Wye-Delta (Star-Delta) Motor Starting Diagram
This is a free printable wye delta diagram: download the diagram as SVG or open it and print to paper or PDF.
A wye-delta diagram shows how a three-phase induction motor is started in star (wye) configuration then switched to delta, reducing inrush current to roughly one-third of direct-on-line starting current.
Wye-delta starting, also called star-delta starting in IEC countries, is one of the oldest and most widely used reduced-voltage starting methods for three-phase squirrel-cage induction motors. The technique exploits a fundamental property of three-phase systems: when motor windings are connected in star (wye), each winding receives only 57.7% of line voltage (line voltage divided by the square root of 3). Because torque is proportional to the square of voltage, starting torque in star is one-third of what it would be in delta. More importantly for the supply network, starting current is also reduced to one-third of delta starting current.
The diagram typically shows three main contactors: the main contactor (KM1) that connects the motor to the supply throughout operation, the star contactor (KM2) that short-circuits the motor neutral point during starting, and the delta contactor (KM3) that reconnects the windings in delta for normal running. A timer relay (typically 5–15 seconds, set to suit load inertia) triggers the transition from star to delta once the motor has accelerated to approximately 70–80% of synchronous speed.
Mechanical and electrical interlocking between KM2 and KM3 is mandatory: closing both simultaneously short-circuits two phases of the supply and will destroy the contactors and potentially cause injury. Most modern designs use both a mechanical interlock (a lever linkage between contactor bodies) and an electrical interlock (normally-closed auxiliary contacts wired in series with the opposing coil circuit).
The main variants are open-transition and closed-transition. Open-transition (the standard wye-delta circuit) briefly disconnects the motor from the supply during changeover, causing a transient current spike that can exceed direct-on-line inrush for a few milliseconds. Closed-transition inserts resistors or an impedance during changeover to prevent this spike, and is preferred for sensitive loads or large motors. Wye-delta starting is suitable for motors where the connected load allows starting under light or no load, because starting torque at one-third means heavily loaded conveyors or compressors may not start successfully.
How to wire wye delta diagram
- Confirm motor winding configuration Verify the motor nameplate shows both star and delta voltage ratings (e.g., 400V/690V or 230V/400V). The delta voltage rating must match your supply voltage. All six winding terminals (U1, V1, W1, U2, V2, W2) must be accessible on the terminal board — motors with only three terminals cannot be wye-delta started.
- Size the contactors and overload relay The main contactor (KM1) carries full line current and is rated at 100% of motor full-load current (FLC). The star contactor (KM2) carries winding current, which is FLC divided by the square root of 3, so it can be rated at 58% FLC. The delta contactor (KM3) also carries 58% FLC. The overload relay is set to motor FLC and installed in the main circuit, not the star or delta branch.
- Wire the main contactor to the supply and motor terminals U1, V1, W1 Connect the three supply phases (L1, L2, L3) through the main contactor KM1 to motor terminals U1, V1, W1 respectively. Route this wiring through the overload relay thermal elements. The overload relay output feeds KM1 coil control circuit.
- Wire the star contactor to short-circuit the neutral point Connect the star contactor KM2 across motor terminals U2, V2, W2. When KM2 closes, it joins these three terminals together, forming the star neutral point. Install a mechanical interlock and wire normally-closed auxiliary contact of KM3 in series with the KM2 coil to prevent simultaneous closure with the delta contactor.
- Wire the delta contactor to reconnect windings in delta The delta contactor KM3 connects U2 to V1, V2 to W1, and W2 to U1 (the exact cross-connection depends on the manufacturer's wiring diagram for the motor terminal labelling convention). Install mechanical and electrical interlocks with KM2. Wire a normally-closed auxiliary of KM2 in series with the KM3 coil circuit.
- Connect and set the timer relay Wire the on-delay timer relay coil in parallel with the main contactor KM1 coil (so timing starts when the motor starts). The normally-open timed contact de-energises KM2 and energises KM3 after the set delay. Adjust the timer using a clamp ammeter: the motor should have stabilised in speed before switching; a sharp current transient at changeover indicates the motor has not yet accelerated sufficiently.
- Test, commission, and verify interlock operation Before energising with the motor connected, verify the mechanical interlock prevents KM2 and KM3 closing simultaneously by attempting to close them by hand. Energise and confirm sequence: KM1 and KM2 close first (star start), then KM2 opens and KM3 closes (delta run) at timer expiry. Check overload relay trip current with a clamp meter under full load.
Specifications
| Starting current (line) vs DOL | Approximately 33% of direct-on-line starting current |
|---|---|
| Starting torque vs DOL | Approximately 33% of full delta (DOL) starting torque |
| Star phase voltage (400 V system) | 230.9 V (400 V ÷ √3) |
| Typical timer range | 5–15 seconds (adjusted to suit motor and load inertia) |
| Minimum motor terminal accessibility | Six individual terminals (U1, V1, W1, U2, V2, W2) |
| Applicable motor types | Three-phase squirrel-cage induction motors only |
| Recommended transition type for motors above 55 kW | Closed-transition (with transition resistors) to limit re-acceleration transient |
| Relevant standards | IEC 60947-4-1 (contactors), IEC 60034-1 (motors), NEC Article 430 (US) |
Safety warnings
- Fixed electrical installation work must be carried out by a licensed or registered electrician in accordance with applicable local regulations and wiring standards (NEC/NFPA 70, BS 7671, AS/NZS 3000, IEC 60364). Non-compliance can result in legal liability, fire, or death.
