Airgap winding for large turbogenerators

Modern turbogenerators have large airgaps for system stability. This airgap space can be used to accommodate the a.c. windings of the machine, instead of placing them in slots. In the paper, the disadvantages of windings in slots are discussed, together with problems that will result from putting the winding in the airgap. Using a published design for a conventional 750 MW machine and converting this to an airgap-winding design, it is shown that the size and weight of the total machine are considerably reduced, with corresponding reductions in cost and transportability. Since it is desirable that a practical airgap winding should pass through the stator bore, so that it can be prefabricated as a complete cylinder, the normal winding is discussed and is shown to be difficult to make as a cylinder. A concentric winding, proposed by the author, which has normal winding factors in the active region and which can be made as a cylinder, is described. Simplified calculations for forces between phase bands and for leakage reactance are given. These forces will have to be calculated in detail later, but the simplified calculation shows that they can easily be contained by modern reinforced resins. The leakage reactance is shown to be considerably reduced; this is important as an indication that there will be little need for transposition in the radial direction. A simple method of threading the rator through the airgap winding is described.