After the three winding pairs are identified and assigned the correct polarity, the motor can be linked in star or delta without one phase opposing the other two. Use terminal leads for the accessible conductors, terminal markings for identifiers such as U1, and winding start and winding end only when the test procedure has established polarity.
Common fixes that miss winding polarity
The number that matters during a polarity fault is the current produced when one winding's magnetic field opposes the intended rotating field. This is heat, not logic: changing terminology, rearranging links without a test, or swapping two supply phases cannot correct a reversed pair inside the terminal identification.
| Attempted fix | What it changes | Why it may fail |
|---|---|---|
Exchange two incoming phases, such as L1 and L2
|
Reverses the phase sequence and therefore the motor's direction of rotation | It reverses the complete rotating field. It does not repair one winding whose two terminal markings are interchanged. |
| Move the star or delta links | Changes the electrical interconnection of the three windings | A correct link pattern still produces a faulty field if one winding pair has the wrong polarity designation. |
| Rename a suspected lead from its physical position | Changes documentation only | Terminal-box position does not establish which two leads belong to one winding or their relative polarity. |
| Call every lead a “phase” | Uses an imprecise description | A six-lead motor exposes two ends of each winding. The phase winding, its accessible lead, and its terminal marking are different objects. |
If the only symptom is correct operation with the wrong shaft direction, exchange two supply phases according to the applicable machine procedure. If the motor stalls or develops abnormal current when a star-delta circuit changes state, inspect the winding pairing, polarity, link pattern, and changeover wiring rather than treating the problem as a simple rotation error.
Current direction and winding polarity
Each stator phase winding has two accessible ends. The markings pair them as U1-U2, V1-V2, and W1-W2. The numeral distinguishes the two ends; it is not a measured electrical value.
Calling the ends “beginning” and “end” describes winding polarity. It does not mean that current can physically flow in only one direction. With an AC supply, instantaneous current reverses every cycle. The start/end assignment provides a consistent polarity reference so that the three winding magnetomotive forces form the intended rotating field when connected.
A motor may still run when all three winding pairs are reversed consistently because their relative relationship remains coordinated. Reversing only one pair is different: that winding's magnetic contribution is inverted relative to the other two. Depending on the connection and operating circuit, the result can include failure to accelerate, stalling during a star-delta transition, excessive current, vibration, or abnormal heating.
| Quantity or designation | Meaning | Where to read or establish it |
|---|---|---|
U1, V1, W1
|
First marked ends of the three winding groups | Terminal markings, lead labels, or verified test records |
U2, V2, W2
|
Corresponding second marked ends | Terminal markings, lead labels, or verified test records |
| Winding pair | Two leads with continuity through one phase winding | De-energized continuity or resistance measurement |
| Relative polarity | Start/end relationship among the three windings | Approved winding-polarity test procedure |
| Operating current | Electrical loading after connection | Measurement at each supply conductor and comparison with the motor rating data |
English terms for the six leads
The precise external-connection term is terminal lead. The letters and numerals printed on the conductors or terminal board are terminal markings. In winding work, coil ends can describe the ends of a coil or winding group, while stator connection leads and stator termination points describe the same connection area more generally.
For troubleshooting instructions, use the exact marking whenever possible: “Exchange U1 and U2 after confirming that they are the two ends of the U winding.” If the mechanism matters, write: “The relative polarity of one phase winding is reversed because its start and end markings are interchanged.” That wording distinguishes a single-winding polarity error from a complete phase-sequence reversal.
The cited terminology separates two concepts:
- Terminal lead: the conductor or connection point available for external motor wiring.
-
Terminal marking: the identifier assigned to that lead, such as
U1.
U1 is therefore not the name of a physical type of lead; it is the marking of a particular terminal lead. IEC terminal marking uses the letter-and-number form. The referenced IEC document is IEC34-8. Under the stated NEMA comparison, NEMA MG1 uses “terminal leads” for the connection points and “terminal markings” for their identifiers.
IEC-to-NEMA marking cross-reference
The stated cross-reference maps the six IEC markings to numerical NEMA-style identifiers. Preserve the marking system used on the actual motor rather than silently translating labels during field work.
| Winding group | First IEC marking | Second IEC marking | Stated numerical equivalents |
|---|---|---|---|
| U / phase a | U1 |
U2 |
1 and 4
|
| V / phase b | V1 |
V2 |
2 and 5
|
| W / phase c | W1 |
W2 |
3 and 6
|
This table identifies corresponding designations; it does not prove the labels on an unknown motor. Rewinding, lead replacement, lost sleeves, or previous field relabeling can leave a motor with internally incorrect markings. Test continuity and relative polarity before relying on translated identifiers.