- Always isolate the supply, lock out and tag out the isolator, and verify the circuit is dead with an approved voltage indicator before touching any wiring or contactors. Three-phase voltages are lethal.
- Mechanical and electrical interlocking between the star and delta contactors is not optional — simultaneous closure creates a phase-to-phase short circuit that will destroy equipment and can cause an arc flash event.
- Never work on a motor starter while it is energised. Contactors can re-energise unexpectedly if the control circuit is live. Follow a full lockout/tagout procedure including proving dead on all six terminals.
- The transition from star to delta produces a brief high-current transient. Verify that upstream protection (fuses, circuit breakers) is rated for motor starting duty and not just continuous current, to prevent nuisance tripping.
Tools needed
- Insulated screwdrivers (flat and Pozidriv/Phillips)
- Calibrated clamp ammeter (True RMS, rated for the supply voltage category)
- Approved voltage indicator / non-contact tester (IEC 61243-3 or equivalent)
- Torque screwdriver or torque wrench for terminal connections
- Multimeter (CAT III minimum, CAT IV preferred for supply-side work)
- Wire stripper and ferrule crimping tool
- Lockout/tagout padlock and personal danger tag
- Stopwatch or timer for verifying transition timing
Common mistakes
- Setting the star-to-delta timer too short: the motor has not yet accelerated, so the transition transient is severe and can trip upstream protection or damage contactors.
- Omitting interlocking between KM2 and KM3: this is the most dangerous wiring error and is explicitly prohibited by all major wiring standards.
- Connecting the overload relay in the star or delta branch instead of the main circuit: current in these branches is only 58% of line current, so the relay will not detect overload correctly.
- Using a motor with only three accessible terminals: wye-delta starting requires all six winding terminals to be individually accessible at the terminal board.
- Setting the overload relay to a value above motor FLC to avoid nuisance trips during starting: this leaves the motor unprotected against sustained overload. Correct practice is to set it to nameplate FLC and address the nuisance trip cause.
Troubleshooting
- Overload relay trips every time during the star-to-delta transition
- Cause: Timer set too short; motor has not accelerated sufficiently before switching, causing a large transient current spike Fix: Increase the timer delay in 1–2 second increments while monitoring the current waveform with a clamp meter until the transition transient is minimised. Also verify overload relay class setting (Class 10 or 20) is appropriate for the motor.
- Motor hums and fails to accelerate past star speed even after the timer expires
- Cause: Load torque exceeds available star starting torque, or the delta contactor is failing to close Fix: Check the delta contactor coil voltage and auxiliary contact continuity. If the contactor operates correctly, the load may be too heavy for wye-delta starting and a different starting method (soft starter or VFD) should be considered.
- Contactors chatter or arc excessively during star-to-delta changeover
- Cause: Open-transition switching; brief disconnection causes voltage transient as motor EMF and supply re-synchronise out of phase Fix: Measure the transition using a power analyser. If current spikes are excessive, consider converting to a closed-transition circuit with transition resistors, or use an electronic soft starter instead.
- Motor runs hot in the star configuration during starting
- Cause: Timer set too long, keeping the motor in star at reduced torque while the load demands more current than the star winding can efficiently supply Fix: Shorten the timer delay so the transition to delta happens once the motor has reached approximately 70–80% of synchronous speed. Check the motor nameplate for the duty cycle and starting frequency limits.
- Star contactor and delta contactor both show burn marks on the contacts
- Cause: Interlock failure — both contactors closed simultaneously, creating a phase short circuit Fix: Replace both contactors. Inspect and restore the mechanical interlock linkage. Verify normally-closed auxiliary contacts of each contactor are wired in series with the opposing coil and confirm operation before re-commissioning.
Frequently asked questions
What is the difference between wye and star connections in motor starting?
Wye and star are two names for the same connection topology where three winding ends meet at a common neutral point. IEC and European standards use 'star'; North American standards tend to use 'wye'. The electrical behaviour is identical. Both terms appear on wye-delta diagrams depending on regional convention.
Why does wye-delta starting reduce inrush current to one-third?
In star connection each winding sees line voltage divided by the square root of 3 (approximately 57.7% of line voltage). Because impedance is fixed, current is proportional to voltage. Starting current in star is therefore 57.7% of delta voltage divided by the same impedance, and since current scales with voltage, total line current is one-third of direct delta starting current.
How long should the timer be set for wye-delta starting?
Timer setting depends on motor size and driven load inertia; a typical range is 5–15 seconds. The motor should reach at least 70% of synchronous speed before switching to delta. Setting the timer too short causes a large transition transient; too long wastes time in reduced-torque mode and can overheat the motor windings.
What loads are not suitable for wye-delta starting?
Loads that require high starting torque under load are unsuitable: reciprocating compressors, heavily laden conveyors, mixers with viscous material, and ball mills. These require the full delta torque from standstill. Wye-delta is best suited to centrifugal pumps, fans, and centrifuges that start unloaded or against low initial resistance.
Is wye-delta starting still used, or have soft starters replaced it?
Both are still widely used. Wye-delta is favoured where simplicity, low cost, and maintainability matter; the components (contactors, timer, overload relay) are universally understood by electricians worldwide. Electronic soft starters and variable-frequency drives (VFDs) offer smoother ramp-up and closed-loop control, but at higher cost and greater complexity.
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