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.
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