3-Phase Motor Winding Diagram: Star (Y) and Delta (Δ) Winding Connections
This is a free printable 3 phase motor winding diagram: download the diagram as SVG or open it and print to paper or PDF.
A detailed reference for understanding three-phase induction motor winding diagrams, covering terminal numbering, star and delta connections, and how to read a motor nameplate correctly.
A three-phase induction motor has three sets of windings — one for each phase — each with two terminals. Following IEC 60034-8, these terminals are designated U1, V1, W1 (the start of each winding) and U2, V2, W2 (the end of each winding). These six terminals are typically brought out to a terminal block inside the motor's terminal box (junction box), allowing the windings to be connected in either star (Y) or delta (Δ) configuration externally.
In a star connection, the three winding ends — U2, V2, and W2 — are connected together to form a neutral point. The three-phase supply connects to U1, V1, and W1. Each winding sees the phase-to-neutral voltage (e.g., 230 V on a 400 V / 230 V supply system). Star connection is used for lower-voltage operation, reduced starting current (in star-delta starters, the motor starts in star), and in motors rated for two voltages.
In a delta connection, the windings are connected end-to-start in a loop: U1 connects to W2, V1 connects to U2, and W1 connects to V2. The three-phase supply connects to the three junction points. Each winding sees the full line-to-line voltage (e.g., 400 V on a 400 V supply). Delta connection delivers higher torque and is used during run mode in star-delta starters, and in motors designed to run continuously in delta.
A motor nameplate showing '230/400 V Δ/Y' means the motor runs in delta at 230 V supply or in star at 400 V supply — the winding voltage is the same in both cases (230 V per winding). A nameplate showing '400/690 V Y/Δ' means star at 400 V, delta at 690 V. Reading the nameplate correctly before making any terminal connections is essential — reversed voltage can destroy the motor windings.
This page is a generic educational reference. Always refer to the motor's nameplate, manufacturer datasheet, and the applicable installation standard (IEC 60364, NEC/NFPA 70, AS/NZS 3000, or local equivalent). Motor wiring must be carried out by a qualified electrician.
How to wire 3 phase motor winding diagram
- Read the motor nameplate Locate the motor nameplate and record: supply voltage, frequency, rated current (full load amps), power rating (kW), insulation class, enclosure rating (IP code), and connection type (Y or Δ). Confirm the available supply voltage matches one of the nameplate voltage ratings. Do not proceed until the connection type for the available voltage is confirmed.
- Isolate and lock out the supply Switch off the motor control circuit (DOL starter, VFD, or star-delta contactor panel). Isolate the main supply at the distribution board or motor control centre (MCC). Apply a personal lockout lock and danger tag. Use a calibrated multimeter to verify all three phase conductors at the motor terminal box are dead before opening the terminal box cover.
- Open the terminal box and identify terminals Remove the terminal box cover. Locate the six terminals: U1, V1, W1, U2, V2, W2. Inspect existing link bars or copper jumpers — these create the star or delta connection. Photograph the existing arrangement before disturbing anything. Verify terminal labels are legible; if not, use a continuity tester to confirm winding pairs.
- Configure star (Y) connection For star connection: fit link bars (shorting bars) across U2, V2, and W2 to form the neutral star point. The three supply conductors connect to U1, V1, and W1 respectively. No connection is made to U2, V2, or W2 from the supply. Many motors are supplied with a set of link bars for this purpose.
- Configure delta (Δ) connection For delta connection: remove the star point links. Connect U1 to W2 (using a link bar or short cable), V1 to U2, and W1 to V2. The three supply conductors connect to U1 (and W2), V1 (and U2), and W1 (and V2) — the three junction points. Confirm the nameplate permits delta connection at the available supply voltage before proceeding.
- Connect supply conductors and earth Connect the three-phase supply conductors (L1, L2, L3) to U1, V1, W1 respectively using the terminal screws or bolts provided. Ensure the protective earth (PE) conductor is firmly connected to the motor earth terminal — do not omit the earth connection. Replace the terminal box cover and all fasteners.
