Power Diagram of Star Delta Starter

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The power diagram of a star-delta starter shows how three contactors — main, star, and delta — interconnect to apply reduced voltage to a three-phase motor during starting, then reconnect the windings at full voltage for running.

A star-delta (wye-delta) starter is a reduced-voltage starting method for three-phase squirrel-cage induction motors. By connecting the motor windings in star (wye) during starting, each winding receives only 58 % (1/√3) of the line voltage, reducing starting current to approximately one-third of the direct-on-line (DOL) starting current. Once the motor has accelerated to near full speed, a timed transition reconnects the windings in delta at full line voltage for normal running.

The power circuit of a star-delta starter consists of three contactors: the main contactor (KM1), the star contactor (KM3), and the delta contactor (KM2). Understanding the power circuit is distinct from the control circuit; this explanation focuses on the power wiring.

In the star (starting) position, KM1 and KM3 are closed, KM2 is open. KM1 connects the three supply lines (L1, L2, L3) to the motor's first set of winding terminals (U1, V1, W1). KM3 simultaneously shorts the motor's second set of winding terminals (U2, V2, W2) together, forming the star point. Current flows through each winding from a supply line, through the winding, and returns via the star point. Each winding sees line-to-star voltage = line voltage / √3.

In the delta (running) position, KM1 remains closed. KM3 opens. After a brief open-circuit transition pause (to allow KM3 to fully interrupt before KM2 closes), KM2 closes, connecting the motor's winding end terminals (U2, V2, W2) in a ring with the start terminals (U1, V1, W1). In delta, each winding is connected directly across a pair of supply lines, receiving full line voltage. The delta contactor (KM2) carries less current than the main contactor — each delta contactor phase carries 1/√3 of the total line current.

Motor terminal identification is critical. The motor must have six accessible terminals (two per winding: start and end). The motor nameplate must state that the delta voltage matches the supply voltage — if the motor nameplate shows 230 V/400 V and the supply is 400 V, the motor can only be run in star permanently (not suitable for star-delta starting at 400 V, which requires a delta-voltage-rated motor). For a star-delta starter on a 400 V supply, the motor delta winding voltage must equal 400 V (meaning the motor nameplate shows 400 V/690 V).

How to wire power diagram of star delta starter

Verify motor suitability for star-delta starting Confirm the motor has six accessible terminals (U1, V1, W1, U2, V2, W2 — or T1 through T6 in North American notation). Confirm the motor nameplate shows a delta voltage equal to the supply voltage (e.g. on a 400 V supply, the motor must show 400 V delta / 690 V star). If the motor nameplate shows 230 V delta / 400 V star on a 400 V supply, star-delta starting at 400 V is not possible.
Select and size the three contactors The main contactor (KM1) must be rated for AC-3 duty at the full motor line current. The delta contactor (KM2) carries 1/√3 of line current (approximately 58 %) — it can be rated at 58 % of the main contactor rating. The star contactor (KM3) carries the same current as the delta contactor during starting, but only for the star period — it can be rated at 58 % of the main contactor rating or matched to the delta contactor. Many manufacturers supply purpose-built star-delta contactor assemblies with pre-wired interlocks.
Wire the main contactor power circuit Connect the three supply lines L1, L2, L3 to the top input terminals of the main contactor KM1. Wire the output terminals of KM1 to the motor's start terminals: KM1/T1 to motor U1, KM1/T2 to motor V1, KM1/T3 to motor W1. These three wires supply power to the motor's winding start points in both star and delta conditions.
Wire the star contactor power circuit The star contactor (KM3) shorts the motor's winding end terminals together to form the star point. Connect one set of KM3 terminals (three terminals) to motor U2, V2, and W2 respectively. Connect the other set of KM3 terminals together to form the star point — all three terminals on that side are joined to each other (but connected to nothing else). When KM3 closes, U2, V2, and W2 are all shorted together.
Wire the delta contactor power circuit The delta contactor (KM2) connects the winding end terminals (U2, V2, W2) to the supply lines in a cross-connected fashion to form the delta. The connection is: motor U2 to supply line L3 (or the junction with V1 on the KM1 output), motor V2 to L1 junction (W1 side), motor W2 to L2 junction (U1 side). The exact cross-connection depends on the motor's winding direction and the desired rotation — consult the motor terminal diagram. The goal is that when delta is formed: U1–U2 winding connects across L1–L2, V1–V2 across L2–L3, W1–W2 across L3–L1.
Install mechanical and electrical interlocks between KM2 and KM3 It is essential that the star contactor (KM3) and delta contactor (KM2) can never be closed simultaneously. Simultaneous closure short-circuits two supply lines through the motor windings. Fit mechanical interlock links between KM2 and KM3 actuators. Additionally wire the NC auxiliary contact of KM2 in series with the KM3 coil circuit, and the NC auxiliary contact of KM3 in series with the KM2 coil circuit (electrical interlock).
Set the timer relay and test Set the star-to-delta transition time on the timer relay. The correct time is typically when the motor has accelerated to approximately 80–90 % of full speed — for most lightly loaded motors this is 5–15 seconds. Too short a transition leaves the motor below full speed at the delta transition, causing a large current surge. Too long a transition keeps the motor in reduced-torque star mode unnecessarily. Monitor motor current during the first start and adjust the timer as needed.

