Motor terminal markings are the language used to connect a winding, select star or delta operation, and confirm the direction of rotation. The labels may look simple—U1, V1, W1, U2, V2, and W2—but a wrong link or phase connection can cause reverse rotation, excess current, poor starting performance, or equipment damage. Always use the motor’s nameplate and connection diagram as the final authority.
What the terminal markings mean
IEC 60034-8 defines rules for identifying winding connection points, marking motor terminals, and relating terminal markings to direction of rotation. For a common three-phase motor with six accessible leads, the first set is often identified as U1, V1, and W1, while the corresponding winding ends are U2, V2, and W2.
| Common marking | Typical role |
|---|---|
| U1, V1, W1 | Beginning terminals of the three stator phase windings. |
| U2, V2, W2 | Corresponding ends of the three stator phase windings. |
| L1, L2, L3 | Supply phase designations used on the installation side. |
| PE | Protective earth connection to the motor frame. |
Some motors use T1 through T6, numbered terminals, fewer accessible leads, or additional terminals for heaters, thermistors, brakes, or thermal switches. Do not convert between marking systems by memory. Read the diagram supplied with the specific motor.
Star and delta connections
A six-lead motor can often be connected in star, also called Y, or delta, also called Δ. The correct connection depends on the voltage shown on the nameplate and the supply voltage. A dual-voltage motor may be marked, for example, with a lower voltage for delta and a higher voltage for star, but the exact relationship must be confirmed on the plate and diagram.
In a star connection, the three winding ends are joined together and the supply is connected to the three winding beginnings. In a delta connection, the winding beginnings and ends are linked in a closed loop, with the supply connected to the three junctions. The links must be installed in the exact positions shown by the manufacturer; a visually neat arrangement can still be electrically wrong.
| Before connecting | What to verify |
|---|---|
| Motor nameplate | Voltage, frequency, phase count, connection symbols, current, and rotation information. |
| Terminal diagram | Link positions for star or delta and the intended L1-L2-L3 connections. |
| Supply | Measured voltage, phase sequence, frequency, protective earth, and available protection. |
| Driven machine | Permitted rotation, starting load, direction-sensitive parts, and guarding. |
How phase sequence affects rotation
For a three-phase induction motor, the phase sequence creates the rotating magnetic field. If the motor rotates in the wrong direction, disconnect and isolate the supply, then interchange any two phase conductors on the supply side. Do not change random links inside the terminal box. After the change, verify the direction again using the manufacturer’s stated viewing convention, commonly the drive-end view.
A brief bump test may be appropriate during commissioning when the equipment manufacturer permits it and the coupling, guards, and load are safe. On a pump, fan, gearbox, screw conveyor, or threaded mechanism, even a short reverse rotation can be harmful. Never perform a rotation check with exposed rotating parts or an unsecured machine.
Terminal boxes and auxiliary devices
Keep power connections separate from auxiliary circuits. PTC thermistors, thermostats, anti-condensation heaters, brakes, encoders, and space heaters may have their own terminals and wiring diagrams. They must be connected to compatible control or protection equipment, not directly bridged or connected to the motor supply unless the diagram specifically requires it.
Check cable size, lugs, clearances, gland sealing, and tightening torque. Ensure the protective earth is connected to the marked earth point and that unused cable entries are sealed. A correct winding connection does not compensate for a loose terminal, damaged insulation, or a poorly sealed terminal box.
Star-delta starters need extra care
A star-delta starter requires six motor leads and a control system that changes from star to delta after starting. The contactors need electrical and mechanical interlocking so that star and delta cannot close together. The motor, starter, links, and overload protection must be selected as one system. If the motor has only three accessible leads, it is not suitable for a conventional external star-delta connection.
Commissioning checklist
- Photograph the original terminal arrangement before removing links or cables.
- Compare the nameplate voltage and frequency with the actual supply.
- Follow the exact star, delta, or starter diagram for that motor.
- Check protective earth, cable glands, terminal tightness, and auxiliary circuits.
- Confirm the driven machine is safe for a brief direction check.
- Measure current and observe vibration, noise, and temperature after startup.
For related selection information, review our guides to reading a motor nameplate, star-delta starting, and motor maintenance. Correct terminal identification is a small commissioning step with a large effect on motor safety and reliability.