By Mike Howell, PE
EASA Technical Support Specialist

Identifying unmarked leads in three-phase motors is a common task for service center technicians. 

When working with 6-lead windings, read Chuck Yung’s Currents article from May 2008, “Identifying Unmarked Leads of 6-Lead Motors With 1 or 2 Windings.” For 9-lead windings, read Tom Bishop’s Currents article from April 2011, “Identifying 9 unmarked leads of three-phase motors.” 

This article provides one way to assign lead numbers for a 12-lead machine with unmarked leads where the winding is designed to be configured as shown in Figure 1. Use of 12-lead windings allows original equipment manufacturers (OEMs) to facilitate multiple voltage ratings and starting methods from the same winding. But if some or all the lead markings are missing, assigning valid markings to the leads can be quite a challenge. While the procedure that follows in this article works well, it does require some specialized equipment and at least partial disassembly of the machine.

A 4-pole conventional winding with 12 groups of stator coils will be used in this example but the procedure can be applied to any valid 12-lead winding, accounting for changes in the total number of groups and their displacement in the stator. 

What You Will Need

• Multimeter (for continuity checks)
• DC Power Supply (to energize windings and check polarity)
• Thermal Camera (to visually identify energized coil groups)
• Pole Detector (magnet in gimbal, compass, or digital tool to check magnetic polarity)
• Winding Connection Template (EASA's Internal Connection Diagrams for Three-Phase Electric Motors)

Let’s Go Step-by-Step

1. Identify Lead Pairs

• Use a multimeter to find six groups of two leads with continuity. These represent the six circuits within the stator.
• Mark or tape each pair for easy identification.

Figure 3 represents 12-lead tags numbered for NEMA and IEC, the 12 leads sorted in six pairs, and a blank Winding Connection Template for a 4-pole winding with 12 groups of coils.

2. Tag Initial Leads

• Select any two leads and tag them as U1 and U2 (IEC) or 1 and 4 (NEMA). These will remain fixed throughout the process.

3. Energize and Visualize

• Pass direct current (DC) through the first pair (1 and 4). If the winding is correctly configured, the temperature will increase in one-half of one phase.
• Use the thermal camera to observe which coil groups heat up and label those on the Winding Connection Template.

Figure 4 represents currents flowing through the first pair and the energized groups indicated on the Winding Connection Template.

4. Series Connection for Phase Identification

• Select another pair of leads and connect them in series with the first pair.
• Pass DC current through the new series circuit.
• Use the thermal camera to check for four coil groups heating up.
• The correct phasing for a 4-pole winding with 12 groups will show four groups spaced 90° apart. 
• Try different pairs until the correct groups for the first phase are identified.

Figure 5 includes thermal scan after energizing the first pair (left) and then the correct two pairs for the first phase. Note that in this 4-pole winding, there is a skip-pole connection since the energized groups on the left are 180 degrees apart.

5. Polarity Check

• Use the pole detector to verify magnetic polarity (NSNS or SNSN) of each energized group.
• If exactly four poles (number of poles in machine) are found, the phase is correctly identified.
• If not, reverse the connections of the second pair and repeat the polarity check.

Figure 6 represents the correct groups and polarity for the first phase. If the wrong second pair of leads were selected, the energized groups would not be 90 degrees apart. If the winding was connected adjacent pole, the incorrect polarity of the second set of leads would produce six poles. If the winding was connected skip pole, incorrect polarity of the second set of leads would produce eight poles.

6. Marking and Documentation

• Once the first phase is confirmed, mark the leads as 1 (U1), 4 (U2), 7 (U3), and 10 (U4).
• Mark the stator (using adhesive notes, steel-friendly marker, etc.) to indicate the center and polarity of each group.

7. Repeat for Remaining Phases

• Select four more leads, connect in series, and repeat the energizing, thermal imaging, and polarity check steps.
• Ensure the polarity alternates with the previously identified phase.
• Mark these as the next phase, e.g. 3 (W1), 6 (W2), 9 (W3), 12 (W4) or 2 (V1), 5 (V2), 8 (V3), 11 (V4).

8. Testing and Verification

• After marking all 12 leads, open stator impedance tests and surge tests can be used to increase confidence in the lead markings.
• Once assembled, run the motor at its designed voltage(s) and start method(s) to ensure proper operation.

Tips and Practical Advice

• Thermal Camera Use: Greatly accelerates identification by visually confirming which coils belong to each phase. This is much quicker than a trial-and-error approach. There are many inexpensive options available that are sufficient for this activity and can be used for various other purposes, such as inspection work and core testing.
• Polarity Tools: Inexpensive pole detectors or digital tools are sufficient for checking magnetic orientation. While many smartphones can now also do this, be aware that the field strength will probably exceed limits, making this an impractical option.
• DC Supply Settings: Use enough current to observe temperature changes, but do not exceed the machine’s rated current. For large machines, a high-current supply is ideal; for small machines, even a battery (or battery charger) may suffice (though slower). If you don’t have a large DC supply, energize the circuits with AC to identify the groups and then with low-voltage DC for the polarity check.
• Permanent Marking: Always mark leads clearly to prevent future confusion.

Conclusion
The EASA procedure for identifying unmarked leads in 12-lead three-phase motors helps to make a challenging task manageable and efficient. As with most tasks, there are different ways to do this, and you should use what works best for you. EASA is always looking for better ways to help members, so feel free to share your method if you feel it saves time. For example, one member utilizes a single-phase autotransformer and oscilloscope for identifying unmarked leads. That method may be shared in a future article.