Specifications

Starting current (star-delta vs DOL)	Approximately 1/3 of DOL starting current
Starting torque (star-delta vs DOL)	Approximately 1/3 of DOL starting torque
Winding voltage in star (on 400 V supply)	231 V (400 V / √3)
Winding voltage in delta (on 400 V supply)	400 V (full line voltage)
Delta contactor current rating	58 % of main contactor rating (1/√3 of line current)
Typical star-to-delta transition time	5–15 seconds (load dependent)
Motor requirement	Six terminals accessible; delta winding voltage = supply voltage
Overload relay setting (in main circuit)	Motor nameplate FLA

Safety warnings

Star-delta starter wiring involves three-phase mains voltage (typically 400 V or 480 V). All installation and maintenance work must comply with the applicable electrical code (NEC/NFPA 70, BS 7671, AS/NZS 3000, or IEC 60364) and must be performed by a licensed electrician. Always isolate all three supply phases at the upstream disconnecting means and verify all three phases are dead before working on the starter.
Simultaneous closure of the star contactor (KM3) and delta contactor (KM2) creates a direct short circuit across supply lines through the motor windings. Both mechanical and electrical interlocks between KM2 and KM3 are mandatory — never commission a star-delta starter without verifying both interlocks are fitted and functional.
The six motor terminal wires inside the star-delta starter carry mains voltage whenever the starter is energised. Correct identification of motor terminal connections (U1, V1, W1, U2, V2, W2) is critical — an incorrect delta wiring arrangement will cause a short circuit on closure of KM2.
Do not attempt to modify the timer relay setting while the motor is running. The transition from star to delta while the motor is at low speed produces a severe current and torque surge that can damage the motor windings and driven equipment.
Motor winding access covers and the control panel door must be secured with an appropriate interlock or padlock to prevent access to live parts. Follow local electrical safety regulations for work on live parts.

Tools needed

Digital multimeter (voltage measurement and continuity)
Clamp-on ammeter (motor current verification during star and delta phases)
Insulated screwdrivers
Torque screwdriver and torque wrench (terminal tightening to specification)
Tachometer or motor speed meter (optional, for verifying speed at transition)
Phase rotation meter (verifying correct phase sequence)

Common mistakes

Using a motor whose delta voltage rating does not match the supply voltage — attempting star-delta starting when the motor is rated 230 V delta on a 400 V supply will overload the delta windings when they reach full voltage.
Omitting the mechanical interlock between KM2 and KM3, relying only on the electrical interlock. A welded contact on KM3 will not be overridden by the electrical interlock alone, causing a short circuit when KM2 is commanded to close.
Setting the transition timer too short — transitioning to delta while the motor is still at very low speed produces an inrush that can approach or exceed DOL starting current, negating the purpose of the reduced-voltage starter.
Incorrectly wiring the delta contactor connections, connecting winding ends to the wrong supply lines and creating a miswired delta that short-circuits on closure.
Placing the overload relay in the individual motor phase wires rather than the main supply lines without adjusting the overload setting. In this position, the overload must be set to 0.58 × motor FLA (not the full FLA) because the individual conductors carry only 58 % of line current in delta running.
Failing to verify motor rotation direction before coupling to a driven load — in a three-phase starter, swapping any two input phases will reverse rotation.

