Core lamination groundings of large stators

As we know that the laminations are insulated from each other to prevent the eddy current circulations between the adjacent laminations. Once lamination staking done, how we are gonna earth the core of any transformer or motor? For a large 11KV motor, I have seen a guide bar welded to the laminations at the core back side & that is grounded to body but that eventually lead to the shorting of insulations at one point.

For motors there is typically a welded bar on the back or the whole stack is welded at various positions around the exterior of the stator, to hold the stack together. Eddy currents can circulate but the intensity is much less because the weld is on the outside of the winding area, not where the magnetic fields are intense. The electromagnetic fields are most intense in the iron covered by the windings and close to the rotor and air gap. The electromagnetic field is not as strong on the back side of the iron, so that is where the mechanical connection is made and where grounding can be accomplished.

All the magnetic cores for generators are made by several laminations (0,5 mmm thick for large turbo generators) to avoid the known phenomena of Eddy current or Foucault current circulation.

Thus the currents circulating in each lamination, induced by the variable main magnetic field, are very low because the core is made by thin laminations. The main magnetic field in a core is distributed thought the full corona cross section, links the bars of stator winding and close thought the air gap on the rotor poles. Across the teeth the main magnetic field is more intense.

The welded bars on the back cause only a variation of reluctance of magnetic circuit and so a different distribution of magnetic flux.

All the laminations making the magnetic core are short circuited by the welded wedges on the back but only in one point. To close a current loop between two or more lamination it is necessary to create another contact point (short circuit). In this case the high circulating currents will cause an overheating of affected laminations.

The leakage variable flux (which nothing has to do with the main flux) generated by the current in the stator winding bars also interest the magnetic core in the area of teeth/slots.

The core of a large generator is secured electrically to ground via the frame to which is connected (in turbo generators) by mean of mechanical damping systems.

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