Star Delta Motor Connection

Star Delta Motor Connection — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable star delta motor connection: download the diagram as SVG or open it and print to paper or PDF.

A star-delta motor connection shows how a three-phase induction motor's stator windings are reconfigured from star (wye) to delta during starting to limit inrush current and reduce mechanical stress.

Star-delta starting is the most common reduced-voltage starting method for three-phase induction motors in industrial and commercial installations. Its purpose is to limit the high inrush current that occurs when a large motor is started directly on-line — direct-on-line starting draws 6–8 times the full-load current, which stresses the electrical supply and causes voltage dips that affect other equipment on the same bus.

The motor must have six accessible stator winding terminals (U1, V1, W1 for the winding start terminals, and U2, V2, W2 for the winding ends) to permit a star-delta connection. Motors with only three terminals are already internally connected and cannot be used with this starting method.

In the star (wye) configuration, the three winding ends (U2, V2, W2) are connected together to form a common neutral point. Each winding is then connected between one supply phase and this neutral. The voltage across each winding in star is the phase-to-neutral voltage — for a 400 V three-phase supply, that is approximately 231 V per winding. Starting in star reduces starting torque to one-third of direct-on-line torque and limits starting current to approximately one-third of direct-on-line starting current.

In the delta configuration, the three windings are connected end-to-end in a closed loop. Each winding sees the full phase-to-phase voltage (400 V on a 400 V supply). Torque and full-load current are as designed for the motor's nameplate rating.

The transition from star to delta typically occurs at a preset time (2–15 seconds depending on load) set on a star-delta timer. During transition, there is a brief open-transition period where the motor is disconnected from the supply before being reconnected in delta. This causes a current transient as the motor re-accelerates. Closed-transition star-delta starters include a resistor to limit this transient.

The power circuit requires three contactors: a main (KM) contactor, a star (KY) contactor, and a delta (KD) contactor, plus a thermal overload relay sized for the full-load current.

How to wire star delta motor connection

Verify motor terminal configuration Check the motor nameplate for voltage rating and terminal arrangement. The motor must have six accessible terminals (U1, V1, W1, U2, V2, W2) and must be rated for delta connection at the supply voltage. Open the terminal box and identify the six terminals using the motor manufacturer's terminal diagram.
Connect the main contactor (KM) to the supply and motor The three supply phases (L1, L2, L3) connect to the main contactor (KM) input terminals. The KM output terminals connect to the three motor start terminals: L1 → U1, L2 → V1, L3 → W1. This connection remains constant in both star and delta operating modes.
Connect the star contactor (KY) The star contactor (KY) shorts the three motor end terminals together to form the star neutral point. Wire U2, V2, and W2 to the three poles of the KY contactor; when KY closes, it connects all three to a common point, placing the windings in star. This contactor must be interlocked electrically with the delta contactor (KD) so both cannot close simultaneously.
Connect the delta contactor (KD) The delta contactor (KD) reconfigures the windings into a closed loop. Connect: U2 to W1 (or L3), V2 to U1 (or L1), W2 to V1 (or L2). In practice, the delta contactor connects the motor end terminals to the supply phases in a cross-connection pattern that closes the delta loop. Verify the exact connection sequence against the starter manufacturer's wiring diagram for the specific contactor arrangement.
Include electrical interlocking between KY and KD A normally closed (NC) auxiliary contact of KY must be wired in series with the KD coil circuit, and an NC auxiliary contact of KD must be in series with the KY coil circuit. This prevents both contactors from energising simultaneously — a potentially catastrophic line-to-line short circuit through the motor windings.
Set the thermal overload relay The thermal overload relay (OL) must be sized for the motor's full-load current (FLC) as stated on the nameplate. Position the overload relay in the main contactor circuit (after KM, before the motor) so it monitors line current in both star and delta modes. Note that in star connection the motor draws only about 58% of delta full-load current — the overload will not trip in star even if running longer than planned, so do not rely on the overload for star-mode protection.
Set the transition timer and test under no-load Set the star-delta timer to an initial value (3–6 seconds is a common starting point). Apply power with no load on the motor and observe the start sequence: KM and KY should close simultaneously on start, the motor accelerates in star, the timer times out, KY opens, and KD closes. Listen for the transition and check that the motor continues to accelerate to full speed in delta.

Specifications

Starting current (star-delta vs. direct-on-line)	Approximately 1/3 (33%) of DOL starting current
Starting torque (star-delta vs. direct-on-line)	Approximately 1/3 (33%) of DOL starting torque
Winding voltage in star (400 V supply)	231 V (phase-to-neutral)
Winding voltage in delta (400 V supply)	400 V (phase-to-phase)
Motor terminal requirement	Six externally accessible terminals (U1, V1, W1, U2, V2, W2)
Motor nameplate connection requirement	Motor must be rated for delta connection at the supply line voltage
Typical star-period timer range	2–15 seconds, set to achieve near-full-speed acceleration before delta transition
Applicable standards	IEC 60947-4-1 (contactors and motor starters), IEC 60034 (motor ratings), NEMA ICS 2

Safety warnings

Star-delta motor starters must be installed by a licensed electrician in accordance with IEC 60364, NEC/NFPA 70, BS 7671, or AS/NZS 3000. Three-phase motor circuits operate at voltages (typically 380–415 V) that are immediately lethal. Always isolate, lock out, and verify dead before working on any part of the circuit.
Electrical interlocking between the star and delta contactors is mandatory — not optional. If KY and KD close simultaneously, a direct phase-to-phase short circuit occurs through the motor windings, potentially destroying the motor, contactors, and causing arc flash.
Never attempt to increase the star-period timer setting to the point that the motor runs continuously in star. Star connection exposes each winding to the full supply voltage phase-to-neutral, but the winding is rated for full phase-to-phase voltage. Prolonged star operation will overheat and destroy the motor.
High-voltage, high-current arc flash is possible inside motor control panels. Adhere to arc flash risk assessments, use appropriate PPE (arc flash rated face shield, gloves, arc-rated clothing), and follow lock-out/tag-out procedures.
The transition from star to delta produces a current transient. Ensure the mechanical system (shaft, coupling, driven machine) is designed to withstand the associated torque impulse. Sudden torque steps can shear couplings or damage gearboxes on sensitive machinery.