Troubleshooting

Motor starts in star but does not transition to delta: Cause: Timer relay faulty, electrical interlock blocking KM2, or KM3 not releasing Fix: Observe the timer relay output contact at the transition time. If the output does not change, the timer is faulty. If it changes but KM2 does not close, check the electrical interlock (NC contact of KM3 in series with KM2 coil) and verify KM3 has fully released. Measure voltage across KM2 coil — if coil voltage is correct but contactor does not pick up, the contactor coil or mechanism is faulty.
Large current surge and noise at star-to-delta transition: Cause: Transition occurring too early (motor not yet at speed), or open-circuit transition time too long Fix: Increase the star transition time on the timer relay. Monitor motor current and RPM at the transition point if possible. The motor should be at 80–90 % speed before transitioning. Alternatively, consider upgrading to a closed-circuit transition starter if the load torque is variable or the transition current is unacceptably high.
Overload relay trips during star-to-delta transition: Cause: Transition surge current exceeds overload relay thermal capacity, or motor is mechanically overloaded Fix: Extend the star period (increase timer delay) to allow better acceleration before transition. Verify the mechanical load is not too high for star-delta starting — some loads with high breakaway torque or inertia are not suitable. If the overload relay setting is at the lower end of its range, a brief transition surge may trip it — consider a Class 20 relay or adjust the setting within allowable limits.
Motor hums and does not accelerate in star phase: Cause: Motor insufficiently loaded to start in reduced-torque star configuration, or winding fault Fix: Star-delta starting produces only one-third of the DOL starting torque. Verify the load torque at startup — a compressor starting against pressure, or a conveyor with heavy static load, may not start in star. Consider using DOL starting for this application. Also check for a missing supply phase (one phase open) which drastically reduces starting torque.

Frequently asked questions

Why must the motor be a dual-voltage motor for star-delta starting to work?

Star-delta starting only works if the motor's windings, when connected in delta, receive the full supply line voltage. On a 400 V supply, the delta configuration delivers 400 V per winding — so the motor must be rated 400 V delta (often expressed as 400/690 V, meaning 400 V delta or 690 V star). A motor rated only 230/400 V on a 400 V supply can only be run permanently in delta at 400 V; connecting it in star-delta at 400 V would overload the delta windings.

What is the transition from star to delta and why must there be a pause?

When the timer signals the end of the star period, the star contactor (KM3) opens first. A brief pause (typically 50–100 ms) allows the star contactor's contacts to fully interrupt before the delta contactor (KM2) closes. If both close simultaneously, a momentary short circuit occurs across the supply lines through the motor windings. This pause is enforced by the timer relay or by mechanically and electrically interlocking KM2 and KM3.

How much does star-delta starting actually reduce starting current?

Star-delta starting reduces the starting current to approximately one-third (33 %) of the DOL starting current. For a motor with a DOL inrush of 600 A, star-delta starting draws approximately 200 A. However, the starting torque is also reduced to one-third of DOL torque. If the load requires high starting torque (compressors, loaded conveyors), star-delta starting may not provide sufficient torque to accelerate the load and the motor may stall in the star phase.

What is the purpose of the overload relay in a star-delta starter?

The thermal overload relay protects the motor windings from sustained overcurrent. In a star-delta starter, the overload relay is typically placed in the main contactor circuit (before the star/delta split) to monitor total line current, or in the individual motor winding circuits. If placed in the main circuit, the relay is set to the motor's full-load line current. Some configurations require the overload to be set to 0.58 × FLA when placed in the individual phase lines.

What is the difference between an open-circuit and closed-circuit transition star-delta starter?

An open-circuit transition starter opens the star contactor, briefly disconnects the motor from the supply (creating an open-circuit transition), then closes the delta contactor. The brief interruption causes a transient voltage and current spike as the motor's residual magnetic field reconnects to the supply. A closed-circuit transition uses resistors or reactors to maintain supply continuity during the changeover, eliminating the reconnection transient at the cost of added components.

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