Star-Delta Connection Diagram: Complete Motor Starting Guide

Star-delta (also called wye-delta or Y-D) starting is the most common reduced-voltage starting method for three-phase induction motors. By starting the motor in star configuration and switching to delta at full speed, the starting current is reduced to approximately one-third of the direct-on-line (DOL) current. This guide covers the theory, wiring diagrams, timer settings, and practical installation details.

Why Star-Delta Starting?

When a three-phase induction motor starts DOL (direct-on-line), it draws 6 to 8 times its full-load current. For large motors (typically above 7.5 kW / 10 HP), this inrush current causes:

• Voltage dips affecting other equipment on the same supply
• Mechanical shock to driven equipment (pumps, fans, compressors)
• Tripped breakers or blown fuses if the supply cannot handle the inrush
• Utility penalties -- many power companies charge penalties for high starting currents

Star-delta starting reduces the starting current to approximately 33% of DOL current by applying reduced voltage to the motor windings during startup.

How Star-Delta Starting Works

Star (Wye) Configuration

In star configuration, each motor winding receives the phase voltage divided by the square root of 3 (approximately 1.732). For a 400V supply:

• Line voltage: 400V
• Winding voltage in star: 400V / 1.732 = 231V
• Starting current: approximately 1/3 of DOL current
• Starting torque: approximately 1/3 of DOL torque

Delta Configuration

In delta configuration, each motor winding receives the full line voltage:

• Line voltage: 400V
• Winding voltage in delta: 400V
• Running current: normal full-load current
• Running torque: full rated torque

The Transition

A star-delta starter switches from star to delta after the motor has accelerated close to full speed (typically 5 to 15 seconds). During transition, there is a brief interruption of power supply (open transition) which causes a current spike. Closed-transition starters use resistors to avoid this spike but are more complex.

Star-Delta Starter Components

A basic star-delta starter requires:

1. Main contactor (KM1) -- Connects the motor to the supply (always energized during run)
2. Star contactor (KM2) -- Short-circuits the motor winding ends to form star configuration
3. Delta contactor (KM3) -- Connects the motor winding ends in delta configuration
4. Star-delta timer -- Controls the transition timing from star to delta
5. Overload relay (F1) -- Protects the motor from overcurrent (set to 58% of FLC for star connection)
6. Start/Stop pushbuttons -- Operator controls
7. Main fuse or circuit breaker -- Short circuit protection

Interlocking

The star and delta contactors must NEVER be energized simultaneously -- this would create a short circuit between phases. Mechanical and electrical interlocking is mandatory:

• Mechanical interlock -- A physical mechanism between KM2 and KM3 that prevents both from closing
• Electrical interlock -- Each contactor's normally-closed auxiliary contact is wired in series with the other contactor's coil

Power Circuit Wiring Diagram

The power circuit connects as follows:

From Supply to Motor

1. Three-phase supply (L1, L2, L3) feeds through the main fuse/breaker
2. Main contactor KM1 connects L1 to U1, L2 to V1, L3 to W1 (motor winding starts)
3. Delta contactor KM3 connects U2 to V1, V2 to W1, W2 to U1 (delta bridge)
4. Star contactor KM2 shorts U2, V2, W2 together (star point)
5. Overload relay F1 is placed between KM1 and the motor terminals

Motor Terminal Connections

A star-delta motor must have all six terminals accessible in the terminal box:

• U1, V1, W1 -- Winding starts (connected to supply through KM1)
• U2, V2, W2 -- Winding ends (connected to either star point via KM2 or delta bridge via KM3)

The standard terminal arrangement in the motor terminal box is:

U1  V1  W1
W2  U2  V2

For star: Link U2-V2-W2 together (bottom three terminals shorted) For delta: Link U1-W2, V1-U2, W1-V2 (diagonal links)

Control Circuit Wiring Diagram

The control circuit operates at the supply voltage (or stepped down via control transformer):

Start Sequence

1. Press START pushbutton (S2)
2. Timer relay energizes and starts counting
3. Main contactor KM1 energizes (connects motor to supply)
4. Star contactor KM2 energizes (motor starts in star)
5. After timer delay, timer switches:
   • De-energizes KM2 (opens star contactor)
   • Brief delay (50-100ms) for arc extinction
   • Energizes KM3 (closes delta contactor)
6. Motor runs in delta at full voltage

