Blake Parker
Technical Education Committee Member
High-Speed Industrial Service
When looking at a stator core that requires repair, it can be easy to jump to conclusions. There are many factors to consider when re-stacking a stator. Those include the materials, core compression, length of the core, vents, spacers, vent construction and more. This article focuses on taking proper measurements when re-stacking a stator core and how to go about stacking the stator to ensure those dimensions are met.
Stator cores that feature vents typically do so to provide cooling to the stator and windings. (Figure 1) While it may seem simple, there are some pitfalls that can be very costly if mistakes are made when re-stacking a stator. While technicians are normally concerned about the magnetic field necessary to rotate the rotor and connected load, there is a magnetic axial component that also occurs. Failure to re-stack the stator properly can cause an imbalanced axial magnetic pull and prevent the rotor from being able to run on magnetic center.
There have been a variety of attempted methods to re-stack in the past. Following are a few of those approaches plus potential issues with each approach.
The first is to replace the laminations with the exact same number in each stack. There are a few problems with this. First, counting each lamination requires a significant amount of time. Longer stator cores feature thousands of laminations. Maintaining an accurate count is very difficult, increasing risk with regard to quality. Second, when replacing the laminations, the thickness will not be the same. Even if it is just 0.00025”(0.00635 mm) different if the stack contains 1000 laminations, that’s a 0.250” (6.35 mm) difference. When recoating the laminations, the coating will add thickness to the lamination once again throwing off the total stack height. Third, stacking efficiency is going to be different depending on the cleanliness of the core, the alignment method and the pressures used to compress the core. There are just too many variables to control for this method to work well.
The second approach is to weigh each stack of laminations as removed. Many of the same variables affect this process as well. Lamination thickness inconsistency is one variable. And, did the lamination provider add a feature or remove one that was unnecessary therefore eliminating or adding weight? Was there corrosion build-up or material removed from a rotor that drug the stator? As you can see once again, this method does not provide reliable results.
Let’s now jump to the recommended method. This method is to measure the individual stack, and then the cumulative, meaning the stack plus the vent and then the next building up to the overall height. The tolerance for the height of the stack and the cumulative stack is plus or minus the thickness of one lamination.
The reason we measure the packet then the cumulative stack is because if we are off on the individual packet height, it will result in having to make a large-scale correction on the last packet of laminations. Doing so results in an axial magnetic pull and can also cause alignment issues with the vents of the rotor. The reason we don’t just look at the overall is the same; a balance is required for the individual packets and overall length. This method considers the variation in thickness of laminations and stacking efficiencies. (Figure 2)
Starting with an accurate measurement of the first stack and vent location is critical. Checking for skew and any asymmetry issues are critical as well. Failure to align the vents properly can cause excess noise issues or cooling issues. Some motors feature stator and rotor vents that align perfectly. Other motors will whistle or “scream” causing noise issues. Those motors have offset vents to prevent that phenomenon.
The recommended tool for measuring stack heights is dial or digital calipers (Figure 3). Align the calipers with a vent, as many times there is swelling or waviness between the vents. A steel rule can be used; also, a tape measure should be the last resort. Many times, especially if the rotor drug the core iron, it is necessary to take the measurement from the back iron of the stator. If that is not possible, the lamination damage should be cleaned up to provide the most accurate measurement possible (Figure 4).
It is recommended that a second person verifies the core dimensions. Measure the cumulative height; don’t just add the columns as shown in the tables below as errors can be found this way. This also prevents rounding errors.
This table (Table 1) shows the compounding effect of paying attention to the packets only and erring on the side of additional height when deciding as to how many laminations to add. Lams and vents 4-19 are hidden to allow the chart to fit on the page. The net effect in the last packet is 0.254” (6.45 mm) shorter resulting in a weakened axial magnetic field at that end of the machine. It would be expected to see magnetic center issues when this motor is test run.
This table (Table 2) shows what a proper re-stack should look like. You’ll notice some are a little thicker, some a little thinner but none are off by more than 0.019” (0.48 mm) and the net effect of the total length is within 0.005” (0.13 mm). This motor will run without an axial magnetic pull.
Re-stacking stators is a great offering for your business. Keep in mind measurement is only one piece of the equation; other factors such as materials and compression need to be considered as well. Done properly, you can serve your customers well and grow your business with this service offering.
AVAILABLE IN SPANISH
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100- Section 2: Mechanical repair
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