Symptoms and diagnostic decisions
| Observed symptom | Likely distinction | Deciding check |
|---|---|---|
| Motor runs normally but rotates in the wrong direction | Incoming phase sequence is reversed relative to the required rotation | Verify the required direction, then exchange two incoming phases under the machine's approved procedure. |
| Motor stalls when changing from star to delta | Delta links, changeover wiring, winding pairing, or one winding's polarity may be wrong | De-energize the circuit and trace every contactor output to the marked motor lead; then verify all three winding pairs and their relative polarity. |
| One measured winding pair differs from the other two | Wrong pairing, connection fault, or winding condition | Separate all six leads and repeat the resistance or continuity measurements directly at the motor. |
| Markings disagree with polarity testing | One or more lead labels are misplaced | Record the test-established pairs and polarity before assigning corrected markings. |
| Abnormal current or heating follows connection | Connection, supply, mechanical load, or winding fault remains | Compare conductor currents, supply conditions, connection diagram, mechanical condition, and motor rating data. |
A resistance test establishes pairing but not start/end polarity. Three plausible resistance pairs can still be labeled incorrectly. The polarity test is the step that decides whether the “1” ends have the intended relative orientation.
Terminal identification procedure
- Isolate the motor. Remove power using the site's electrical isolation procedure, prove the circuit de-energized, and separate the six motor leads from star-delta contactors, links, and supply wiring. Do not energize an unidentified six-lead winding arrangement; a wrong connection can produce damaging current and heat.
- Record existing markings. Photograph or transcribe every sleeve, terminal-board label, link, and conductor destination. Treat this as the as-found condition, not proof that the markings are correct.
- Find the three winding pairs. Use a suitable de-energized continuity or resistance test to identify the three isolated circuits. Establish the pairs before assigning U, V, or W names. Each valid pair represents the two terminal leads of one phase winding.
- Check measurement symmetry. Compare the three winding measurements using the applicable motor test limits and instrument method. A markedly different result requires investigation before a polarity or connection test proceeds.
- Establish relative polarity. Apply the organization's approved winding-polarity test method and test equipment. The method must identify the relative start and end of each winding; continuity alone cannot do this.
-
Assign terminal markings. Mark the three coordinated first ends as
U1,V1, andW1, and their paired second ends asU2,V2, andW2. Keep a test record linking every new marking to the measured winding pair. - Reconnect from the motor diagram. Select star, delta, or a star-delta circuit only when it agrees with the motor rating data and the intended supply. Trace automatic changeover wiring contact by contact rather than inferring it from conductor color.
- Perform controlled commissioning checks. Verify link placement and insulation status under the applicable procedure, then monitor rotation, current in each supply conductor, acceleration, sound, vibration, and heating during the authorized test.
Star and delta connection reference
For the stated IEC six-lead arrangement, a star connection joins U2, V2, and W2; the supply connects to U1, V1, and W1. A delta connection forms three junctions: U1 with W2, U2 with V1, and W1 with V2. The supply connects to the three junctions.
| Connection | Required junctions | Supply points |
|---|---|---|
| Star |
U2-V2-W2 joined together |
U1, V1, W1
|
| Delta |
U1-W2, U2-V1, W1-V2
|
Each of the three junctions |
This connection table defines topology, not suitability. Read the permitted connection and supply relationship from the motor's rating plate and connection diagram. A six-lead terminal box provides access to the winding ends, but the application data decides which connection may be energized.
Verification and documentation
Verification must separate correct rotation from correct winding identity. A motor can rotate in the required direction while labels remain wrong, and a correctly marked motor can rotate backward because the incoming phase sequence is wrong.
- Confirm that continuity exists only within the recorded pairs
U1-U2,V1-V2, andW1-W2while the links and external circuit are disconnected. - Confirm that the approved polarity test places all three “1” ends in the same relative orientation.
- Compare the final links against the selected star or delta diagram and the motor rating data.
- Trace the supply and, where fitted, every star-delta changeover connection to its terminal marking.
- During the controlled run, confirm required rotation and compare current among the supply conductors. Investigate imbalance, failure to accelerate, stalling at transition, or rapid heating before further operation.
- Update the terminal schedule with the as-left markings, winding pairs, connection, test method, measurements, and observed rotation.
FAQ
What happens if U1 and U2 are reversed on one winding?
That winding has opposite relative polarity to the other two. The motor may fail to accelerate, draw abnormal current, vibrate, heat rapidly, or stall during a star-delta changeover.
What happens if I swap L1 and L2 at the motor?
Exchanging two incoming phases reverses the overall phase sequence and normally reverses rotation. It does not correct terminal markings or a single winding pair whose start and end are interchanged.
What happens if I identify winding pairs using continuity only?
Continuity can establish U1-U2, V1-V2, and W1-W2 as pairs, but it cannot establish which end is “1” or “2.” Complete an approved relative-polarity test before assigning those terminal markings.
When should I stop testing and contact official support?
Stop when the winding pairs cannot be separated, polarity results repeat inconsistently, one winding measurement differs materially, or abnormal current, stalling, or rapid heating remains after the wiring is traced. Keep the motor isolated and provide the manufacturer or authorized service channel with the rating-plate data, connection diagram, as-found markings, measurements, and polarity-test results.