Motor Connection 3 Phase: Terminal Box Wiring for Star and Delta Configurations
This is a free printable motor connection 3 phase: download the diagram as SVG or open it and print to paper or PDF.
Three-phase motor connection covers the identification of motor terminals U1 V1 W1 U2 V2 W2, the correct link configuration for star or delta at the nameplate voltage, and the protective earth bonding required for safe motor installation.
Three-phase motor connection begins inside the motor terminal box where six terminals expose both ends of each stator winding. The naming convention follows IEC 60034-8: U1 and U2 are the start and end of the first winding, V1 and V2 of the second, and W1 and W2 of the third. The physical arrangement in the terminal box follows a standard layout — U1, V1, W1 across the top row and W2, U2, V2 across the bottom row — designed so that adjacent terminals in the upper and lower rows always belong to different windings, making delta connection straightforward with horizontal links. Star connection is achieved by installing three short link bars across the bottom row, connecting W2 to U2, U2 to V2, and V2 back to W2 — effectively joining all three winding ends at a common neutral point. The supply conductors L1, L2, L3 connect to the upper terminals U1, V1, W1. On a 400 V three-phase supply with star connection, each winding sees 231 V line-to-neutral. Delta connection uses six links or three longer links to connect the end of each winding to the start of the adjacent winding: U2 connects to V1, V2 connects to W1, and W2 connects to U1. This diagonal configuration applies full 400 V line-to-line across each winding, producing higher torque than star at the same supply voltage. The standard IEC nameplate voltage notation 230/400 V means 230 V in delta and 400 V in star — both produce equivalent power at their respective voltages. The notation 400/690 V means 400 V delta and 690 V star. Selecting the wrong configuration for the supply voltage over-energises or under-energises the windings: star connection on a voltage that should use delta operates the motor at one third power but will not damage it; delta connection on a voltage intended for star instantly saturates the magnetic core, produces excessive current, and destroys the winding insulation within seconds. The protective earth conductor connects to the designated PE terminal — typically a green/yellow marked stud in the terminal box — and bonds the motor frame to the installation earth bar.
How to wire motor connection 3 phase
- Read nameplate and determine configuration Identify supply voltage and motor nameplate voltage rating. If the supply voltage matches the lower nameplate value (e.g. 400 V for a 400/690 V motor), configure delta. If it matches the higher value (690 V), configure star. Confirm with the nameplate wiring diagram.
- Open terminal box and identify terminals Remove terminal box cover. Identify U1, V1, W1 in the upper row and W2, U2, V2 in the lower row. Remove any existing link bars and inspect terminals for corrosion or damage.
- Install configuration links For star: install three horizontal links across the bottom row connecting W2-U2, U2-V2. For delta: install three diagonal links connecting U2-V1, V2-W1, W2-U1. Torque link bar bolts to specification.
- Connect supply and PE cables Connect L1, L2, L3 conductors to U1, V1, W1 respectively using correctly crimped cable lugs. Connect the PE conductor to the PE terminal stud. Torque all lugs to terminal specification.
- Test insulation and energise Before closing the terminal box, measure insulation resistance between each winding pair and between each winding and PE stud. Confirm all readings exceed 1 megohm. Close the box, energise, and measure current on all three phases.
Specifications
| Standard terminal notation (IEC) | U1, V1, W1 (supply) / U2, V2, W2 (winding ends) |
|---|---|
| Star winding voltage (400 V supply) | 231 V per winding (400 ÷ √3) |
| Delta winding voltage (400 V supply) | 400 V per winding (full line-to-line) |
| PE conductor minimum size | Equal to phase conductor up to 16 mm² (IEC 60364) |
Safety warnings
- Double-check star or delta link configuration against the nameplate before energising — delta-connected windings on a star-voltage supply overheat and fail in seconds.
- Torque all terminal bolts to the manufacturer specification — loose terminals cause arcing under high motor starting current that carbonises insulation irreversibly.
