Motor connection tips for avoiding costly mistakes

The following tips apply to common connections on machines with one speed at power frequency.


Figure 1: Motor connection failure. Courtesy: EASAManufacturers deploy various external connection schemes to produce three-phase induction motors for multiple voltages and/or starting methods, so successful installation depends on using the relevant connection diagram. If this information is lost, damaged, or ignored, a connection mistake could lead to a costly rewind (see Figure 1).

The following tips apply to common connections on machines with one speed at power frequency. If the manufacturer's external connection diagram isn't available, ask a service center for assistance, especially if there are several missing lead tags, multiple speed ratings at power frequency, unconventional numbering, or NEMA-IEC cross-references.

Three leads

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

Six leads

If leads are numbered 1-6, 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.

Figure 2: Line lugs terminated on bus in outlet box. Courtesy: EASA

For a single voltage rating, most 6-lead machines are capable of wye-delta starting (and will run in delta). The exception would be that some large machines have external wye connections to facilitate differential protection.

If the leads are numbered 1-3 and 7-9, 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, which will be a delta-run motor (see Figure 2). Also, some part-winding start motors are numbered incorrectly as 1-6, so keep in mind the starting method you're using.

Nine leads

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

On the lower voltage rating, however, the external connection will be different for wye-connected motors than for delta-connected motors, so it's important to know what you have. If a multi-meter shows continuity between leads 7, 8, and 9, the machine is wye-connected (see Figure 3).

Twelve leads

If the leads are numbered 1-12, 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 the low voltage only. Machines 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, ask a service center; they have reliable procedures for identifying leads.

Mike Howell is a technical support specialist at the Electrical Apparatus Service Association. Courtesy: EASAUncoupled run

If in doubt about an external connection, run the machine unloaded to determine the direction of rotation and no-load current. No-load current significantly above or below the ranges in Table 1 may indicate a connection error or a winding error if the machine has been rewound. (Note: Never operate a roller bearing machine without radial load.)

Table 1: Typical guidelines for no-load current.

 Table 1: Typical guidelines for no-load current. Courtesy: EASA

Some misconnections can cause failures very rapidly, so remember that a delayed start-up is better than an unnecessary motor failure.

-Mike Howell is a technical support specialist at the Electrical Apparatus Service Association (EASA). EASA is a CFE Media content partner.

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