Tom May
Trade-Mark Industrial
For years, electric motor repair facilities have been routinely rewinding the common three-phase electric motor. I can recall the rare occasion where a winder slipped up taking a connection or counting turns, or missed recognizing a “specialty” two-speed or consequent pole connection.
It happens. We have all experienced it and learned from it. Today, winders are facing new challenges when winding AC motors, particularly when it comes to spindle and servo motors.
With the complexity of some of the newer technologies, the chance for error is significantly increased. We have had to become more observant and heighten our awareness. Here are a couple of examples that our service center recently encountered, and where we were able to identify the data and rewind the motor successfully.
Example 1
The first was a metric frame three-phase AC servo motor – 9.3 Nm (6.9 lb-ft) at 3000 rpm – 258 volts, 7.8 amps. The number of poles in the motor is determined by the magnetic poles on the rotor; in this case it was 8 poles. The stator has 36 slots with 18 groups of 1 (6 wound poles) in a 3-layer lap configuration. Each layer has 6 groups, two with a 1-4 span and four with a 1-5 span. There are eight empty slots – four across from each other in each layer. The empty, or skipped, slot locations are consequent poles. This is a part-salient part-consequent pole winding. See the article titled “Designing The Proper Part-Salient Part-Consequent Winding” in the August 2001 edition of Currents. Figure 1 depicts the winding, showing the first layer, and then the subsequent two layers on top of each other.
A winder who is too quick to cut off the opposite connection end, or start removing coils, can quickly become confused. We tag the leads with metal tags before going through the burnout process, and note the location and number of each lead. This allows us to accurately draw the connection diagram as we progress through the stripping process. In addition, the winder took digital pictures of each layer as it was unwound. We compare the turns and wire size in each layer. This particular winding was quite unique in that each layer had 4 north and 2 south-pole polarity coils. Each layer had the same layout. See Figure 1.
Experience tells us that it usually doesn’t matter where the winding starts in terms of slot and coil location, as long as you have the expertise and the equipment to align the encoder after the motor is assembled. A secondary challenge often encountered during the process of rewinding 3-phase AC servo motors is the relatively short coil extensions. Typically, size is an issue for the servo motor designs, so winding extensions are extremely important. In this case, the extension was 1” (25 mm).
Example 2
The second job we would like to share is from a spindle drive stator. Typically you receive just the stator with the winding, and no nameplate data, stator housing or rotor. In this case, accurately identifying the winding extension and shape is critical.
The stator had 36 slots, with 18 coils (6 groups of 3). Here is the challenging part: there were two windings laid on top of one another in the same slots. Each winding had two leads coming out of the same slot. Each winding had the same pitch, but different turns (one winding had 2 turns and one winding had 3 turns). Each winding had 16 wires in hand as illustrated in Figures 2 and 3.
Before burning out the stator, we tagged and identified all leads with metal tags that will not distort during the burnout process. The connection was then lifted and identified. Digital pictures are taken each step of the way (you can never have too many digital pictures). After the burnout oven process, the stator was unwound by hand noting the polarity of each turn (and watching for flipped coils). In order to identify the difference in each winding, we laid out the entire set of coils on the floor – stretching it out full length.
Noting coil polarity is extremely important. In each case we had, the polarity was the same in every coil. By identifying this, we were able to determine that the winding was connected consequent pole.
When winding the coils, it is advisable to mark each winding with a different color sleeving material. You must be able to identify the separate windings at the connection stage. Since only the stator is typically received on these jobs, it is critical to tag the leads exactly the same as they were received. Experience has taught us that even though the numbering system (especially if the factory leads are not on the motor) doesn’t appear to make sense because many maintenance people have their own diagrams, we have been able to do a successful rewind by duplicating their unique coded numbering system.
In summary:
- Be quick to recognize that anything that is not a standard NEMA style motor could be unique in the winding layout.
- Always lift the connection first – cold (pre burnout) if you can.
- Take digital pictures along the way.
- Unwind by hand unless you are absolutely sure of the layout and connection. It’s easier to rewind if you are not guessing
- Make legible, clear, well-documented drawings of the coil layout, lead numbers and all of the data.
- Duplicate wire size (i.e., circular mil area) and mean turn length are critical in maintaining the resistance of the original winding.
- Exact duplication of winding details avoids having the drive “reject” the winding.
- Servo motor windings do not always follow the typical rules of AC motor windings.
- Awareness will assist an experienced winder to verify number of poles (by recognizing the span)
- Obtain as much information about the machine prior to commencing the job to help in identifying pole count, voltage, etc. If you can, get an electrical schematic of the motor control.
- Don’t hesitate to consult with EASA technical support if you are not sure of something.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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