Maintenance Motors & Drives

Avoid Costly Motor Connection Mistakes

EP Editorial Staff | May 16, 2016

By Mike Howell, Electrical Apparatus Service Association (EASA)

Manufacturers deploy various external connection schemes to produce three-phase induction motors for multiple voltages and/or starting methods. Be sure to follow the relevant connection diagram, which is usually affixed to the motor or contained in its manual. If the diagram is lost, damaged, or ignored, you could find yourself dealing with a costly rewind.

The following tips apply to connections commonly encountered on machines with one speed at power frequency. If the external connection information isn’t available, ask your local service center for assistance, especially if several lead tags are missing or there are multiple nameplate speed ratings at power frequency. The service center can also help with unconventional numbering or cross-referencing IEC and NEMA numbering. Caution: The integrity of lead markings is only as good as the electrician who removed the motor from service and quality of the labeling materials at hand.

3 Leads

While three-lead connections are the most straightforward, always check the direction of rotation before finalizing the motor installation, regardless of the lead quantity.

6 Leads

If leads are numbered 1 to 6, the winding can usually be connected wye or delta. On machines rated for two voltages, the wye connection is for the high voltage; the delta connection is for the low voltage.

For a single voltage rating, most six-lead machines are capable of wye-delta starting (and will run in delta). The exception would be some large machines that have external wye connections to facilitate differential protection.

If leads are numbered 1 to 3 and 7 to 9, the winding is capable of part-winding start. When using a different starting method, e.g., soft start, variable-frequency drive, or across-the-line, always connect the machine for run.

Some machines will have 1-1, 2-2, 3-3, indicating a delta-run motor. Also, since some part-winding start motors are numbered incorrectly as 1 to 6, remember the starting method you’re using.

9 Leads

If leads are numbered 1 to 9, the motor is typically rated for two voltages and could be designed with either a wye or delta connection. Using the machine on the higher rating, the external connection is the same either way.

Screen Shot 2016-05-16 at 11.10.39 AM

On the lower voltage rating, though, the external connection will be different for wye- and delta-connected units. Verify what you have! If a multimeter shows continuity between leads 7, 8, and 9, the machine is wye-connected (see Fig. 1).

12 Leads

If leads are numbered 1 to 12, the motor is typically rated for two voltages and could be used with a wye-delta starter at either voltage, or a part-winding starter for low voltage only. Units rated for single voltage may have 12 leads and be suitable for wye-delta or part-winding starts. Twelve-lead induction motors will almost always run connected delta.

Unmarked Leads

If only a couple of leads are unmarked, you may be able to restore numbering by process of elimination. Otherwise, it’s best to ask a service center for assistance, because they have reliable procedures for identifying leads.

Screen Shot 2016-05-16 at 11.10.51 AM

Uncoupled Run

If there’s any doubt about the external connection, it’s a good idea to run the machine unloaded to determine the direction of rotation and no-load current. A no-load current significantly above or below the ranges in Table I may indicate a connection error, or a winding error on rewound motors. (Caution: Never operate a roller-bearing machine without radial load.) MT

Mike Howell is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis. For more information, visit


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