring error creating an internal short Fix: Isolate the circuit. Verify supply voltage on all three phases at the circuit breaker output. Check overload relay reset position. Verify motor terminal connections against the wiring diagram — particularly the delta cross-connections at KM3. Test motor winding resistance for open or short circuits.
Circuit breaker trips immediately when START is pressed: Cause: A short circuit in the main power circuit — wiring error in KM3 delta connections, damaged motor winding, or a contactor contact welded in the wrong position Fix: Isolate the circuit breaker. Disconnect the motor terminals from the starter output. Test insulation resistance between each motor terminal and the other terminals, and each terminal to earth. If motor insulation tests good, trace the power wiring in the starter for connection errors — the delta cross-wiring is the most common error location.

Frequently asked questions

Why is a star-delta starter used instead of DOL starting?

A direct-on-line (DOL) start applies full supply voltage instantly, producing starting current of 6–8 times the motor's full-load current. This current surge stresses the motor windings, supply cables, and switchgear, and can cause voltage dips that affect other equipment on the same supply. A star-delta starter reduces starting current to approximately one-third of the DOL starting current, significantly reducing mechanical shock and supply disturbance.

What is the correct sequence of operation for a star-delta starter?

On pressing START: KM1 (main) closes; KM2 (star) closes simultaneously — motor runs in star. Timer KT begins timing. After the set time elapses: KM2 (star) opens; after a brief pause (in some designs), KM3 (delta) closes — motor runs in delta at full voltage. KM1 remains closed throughout. On pressing STOP: KM1 opens, and if KM3 is closed it opens simultaneously, disconnecting the motor completely.

What happens if the star and delta contactors close simultaneously?

KM2 (star) closing while KM3 (delta) is also closed creates a dead short across two phases of the supply through the motor windings, producing an extremely high fault current that will trip or destroy the circuit protective devices and can damage the contactors, motor windings, and cabling. The electrical interlock using NC auxiliary contacts and a mechanical interlock must both be present to prevent this.

What timer delay should I set for a star-delta starter?

The timer delay should be set to the time required for the motor to accelerate the driven load to approximately 75–90 % of full speed in star connection. For lightly loaded machines this may be 3–8 seconds; for high-inertia loads (fans, pumps, flywheels) it may be 10–20 seconds or longer. Set too short a delay causes a high transition current surge; too long a delay causes excessive motor heating as the under-connected star winding is stressed by the load.

Can a star-delta starter be used with any three-phase motor?

Only with motors that have all six winding terminals accessible at the terminal box: U1, V1, W1 (winding starts) and U2, V2, W2 (winding ends). Motors with only three terminals (already internally connected in delta or star) cannot be star-delta started externally. The motor must also be designed for the full supply voltage in delta — a motor rated 400 V delta / 690 V star on a 400 V supply cannot be star-delta started (it would be over-voltaged in delta).

What is the circuit diagram of a star-delta starter?

A star-delta starter circuit has two parts: the power circuit and the control circuit. In the power circuit, the main contactor (KM1) connects the motor terminals to the supply; the star contactor (KM3) short-circuits the motor's winding ends to form a star (Y) connection at start; after a timer delay the star contactor opens and the delta contactor (KM2) closes to reconnect the windings in delta for normal running. The control circuit uses a timer relay to sequence KM1, KM3, and KM2, with electrical interlocking between KM2 and KM3 to prevent both closing simultaneously. This method reduces starting current to approximately one-third of the direct-on-line value.

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