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3-Phase Motor Delta (Δ) Connection: Remember U1–W2, V1–U2, W1–V2 and Choose from the Nameplate
A delta connection joins the three windings of a 3-phase induction motor end to end in a closed triangle and feeds the supply into its three corners. In the terminal box it takes only three links. This article covers how to remember the pairs, the voltage and current relationships, and what to check on the nameplate before switching on.
- Link U1–W2 · V1–U2 · W1–V2, then connect L1 → U1, L2 → V1, L3 → W1
- Each winding sees the full line voltage, , and the line current is
- Use delta only when the Δ figure on the nameplate equals the supply's line voltage, for example 400/690 V on a 400 V supply
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3-Phase Motor Delta Connection
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How to remember U1–W2, V1–U2, W1–V2
Each phase winding has a start (number 1) and an end (number 2): U1–U2, V1–V2 and W1–W2. A delta connection joins the start of one phase to the end of the previous phase, going round in the order U → V → W → U, and the supply is connected to the three junctions.
- Start of U (U1) joins the end of W (W2) and is fed from L1
- Start of V (V1) joins the end of U (U2) and is fed from L2
- Start of W (W1) joins the end of V (V2) and is fed from L3
What U1 V1 W1 U2 V2 W2 mean, and which wire colours go where
Under IEC 60034-8 the letters U V W name the three phase windings and the number gives the end of the winding: 1 = start and 2 = end. A six-terminal motor therefore has U1 V1 W1 (the starts) and U2 V2 W2 (the ends). Older motors and American motors use other names, which map across as follows.
| Winding | Start – end (IEC) | Older marking | NEMA (US) | Supply conductor (IEC 60445 colour) |
|---|---|---|---|---|
| Phase U | U1 – U2 | U – X | T1 – T4 | L1 brown → U1 |
| Phase V | V1 – V2 | V – Y | T2 – T5 | L2 black → V1 |
| Phase W | W1 – W2 | W – Z | T3 – T6 | L3 grey → W1 |
| Earth | earth terminal in the box ⏚ | – | – | PE green-yellow |
- The leads inside the motor have no standard colour. IEC 60034-8 identifies terminals by letters and numbers, not by the colour of the winding leads. Many manufacturers use one colour for every lead and print the marking on a sleeve; others use colours, but the schemes differ between brands. Go by the printed marking and the diagram inside the terminal-box lid, not by colour.
- The supply cable follows IEC 60445 (in Thailand, TIS 11-2553): L1 brown, L2 black, L3 grey, N blue, PE green-yellow. Connecting brown → U1, black → V1, grey → W1 makes the work easy to inspect, but colour does not guarantee phase sequence: older installations may follow an earlier colour code. Check with a phase-sequence meter or by a test run.
- Direction of rotation under IEC 60034-8. With L1, L2, L3 connected to U1, V1, W1 in phase sequence, the motor turns clockwise seen from the shaft (drive) end. To reverse it, swap any two supply conductors.
- Markings lost? An ohmmeter finds which leads are connected, giving three pairs (the three windings) of similar resistance. Telling the start (1) from the end (2) of each winding needs a polarity test, done the way a motor-rewind shop or the manufacturer's manual describes. If one winding is reversed in a delta, the voltages round the delta no longer cancel and a large current circulates in the loop even with no load.
Why the bottom row of the terminal box reads W2 U2 V2
In a six-terminal box the top row is U1 V1 W1, while the bottom row is shifted by one position to W2 U2 V2 (W2 sits under U1). The shift is deliberate: the same set of links serves both connections.
- Delta (Δ): place three vertical links to get exactly U1–W2, V1–U2, W1–V2
- Star (Y): place the links horizontally to join W2, U2 and V2 into the star point
The supply enters at U1, V1 and W1 in both cases. A side-by-side drawing of both link layouts, including the older U-X, V-Y, W-Z markings, is in the lesson star and delta motor connections.