Stop Sequence

1. Press STOP pushbutton (S1) or overload relay trips
2. All contactors de-energize
3. Timer resets

Control Circuit Details

The control circuit includes:

• Stop button S1 (NC) in series with the control circuit
• Start button S2 (NO) in parallel with KM1 holding contact
• KM1 holding contact (NO) seals in the start command
• Timer coil energized in parallel with KM1
• Timer NO contact (delayed) feeds KM3 coil through KM2 NC interlock
• Timer NC contact (instant) feeds KM2 coil through KM3 NC interlock
• KM2 NC auxiliary in series with KM3 coil (electrical interlock)
• KM3 NC auxiliary in series with KM2 coil (electrical interlock)
• Overload relay F1 NC contact in series with the control circuit

Timer Settings

Star-Delta Timer Configuration

The timer must be set long enough for the motor to reach approximately 80% of rated speed in star configuration. Typical settings:

• Small motors (7.5-15 kW): 5-8 seconds
• Medium motors (15-55 kW): 8-12 seconds
• Large motors (55-200 kW): 10-15 seconds
• High-inertia loads: Up to 20 seconds

Setting the timer too short causes high transition current spikes. Setting it too long wastes energy running in star when the motor is already up to speed.

Transition Delay

Some timers include a separate transition delay (50-100ms) between opening the star contactor and closing the delta contactor. This prevents a direct short circuit during changeover.

Overload Relay Settings

The overload relay setting depends on where it is installed in the circuit:

• In the line (before star/delta): Set to 58% of motor FLC (full-load current)
• In the delta connection: Set to motor FLC / 1.732

The 58% factor accounts for the current reduction in star configuration. Most modern star-delta starters place the overload after the main contactor and before the motor, requiring the 58% setting.

Motor Requirements for Star-Delta Starting

Not every motor can use star-delta starting:

1. Voltage rating: The motor must be rated for the supply voltage in delta. For a 400V supply, the motor must be rated 400V delta / 690V star (shown as 400/690V on the nameplate).
2. Six terminals: All six winding terminals must be accessible. A motor with only three terminals cannot be star-delta started.
3. Load torque: The load torque at starting must be less than the star starting torque (approximately 33% of DOL torque). Star-delta starting is NOT suitable for high starting torque applications like loaded conveyors or positive displacement pumps.

Common Problems and Troubleshooting

Motor Does Not Start in Star

• Check that all six motor terminals are correctly connected
• Verify the star contactor is closing (check auxiliary contacts)
• Measure voltage at motor terminals during star start
• Check the motor nameplate -- it must be rated for star connection at the supply voltage

High Current Spike at Transition

• Increase the star timer duration -- the motor may not be up to speed
• Check for mechanical binding or high-inertia loads
• Consider a closed-transition star-delta starter with transition resistors
• Verify the transition delay between star open and delta close

Motor Runs in Star But Trips on Delta

• Overload relay may be set too low -- verify the 58% setting
• Motor may be overloaded -- check the driven equipment
• Phase loss -- check all three phases are present
• Winding damage -- insulation resistance test the motor

Contactors Chatter or Buzz

• Low control voltage -- check the control circuit supply
• Weak contactor coil -- replace the contactor
• Dirty or pitted contacts -- clean or replace

Comparison: Star-Delta vs Other Starting Methods

Star-delta remains popular because it provides significant current reduction at moderate cost with proven reliability. However, the low starting torque (33% of DOL) limits its use to applications where the motor starts unloaded or lightly loaded.

Creating Star-Delta Diagrams with CircuitDiagramMaker

CircuitDiagramMaker includes a pre-built star-delta motor starter template in the Industrial symbol pack. The template includes the complete power circuit and control circuit with properly labeled contactors, timer, overload relay, and pushbuttons.

You can also use the AI circuit generator to create a star-delta starter -- simply type "star-delta motor starter with overload protection" and the AI will generate both the power and control circuits.

The built-in SPICE simulation lets you verify the control circuit logic before building the panel. Run a transient analysis to see the timing sequence and verify the interlock logic.

Star-Delta Current and Voltage Formulas

Two standard three-phase relationships explain why the star-delta method works, and they apply to any three-phase winding, not just motors.

