Interlamination voltages in large turbogenerators

The magnetic flux in a generator induces a voltage between adjacent laminations of the stator core. These voltages are uniformly low (less than 50 mV) along the length of the core, and so only light interlamination insulation is required to prevent troublesome heating in the core. However, near the end of the core, additional interlamination voltages are induced by the axial magnetic fields in the core-end region and the radial leakage field at the core back. These voltages result from the segmentation of the laminations, and their magnitude depends on the way in which individual laminations are earthed to the stator building bars. These `segmentation voltages¿ are calculated by considering the eddy currents induced in the laminations by the axial magnetic fields, the currents flowing from the laminations to the stator building bars and the currents induced in these bars by the radial leakage fields. It is shown that, if the contact resistance between the laminations and the building bars is relatively high, and varies from contact to contact, the segmentation voltages are an order of magnitude higher than those induced by the main generator flux and hence sufficient to cause troublesome heating in damaged areas of the core.