Chuck Yung
EASA Senior Technical Support Specialist
When taking winding data, the area most prone to error is in identifying the connection. This article includes a reference page (see Figure 1) that I encourage you to print and laminate for the winders to use.
The first step is to verify the lead markings. Do the markings appear to be factory original and are they NEMA or IEC?
The next step is to confirm whether or not the starting method requires more than 3 leads. If the motor has 6 leads, it is essential to determine whether the motor uses wye-delta, part-winding (PWS), or direct-on-line starting. If the motor has only 3 leads, the question becomes: “Is it wye or delta?” With 1 group-end per lead, the connection can only be a 1-wye. With 2 group ends per lead, it could be 2-wye or 1-delta. If the group ends connected to any of the 3 leads are from adjacent groups, the connection must be delta.
Check closely!
With 3 group ends per lead, it can only be a 3-wye, so it has to be a 6-pole or have a pole count that is a multiple of 3. That is the reality check for each connection: Is the number of circuits copasetic for the number of poles? If the poles are not divisible by the number of circuits, check again!
When counting the leads, paralleled leads used to permit the use of smaller lead wire (e.g., two of a 2 AWG lead cable are easier to work with than 1 of a 4/0) are not counted separately. In other words, if the stator has 6 leads, numbered 1-1, 2-2, 3-3, treat it as a 3-lead motor. Document the actual number of leads and the size, to avoid errors.
Reality check: the number of circuits has to make sense for the number of poles. If the motor is dual-voltage, or if your customer requests it be converted for dual voltage, both circuits have to be “legal” for the number of poles. For example, a 2-circuit 460v 6-pole motor cannot be converted to 230/460v without also changing the number of turns and circuits.
EASA’s AC Motor Verification and Redesign Program is also valuable as a quality control check. When you input the winding data, if the airgap density is way outside the expected range, that could mean you recorded the connection incorrectly. If the airgap density (AGD )and the circular mils per amp both seem too low, it probably means the motor has more circuits. Look at the nameplate: if the motor is labeled for only one voltage, and it has more than 3 leads, it should be connected for the “low voltage”delta to operate.
Examples
- 500hp, 4-pole with 12 leads, rated 460v. It’s a safe assumption that no one is going to operate this motor at 230v, so the correct connection is the “low voltage delta” for 460v. Misconnecting it for “high voltage” is a common cause of failure, so that should be part of your failure analysis.
- An IEC motor with 6 leads is labeled for 415v 50 Hz; it is almost certainly intended for wye-delta starting. Input it as a delta connection.
Terminology helps!
Winders don’t all use the same terminology. Connections for 3-phase electric motors are either wye, delta or wye-delta. If it has a wye (a star point), record it as a wye connection. If it has 6 leads, with 3 separate circuits, it is a wye-delta. Some people call this an “independent” connection, but record it as a wye-delta.
Next up is the number of circuits. Use the accompanying reference chart – it avoids confusion, and makes it easier to correctly identify the connection. By using the same terminology we can avoid mistakes and identify cases where the customer incorrectly connected the external leads to run.
If you identify a connection that your reality check says is not possible, have someone double check it before you strip the motor. There are some clever design engineers out there, so you really might have a 3-circuit PWS on a 4-pole!
Verify starting method
If the winding has 6 leads, it could be dual voltage (e.g., 2300/4000v if NEMA; 230/415v if IEC; any voltage ratio where the high voltage is square root of 3 times the low voltage should be a wye-delta); or the motor could use a special starting method to reduce torque and/or starting current. Those methods are either PWS or wye-delta. According to NEMA MG1, the lead markings for wye-delta are 1 through 6. For PWS, the leads should be numbered 1-2-3-7-8-9. But if the lead markings are not original to the manufacturer, be careful. The plant electrician who removed the motor from service might not have followed convention. ALWAYS check with your customer to verify the starting method they are using.
The danger of mistaking the starting method is this: A 2-circuit wye-delta and a 2-circuit delta PWS both operate as a 2-delta in operation. The calculated magnetic flux densities are of no help in deciding which connection is correct. Tip: A wye-delta arrangement requires 3 contactors, while the PWS only requires 2. That makes PWS more frequently used on low-bid jobs like municipal water plants.
The customer will benefit!
The payoff for attention to detail is in customer satisfaction. Connecting a PWS motor for wye-delta because the plant electrician labeled the leads incorrectly might not be your fault – but you will certainly be blamed for it. So the phone call to confirm starting method protects your reputation.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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