Mike Howell
EASA Technical Support Specialist
Induction motor stator cores can be manufactured using single-piece laminations (see Figure 1 left) up to an outside diameter of about 48 inches (1200 mm). For larger stators, or when minimizing scrap material, the stator laminations are segmented (see Figure 1 right). The typical circumferential gap between segmented laminations is only around 0.012 inches (0.3 mm), so it is exaggerated in the included figures. The number of segments chosen by a manufacturer for a given design can depend on several factors, some technical and some economic. For most service center repair activities, machines with segmented lamination stators are processed no differently than those with single-piece laminations. However, there are a few areas worth exploring that could be helpful when working with segmented lamination stators.
Rewinds
Stator rewinds can be performed in the same fashion, whether the stator laminations are single-piece or segmented. However, even though it is a good practice, not all service centers utilize spacers in the bottom of the stator slots for form windings, and the presence of segment joints does increase the likelihood of surface irregularities. For this reason, it is recommended to use a bottom spacer (bottom stick) for segmented lamination designs even if you choose not to use them for single-piece lamination stators. No significant amount of slot space needs to be lost as a 0.009 inch (0.2 mm) separator is sufficient. Glass-epoxy or similar materials are recommended unless stress control materials are being used, in which case a conductive material should be used.
Core Replacement
If replacing a stator core with an outside diameter smaller than 48 inches (1200 mm), consider converting the stator laminations to single-piece design. This will produce more waste, but the laser cutting process and the stacking process will require much less labor. Additionally, the likelihood of mistakes will be greatly reduced. Most laser cut lamination suppliers can assist with this change.
If the stator is to be restacked with segmented laminations, it is important to understand the orientation and stacking sequence of the original laminations. If new laminations are made, the electrical steel properties should be equivalent to the original, and the finished assembly should have an appropriate retained pressure. The number of segments used can vary by design, but we will look at a couple of examples of six-segment stators. For a six-segment stator, each segment will occupy 360°/6 = 60°. The laminations will be located using mating seats along the outside diameter. Two such 60° designs are shown in Figure 2 and in both cases, there are 12 mating seats equally spaced in 30° increments around the outside of the stator. The design on the left in Figure 2 is symmetrical about its centerline while the design on the right is asymmetrical. The symmetrical laminations would be stacked with each layer being indexed from the previous layer by 30° as shown in Figure 3. The advantage of this design is its simplicity, but it only produces 12 joint locations around the periphery of the stator, which is not ideal for minimizing magnetic dissymmetry or for maximizing mechanical integrity. With the asymmetrical lamination design, 24 joint locations are produced around the periphery of the stator as each layer is indexed 15° from the previous layer. However, assembly is more complicated because every other layer must be flipped in addition to being indexed as shown in Figure 4, where each lamination has a red side and a blue side.
Loop Test/Core Testing
Service centers sometimes experience much higher-than-expected test currents when loop testing segmented stators. A procedure for performing stator core tests is provided in Section 7.4 of the EASA Technical Manual. To achieve a back iron magnetic flux density of 85,000 lines/in2 (1.3 T), the procedure assumes a required magnetizing force of 9 ampere-turns per inch (350 ampere-turns per meter) of average back iron circumference. This allows the user to estimate the test current so that cable size can be selected, and power supply capacity can be verified.
The problem that arises with the loop test of a segmented stator is that the flux path around the back iron is interrupted once per segment with an air gap. This significantly increases the reluctance of that path requiring more magnetizing force to achieve the same back iron flux density. Interestingly, the flux does not actually cross those air gaps; rather, it is essentially rerouted through the adjacent laminations. This is because the interlaminar insulation separating the laminations is usually about 0.001 inches (0.03 mm). When this separation thickness is significantly smaller than the gap between segments, the path of least reluctance is through adjacent laminations. Additional magnetizing force is required to cross the insulation thickness, and the adjacent laminations may see localized saturation. The result is typically an increase in current of around 60% to 100% over the same core with single-piece laminations.
Figure 5 shows a simplified finite element model used to demonstrate this effect. The current in the test coil was increased until the laminations experienced a back iron flux density of 85,000 lines/in2 (1.3 T). The magnetizing force in that orange region not influenced by the segmenting is 6.4 ampere-turns per inch (250 ampere-turns per meter). In the purple regions just above and below the segment gap, the flux density is 103,000 lines/in2 (1.6 T) and the magnetizing force is 132 ampere-turns per inch (5,200 ampere-turns per meter). It is also apparent that the magnetic flux density in the segment gap is practically 0 – indicated by the blue shading.
Pole Change Redesigns
When stators are segmented, the magnetomotive force (MMF) produced by the stator winding may not sum to zero at all instants of time. If this sum is not zero, a voltage will be induced into the shaft, which can lead to damaging bearing currents. This is basically the same effect we see when air gaps are not uniform. Manufacturers have learned over the course of many decades that certain combinations of the number of poles and stator segments should be avoided. These are design rules that rarely affect a service center, but one situation where this could become significant is in the event a machine with a segmented stator is being redesigned for a different number of poles. If the resulting combination of poles and stator segments violates these design rules, it is likely that induced shaft voltages could become a problem and, in these cases, bearing insulation should be employed. Polyphase Induction Motors by Paul Cochran provides these rules as follows:
- The number of segments divided by the number of poles should not equal 3/8, 3/4, or 3/2.
- Twice the number of stator slots divided by the number of poles should not equal an odd integer.
- Four times the number of segments divided by the number of poles should not equal an odd integer.
- Two times the number of punching segments divided by the number of poles should not equal A/B, where A is an odd integer, and B is the denominator after reducing A/B to its lowest terms.
Further, most manufacturers will insulate bearings of machines once a certain size is reached due to the likelihood of magnetic dissymmetry from manufacturing variation.
Summing It Up
While most service center repairs of machines with segmented stator laminations require no special attention, we have reviewed a few scenarios that are important to be mindful of. Before making any major changes to a machine like this, we do encourage members to utilize EASA’s technical support staff as a sounding board in an effort to prevent any unforeseen issues.
For Technical Support
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EASA Technical Manual
More information on this topic can be found in EASA's Technical Manual- Section 2: AC Machines
- Section 7: Electrical Testing
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