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Core is the part of the transformer where the magnetic field oscillates. It is made out of a high grade super oriented steel with a small percentage of silicone. This is to make sure that the energy losses that result in heating are minimized. The core steel is rolled into thin sheets, as small as 20 gauge and each side is coated with insulating material, the purpose of these thin sheets is to further minimize losses.
For power transformers, the sheets are stacked together with mitered joints, by varying the width of steel laminations in the leg of mitered core, a circular cross section can be approximated.
The process involves core cutting and stacking equipment to construct the core frame. Automatic core cutting machines have become standard now for power transformer cores. The stacking of the core requires extra care and precision. One has to be careful about:
+ gap at the step lap needs to be small, under 2mm.
+ Control of burrs, thin, steel threads that break loose in oil and create partial discharge later.
+ Control of square and perpendicularity of magnetic circuit.
These are some of the few things that need to be closely monitored to ensure that these problems do not arise. In summary, manual stacking is a common practice in the industry, is cumbersome, time consuming and prone to errors in case of a large power transformer construction.
At GTC, we use state of the art Astronics core cutting assembly unit. Astronics automated, core cutting equipment allows precise cutting of single and multi-leg transformer core.
Integrated with the core cutter is an automated E-stacker. The E-stacker stacks the core steel into final position as it is cut, practically eliminating human errors and lowering losses while increasing reliability. At other VTC plants a different version of core stacking is used for medium power transformers.
A laser measurement system is used at VTC to measure the burr. The laser head is mounted on the measurement table and the head is moved across the laminated steel that is placed underneath the laser beam. The system scans for burr and measures at a predetermined rate.
- No calibration is required for every measurement – data is inputted in the system once to compare the acceptable steel edge versus bad steel edge with burr.
- Accurate Measurement – less human interference involved in measurement
- The instrument can provide least count, up to 0.1 mil – increased the gradient of detecting the burr
- Generates reports in excel format; reports can be saved per job for records. Reports can also be used to track burr and take preventive actions prompting preventive maintenance on machine with respect to tool change with accrued data.