English machine translation of the Persian article. For technical decisions, consult the original article and the manufacturer documentation. Translation updated: 2026-10-09.
Identifying Phase Sequence and Correct Star‑Delta Wiring in Motors with Unmarked Leads
In many motors that are re‑wound and returned to assembly, the lead labels have disappeared or, for any reason, it is unclear which leads correspond to U1, V1, W1 and which to U2, V2, W2.
In this situation, incorrect motor start‑up can cause severe over‑current, vibration, winding overheating, and even motor burnout!
Empirical Method by Alvand Sanat Javid Alborz
Based on practical experience in repairing hundreds of electric motors, there is a very fast and reliable method to identify the leads without reopening the motor.
Procedure
• First, temporarily divide the six winding leads into two groups of three.
• Connect the star (Y) configuration.
• Start the motor without load under full control.
• If the motor draws excessively high current or shows abnormal behavior, the wiring is incorrect.
• At this stage, change the position of only one lead and test the motor again.
• Repeat this stepwise process until the motor runs in star with nominal and completely normal current.
Read more on this topic:
- Electric motor terminal box failure – Is the winding burnt?
- Diagnosing electric motor burnout caused by phase loss
- Electric motor nameplate reading
When the motor starts in star with standard current, the correct winding arrangement is confirmed.
After that:
• The leads are permanently labeled.
• The delta connection can be performed easily.
• Correct wiring is identified without reopening the motor.
Benefits of this method
• No need to reopen the motor
• Significant time savings
• Reduced repair costs
• Prevention of operator error
• Permanent lead labeling possible
• Correct star and delta start‑up
Technical Recommendation from Alvand Sanat Javid Alborz
Before applying full voltage, always monitor start‑up current, rotation direction, and motor sound. After correctly identifying the leads, apply permanent IEC‑standard labeling to avoid future errors.
Alvand Sanat Javid Alborz
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