- Test run and verify direction of rotation Restore supply and run the motor briefly (a few seconds) without load. Confirm the direction of rotation matches the driven equipment requirement (pump, fan, compressor). If rotation is reversed, isolate again, lock out, and swap any two of the three supply phase conductors at the terminal box or at the starter — do not swap the earth conductor.
Specifications
| Terminal designation standard | IEC 60034-8: U1, V1, W1 (winding starts); U2, V2, W2 (winding ends) |
|---|---|
| Star (Y) connection voltage relationship | Winding voltage = line voltage ÷ √3 (e.g., 230 V per winding on 400 V three-phase supply) |
| Delta (Δ) connection voltage relationship | Winding voltage = line-to-line voltage (e.g., 400 V per winding on 400 V three-phase supply) |
| Star-delta starting current reduction | Starting current reduced to approximately 1/3 of direct-on-line (DOL) starting current |
| Star-delta starting torque reduction | Starting torque reduced to approximately 1/3 of DOL starting torque |
| Minimum insulation resistance (cold winding, IEC 60034) | Typically ≥ 1 MΩ at 500 V DC (phase-to-earth); refer to IEC 60034-1 and motor manufacturer for specific requirements |
| Motor protection standard | IEC 60947-4-1 (contactors and motor starters); IEC 60034-1 (rotating machines general requirements) |
| Phase sequence for correct rotation | L1, L2, L3 to U1, V1, W1 respectively — swap any two phase conductors to reverse rotation |
Safety warnings
- LOCKOUT/TAGOUT: Three-phase motors can restart unexpectedly if a star-delta timer completes its cycle or if a VFD is not properly isolated. Always lock out and tag out the entire motor control circuit — both the control supply and the main power supply — before opening the terminal box. Verify all three phase conductors are dead with a calibrated multimeter.
- VOLTAGE MATCHING: Connecting a motor in the wrong winding configuration for the available supply voltage (e.g., delta connection on a supply voltage rated for star) will cause immediate and catastrophic winding failure. Read the nameplate. Confirm the available supply voltage. Match the configuration before energising.
- RESIDUAL VOLTAGE: Large motors and capacitor-run motors can retain a residual voltage after being isolated. Allow adequate discharge time before touching terminals — the exact safe time depends on motor size and capacitor rating. Verify zero volts before touching.
- QUALIFIED PERSONNEL ONLY: Three-phase motor wiring involves high voltages and currents capable of causing death. All installation, configuration, and testing must be carried out by a qualified electrician in accordance with IEC 60364, NEC/NFPA 70, AS/NZS 3000, or the applicable local electrical code.
- EARTH CONNECTION: The motor frame must be solidly bonded to the protective earth (PE) system. In a fault condition, the earth provides the fault current path that causes the protection device to operate. An unconnected motor frame in a fault condition can rise to full line voltage — a lethal hazard.
Tools needed
- Calibrated digital multimeter (CAT III, three-phase voltage and current measurement)
- Non-contact voltage tester (NCV tester)
- Insulated spanners and screwdrivers (sized for motor terminal bolts)
- Clamp meter (for measuring motor running current after commissioning)
- Continuity tester (for winding identification if labels are illegible)
- Personal lockout lock and danger tag
- Motor nameplate photograph or transcribed data sheet
- Insulation resistance tester (Megohm meter, 500 V or 1000 V DC)
Common mistakes
- Failing to read the nameplate before wiring and connecting in delta when the motor is a star-only design for the available voltage — results in instant winding burnout on energisation.
- Swapping the earth conductor with a phase conductor when reversing rotation — only the two phase conductors should be swapped; never the earth.
- Leaving star point link bars in place when reconfiguring to delta, creating a short circuit across the winding.
