Generator and prime mover capacities

Electric generators are of necessity driven by prime movers and the combination must be treated as a single unit when considering the questions of ratings and capacities. The importance of the proper adaptation of the capacities is fully emphasized by the fact that there are in operation in many stations in this country, units in which the output is unnecessarily limited by a discrepancy in the ratings, in that the prime mover is either too small or too large for the generator. The latter may, for example, be designed for unity power factor, while the actual operating power factor may be 0. 8, in which case the full capacity of the prime mover can not be utilized. In the past every effort was made to adjust the ratings of the generators to the station load curves, and the result was that overload capacities of 25, 50 or even 100 per cent had to be guaranteed, usually for a period of two hours. With the growth of the generating station and the improved load conditions this practice is now becoming more and more obsolete and the units are being rated on a maximum or, more properly called, constant continuous rating, which should not be exceeded except during momentary peaks. Steam turbine units have been rated according to this method for some time past, and with entirely satisfactory results. It is also becoming quite common in connection with waterwheel-driven units, and there seems to be no reason why it should not prove to be equally satisfactory for hydroelectric plants, especially for low heads, where the waterwheel efficiency falls off rapidly as the power is reduced below the normal full load. Exceptions to this may, however, be found, as, for example, where heavy short-period peak loads must be provided for, in which case it may be advisable to select a generator having a corresponding overload capacity. For the sake of standardization it seems, however, desirable to give all generators a maximum constant continuous rating at a certain specified temperature.