Tools needed

Digital multimeter (AC voltage, continuity, resistance)
Clamp meter (for measuring motor starting and running current)
Insulated screwdrivers (for control and power wiring)
Torque screwdriver or wrench (for contactor and cable terminal torquing)
Phase rotation tester or phase sequence indicator
Lock-out/tag-out equipment
Personal protective equipment: arc flash rated face shield, insulated gloves, arc-rated clothing

Common mistakes

Wiring the motor end terminals (U2, V2, W2) to the delta contactor in the wrong sequence, creating a reverse-phase delta that runs the motor backwards — not immediately obvious without checking rotation before connecting the load.
Omitting the electrical interlock between KY and KD contactors, relying only on mechanical interlocking that may be insufficient to prevent simultaneous closure during contactor bouncing.
Setting the star period timer too short, switching to delta before the motor has accelerated sufficiently and causing a severe current transient that trips the overload or blows fuses.
Using a motor rated for star connection at the supply voltage (e.g., 400 V star motor on a 400 V supply) — the motor will run correctly in star but cannot be connected in delta without exceeding the winding voltage rating.
Not accounting for the star-period when sizing the thermal overload — in star, the line current is approximately 58% of the delta FLC, so the overload set to the delta FLC will not trip even if the star period is extended significantly.
Running high-torque, high-inertia loads on a star-delta starter without first verifying that sufficient torque is available in star to accelerate the load to near-synchronous speed within the timer period.

Troubleshooting

Motor fails to accelerate to full speed before timer switches to delta: Cause: Star timer set too short, or load breakaway torque is too high for the reduced starting torque available in star Fix: Increase the star timer setting in small increments and observe acceleration. If the motor cannot reach near-synchronous speed even with a longer star period, the load is too heavy for star-delta starting — consider a soft starter or VFD.
Heavy current transient and supply voltage dip at star-to-delta transition: Cause: Open-transition switching where the motor residual flux is out of phase with the supply at the moment of delta connection Fix: Reduce the star period slightly so the motor is still accelerating (less residual flux phase offset) at transition. For sensitive applications, replace the open-transition starter with a closed-transition type that bridges the transition with resistors.
Motor runs in star then trips the overload at delta transition: Cause: Motor not at sufficient speed at transition, causing an excessive delta in-rush current, or overload set too low Fix: Increase the star timer period. Verify the overload relay setting is correctly matched to the motor's nameplate FLC. Check that the motor is not already close to its thermal limit from multiple start attempts in succession.
Contactor burning or pitting at star-to-delta changeover: Cause: Contactors switching under high current at transition, or interlock timing allows brief simultaneous closure Fix: Review the interlock circuit and confirm that KY fully opens before KD closes. Add a short time delay (50–100 ms) between KY opening and KD closing if the control circuit does not already include this. Check contactor contact condition and replace if pitted.
Motor runs with excessive vibration in delta mode: Cause: Phase imbalance, incorrect delta terminal connections, or mechanical issue Fix: Measure line current in each phase in delta mode with a clamp meter. Phase currents should be balanced within approximately 5%. If one phase is significantly lower or higher, suspect incorrect terminal connections or a supply phase issue. Verify motor terminal labelling against the wiring diagram.

Frequently asked questions

What motors can use star-delta starting?

Only three-phase induction motors with six externally accessible stator terminals (U1, V1, W1, U2, V2, W2) designed to run in delta on the supply voltage. The motor nameplate must show a delta connection voltage matching the supply. A motor rated 230/400 V (delta/star) runs in delta on a 230 V supply — it is not suitable for star-delta starting on a 400 V supply.

By how much does star-delta starting reduce inrush current?

Star-delta starting reduces starting current to approximately one-third (33%) of the direct-on-line starting current. It also reduces starting torque to one-third of direct-on-line starting torque. This means it is only suitable for loads that are easy to accelerate — high-resistance or heavily loaded starts may fail to accelerate to full speed before the timer switches to delta.

What is the open-transition current transient?

At the moment of switching from star to delta, the motor is momentarily disconnected from the supply. The motor's residual magnetic flux can be out of phase with the supply, and reconnection causes a transient current spike that can approach or exceed the direct-on-line starting current. Closed-transition starters avoid this by introducing a resistor during the changeover.

How is the star-delta timer set?

The timer should be set so that the motor reaches close to full speed before switching to delta, but not so long that the motor runs continuously in star (which would overheat the windings at delta-rated voltage). In practice, start with a timer setting of 3–6 seconds and adjust based on observed motor acceleration — monitor the current and increase the time if the motor has not stabilised before switching.

Can I use a star-delta starter on a conveyor with a full load at startup?

Star-delta starting is generally not suitable for high-breakaway torque loads. Since starting torque is reduced to one-third of direct-on-line torque, the motor may fail to accelerate the load, causing it to run indefinitely in star — a dangerous condition that will overheat the motor. High-torque loads require soft starters, autotransformer starters, or variable frequency drives.