Star (wye) connection:

• Line voltage = sqrt(3) x phase voltage (approximately 1.732 x phase voltage)
• Line current = phase current

Delta connection:

• Line voltage = phase voltage
• Line current = sqrt(3) x phase current (approximately 1.732 x phase current)

These two relationships combine to explain the one-third current reduction described earlier. When a motor winding is switched from delta to star, the voltage across each winding drops by a factor of 1.732. Because the winding's locked-rotor impedance stays essentially constant over that short a time, the winding current also drops by a factor of 1.732. In delta, line current is 1.732 times winding current; in star, line current equals winding current directly. Combining the 1.732 drop in winding current with the loss of the 1.732 multiplier between winding and line current gives a total 3x reduction in line current between delta and star.

Worked example: A motor with a full-load current (FLC) of 58A has a direct-on-line locked-rotor current of 6 x FLC, or about 348A, when started in delta. Starting the same motor in star instead cuts the line current to roughly 348A / 3 = 116A, or about 2 x FLC. Once the timer switches the motor into delta at speed, the running current settles back down to close to the 58A FLC rating.

Contactor and Cable Sizing for a Star-Delta Starter

The three contactors and the six motor-side conductors in a star-delta starter do not all carry the same current, because each one sits in a different part of the circuit relative to line and phase current.

Because the delta and star contactors carry less than the full line current, many panel builders select smaller frame sizes for KM2 and KM3 than for KM1, which saves panel cost and space. The same logic applies to the six conductors running from the starter to the motor terminal box -- they only ever carry winding current, so they are sometimes one size smaller than the three incoming supply conductors, though local wiring regulations and a voltage-drop calculation should always confirm the final size.

Commissioning and Testing a Star-Delta Starter

Before energizing a star-delta starter for the first time, work through a short commissioning sequence to catch wiring errors before they reach the motor.

Conclusion

Star-delta starting is a reliable, cost-effective method for reducing starting current on three-phase induction motors. The key to a successful installation is proper motor selection (400/690V rating), correct timer settings (enough time to reach speed in star), and mandatory interlocking between the star and delta contactors.

Use the wiring diagrams in this guide as a reference, and always follow your local electrical code and motor manufacturer recommendations for specific installations.

Design star-delta motor starters with CircuitDiagramMaker -- free industrial control panel design tool with 114 industrial symbols and built-in simulation.

Frequently asked questions

Can a star-delta starter be used on a single-phase motor?

No. Star-delta starting relies on switching a motor between two three-phase winding configurations, so it only works on three-phase induction motors with six accessible terminals. Single-phase motors have a single main winding plus a start winding and use different reduced-voltage or capacitor-based starting methods, not star-delta.

Does the star point need to be connected to neutral or earth?

No. The star point formed by shorting U2, V2, and W2 together inside the starter is an internal connection only -- it is not connected to the supply neutral or to earth. It simply lets each winding see line voltage divided by 1.732 during the starting period.

Can a motor run continuously in the star connection instead of switching to delta?

It's not recommended. In star, the motor only develops roughly a third of its rated torque and current, so it cannot deliver full rated output or handle full load. Running continuously in star also means the windings never see the voltage and current levels the motor was rated to run at.

Can an existing DOL starter be converted to star-delta?

Only if the motor itself has six accessible terminals and a dual voltage rating such as 400/690V. Converting means rewiring the motor terminal box links and adding a star contactor, a delta contactor, and a timer relay with mechanical and electrical interlocking -- it's not just a change to the control panel.

Can a PLC replace the timer relay in a star-delta starter?

Yes. A PLC can use a timer function block to open the star contactor and close the delta contactor on the same schedule a dedicated timer relay would use. Most installations still keep a hardwired electrical interlock between KM2 and KM3 as a backup, independent of the PLC logic.

Is it safe to skip the star position and start straight in delta?

Starting straight in delta is simply direct-on-line (DOL) starting -- it defeats the purpose of a star-delta starter. The motor draws full DOL inrush current and torque, which can trip breakers, cause voltage dips, or shock-load driven equipment, the exact problems star-delta starting is installed to avoid.

Interactive diagrams for this guide

• Star Delta Motor Connection
• 3 Phase Star Delta Motor Connection Diagram
• Star Delta Connection In Motor Terminal

Related guides

• 3-Phase Motor Wiring Diagram: Star and Delta Connections
• Motor Control Circuit Diagrams: Star-Delta, DOL, and VFD
• 3-Speed Fan Motor Wiring Diagram: Connections Explained
• Capacitor Wiring Diagram: Start, Run, and Motor Capacitors