Voltage and current in a delta connection
In delta each winding sits directly between two lines, so the winding voltage equals the line voltage. The current at each corner is the phasor difference of two winding currents that are 120° apart.
A motor connected in delta on a 400 V system draws a line current of 15 A. The voltage across each winding is
The current in each winding is
A clamp meter around a supply cable reads the line current, not the winding current. Divide by √3 to get the current in a winding.
Where the comes from, and the full comparison of star and delta, is covered in phasors and three-phase systems: Y and Δ (in Thai).
Choosing delta or star from the nameplate
The common mistake is not placing the links but choosing the connection without reading the nameplate. Its two figures give the line voltage to use in delta (the lower value) and in star (the higher value).
| Nameplate (Δ / Y) | On a 400 V 3-phase supply connect in | If connected wrongly |
|---|---|---|
| 230/400 V (older plates: 220/380 V) | Star (Y) | In delta each winding sees √3 times its rating, the core saturates, the current is very high and it burns out quickly |
| 400/690 V (older plates: 380/660 V) | Delta (Δ) | In star each winding gets only 1/√3 of its voltage, torque falls to about 1/3, and under load it draws too much current and overheats |
Rule of thumb: the Δ figure on the nameplate must equal the supply's line voltage before you connect in delta. A 400/690 V motor on a 400 V supply is exactly the kind that can use a star-delta starter: it starts in star, cutting the starting current to about 1/3, then switches to delta once it is up to speed. How many seconds to set on the timer is worked out numerically in star–delta timer: how many seconds?
Steps to connect delta at the terminal box
- Isolate the supply, apply lockout/tagout and prove that no voltage is present
- Read the nameplate: the Δ figure must match the supply's line voltage
- Fit three vertical links U1–W2, V1–U2, W1–V2 and tighten the nuts
- Connect L1 → U1, L2 → V1, L3 → W1 and the protective earth (PE) to the motor's earth terminal
- Test run. If it turns the wrong way, swap any two supply lines; see forward-reverse control of a 3-phase motor
Continue in the library
- Star and delta motor connections: terminal box, nameplate and what happens when it is wrong
- How the 3-phase induction motor works: rotating field and slip
- The star-delta starter in detail
- Motor starter simulator: wire it yourself on screen
- Direction of rotation under IEC 60034-8, and reversing a star–delta starter
- Read next: star–delta timer, how many seconds? A numerical study
- Read next: what power factor is and why motors lower it
Frequently asked questions
What do U1 V1 W1 U2 V2 W2 mean on a motor?
Under IEC 60034-8 the letters U, V and W are the three phase windings, the number 1 is the start of a winding and 2 is its end, so phase U has terminals U1 and U2. Older motors use U-X, V-Y, W-Z, and NEMA motors in the United States use T1-T4, T2-T5, T3-T6.
What colour are the U1 V1 W1 wires?
IEC 60034-8 identifies terminals by letters and numbers, not by colour, and many manufacturers use one colour for every lead with the marking printed on a sleeve, so go by the marking, not the colour. For the supply cable, the IEC 60445 colours (also used in Thailand under TIS 11-2553) are brown for L1 to U1, black for L2 to V1, grey for L3 to W1, and green-yellow for protective earth.
Which terminals are linked for a delta connection?
Three pairs: U1–W2, V1–U2 and W1–V2. In a standard IEC terminal box this means three vertical links, with L1 to U1, L2 to V1 and L3 to W1.
Can a 230/400 V motor be connected in delta on a 400 V supply?
No. The 230 V figure is the line voltage for delta. On a 400 V supply it must be connected in star. In delta each winding would see √3 times its rated voltage, the current becomes very high and the motor can burn out.
How do line current and winding current differ in delta?
In delta the winding voltage equals the line voltage, and the line current is √3 times the winding current. For example, a 15 A line current means about 8.66 A in each winding.
The motor runs backwards after connecting in delta. What should I do?
Switch off and swap any two supply lines, for example L1 and L2. The links in the terminal box stay as they are.