- Do not rely on visual inspection alone to confirm de-energisation — always test all six terminals with a voltage tester before touching any connection inside the terminal box.
Tools needed
- Phase sequence meter to verify L1-L2-L3 rotation direction before first start
- Insulation resistance tester (megohmmeter, 500 V DC) for winding health verification
- Torque wrench with appropriate socket for M4–M12 terminal bolts
- Clamp-on ammeter for measuring and balancing three-phase running current
Common mistakes
- Installing link bars in the wrong positions — misidentifying the terminal box layout and linking wrong terminal pairs, creating winding short circuits.
- Connecting supply L1 to V1 or W1 instead of U1, which reverses motor rotation and may damage the driven machine.
- Using undersized cable lugs on motor terminals, creating high contact resistance that causes hot spots and insulation damage at the terminal entry.
Troubleshooting
- Motor draws very high current on all phases immediately after start
- Cause: Delta connection applied where star was required — windings are over-voltage Fix: De-energise immediately. Open terminal box and reconfigure links to star. The windings may already be damaged — measure insulation resistance before re-energising.
- Motor runs but current is unbalanced across phases
- Cause: One supply phase has higher impedance or a link bar has high contact resistance Fix: Measure phase-to-phase voltage at motor terminals — must be balanced within 1%. Re-torque all link bars and supply cable lugs. A persistent imbalance above 5% current requires supply voltage investigation.
- Motor runs in wrong direction
- Cause: Supply phase sequence reversed at U1-V1-W1 terminals Fix: De-energise, swap any two supply phase conductors at the motor terminals or at the contactor output. Confirm correct direction before connecting to driven load.
Frequently asked questions
How do I identify U1, V1, W1 and U2, V2, W2 in the terminal box?
IEC standard terminal boxes always label terminals clearly, typically with stamped or moulded identifiers. The upper row is U1, V1, W1 from left to right. The lower row is W2, U2, V2 from left to right. This arrangement ensures that the two terminals of each winding are diagonally opposite each other, not adjacent, preventing accidental short-circuiting when installing link bars. Always cross-check labels before wiring.
What voltage does each winding see in star versus delta on a 400 V supply?
In star on a 400 V supply, each winding is connected between one phase and the neutral point, so it sees 400 V divided by root 3, which equals approximately 231 V. In delta on a 400 V supply, each winding is connected directly between two phase conductors and sees full 400 V line-to-line. This is why a motor must be rated for 400 V delta or 690 V star — both deliver the same voltage per winding when supplied at the appropriate line voltage.
How do I determine which way the motor will rotate?
Motor rotation direction is determined by the phase sequence applied to U1, V1, W1. With a standard phase sequence L1-L2-L3 connected to U1-V1-W1, the motor rotates in the direction indicated by the rotation arrow on the nameplate (usually clockwise when viewed from the drive end). Reversing any two supply phases — for example connecting L1 to V1 and L2 to U1 — reverses phase sequence and thus rotation direction. Use a phase sequence meter to confirm before connecting.
Can a motor rated 400/690 V be run on a 230 V single-phase supply?
No. A three-phase motor requires three-phase supply to create the rotating magnetic field that produces torque. Single-phase supply provides only a pulsating field with no starting torque. It is possible to use a phase converter or VFD to generate three-phase power from single-phase supply, but the motor still requires all three phases at the correct amplitude and phase relationship to operate properly.
What is the purpose of the PE terminal in the motor terminal box?
The PE (protective earth) terminal bonds the motor frame, stator housing, and all electrically conducting parts to the installation earth system. If winding insulation fails, fault current flows through the low-impedance earth path, rapidly tripping the circuit breaker or blowing the fuse rather than causing the motor frame to become dangerously live. The PE conductor must be at least equal in cross-section to the phase conductors up to 16 mm² per IEC 60364 Table 54.2.
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