- Under-sizing the motor cable — always size cable based on motor full-load amps (FLA) from the nameplate, the installation method (clipped, buried, in conduit), and ambient temperature derating, per the applicable wiring standard.
- Setting the overload relay to the circuit breaker rating rather than the motor FLA — the overload relay must be set to the motor nameplate FLA to provide correct thermal protection.
- Not verifying rotation direction before coupling the motor to the driven load — connecting a pump in reverse rotation on first start can damage impellers, mechanical seals, or the driven system.
Troubleshooting
- Motor trips overload relay on starting (DOL)
- Cause: Overload relay set too low, motor mechanically overloaded, one supply phase missing (single phasing), or motor winding fault Fix: Check all three supply phases are present with a multimeter. Verify overload relay is set to nameplate FLA. Uncouple the load and run the motor unloaded — if it starts freely, the issue is mechanical overload. If it still trips unloaded, measure winding resistance on all three windings; an open circuit or very low resistance indicates a winding fault requiring rewinding.
- Motor runs but draws higher-than-rated current on all three phases
- Cause: Motor wired in delta at a voltage that requires star connection (winding overvoltage), mechanical overload, or low supply voltage Fix: Isolate and lock out. Verify the winding connection matches the nameplate for the available supply voltage. Measure supply voltage under load — if significantly below nameplate rating, the motor draws higher current to maintain torque. Check driven equipment for mechanical binding or overload.
- Motor vibrates excessively after wiring
- Cause: Single phasing (one supply phase missing or loose connection), incorrect phase sequence causing reverse rotation on a three-phase load, or unbalanced supply voltage Fix: Check all three supply phase connections are secure at the motor terminal box and at the starter. Measure phase-to-phase voltage on all three combinations — values should be balanced within a few percent. If two phases are equal but one is zero or low, trace the open circuit back through the supply.
- Insulation resistance test fails (low reading between winding and earth)
- Cause: Moisture ingress into motor windings, damaged winding insulation, or contamination in the terminal box Fix: Dry the motor by running it unloaded on reduced voltage if possible, or by placing a heat lamp inside the terminal box for several hours (with power fully isolated). Retest. If the reading remains low after drying, the winding insulation may be permanently compromised and the motor requires rewinding or replacement.
Frequently asked questions
What do the terminal designations U1, V1, W1, U2, V2, W2 mean?
These are IEC 60034-8 terminal designations. U, V, and W identify the three phase windings. The numeral 1 indicates the start of each winding, and 2 indicates the end. Having both ends of each winding available at the terminal block allows external star or delta connection without opening the motor.
What is the difference between star and delta voltage ratings on a nameplate?
A motor nameplate showing '400/690 V Y/Δ' means: connect in star (Y) for a 400 V supply, connect in delta (Δ) for a 690 V supply. In both cases, each individual winding sees the same voltage (approximately 231 V in star on 400 V, or 690 V in delta on 690 V). The voltage per winding determines insulation stress.
Why does star-delta starting reduce motor inrush current?
During star starting, each winding receives the phase voltage (1/√3 of line voltage). This reduces starting current to approximately one-third of the direct-on-line (DOL) starting current. Torque is also reduced to one-third of DOL torque. The motor transitions to delta once it reaches approximately 70–80% of full speed.
How do I identify which terminal is U1 if the labels are worn off?
Use a continuity tester to identify which terminals are connected to the same winding (pairs: U1-U2, V1-V2, W1-W2). Correct phase assignment and start/end identification then requires either winding resistance testing and back-EMF phase-check methods — this work should be performed by a motor rewind specialist or qualified electrician with appropriate test equipment.
Can I run a star-rated motor in delta at the same voltage?
No. Connecting a motor in delta at the voltage it was rated for in star applies the full line voltage (√3 times higher) across each winding. This causes approximately three times the rated current to flow, rapidly overheating and destroying the winding insulation. Always match the connection to the supply voltage per the nameplate.
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