When machine parameters are expressed in a properly selected per-unit system, the parameters of a wide range of machine sizes fall into relatively narrow ranges even though the actual values of these same parameters may vary over extremely wide ranges. The per-unit system thereby makes possible an easy comparison between different machines, essentially independent of size, without the use of conversion factors, and facilitates the identification of gross calculation errors. The proper selection of base values is fundamental to the usefulness of a per-unit system. Although per-unit quantities have been used for many years by designers of rotating machinery, the base quantities were not standardized, and it was necessary to define them in each publication. A working group of the Rotating Machinery Committee was created to propose a set of standard definitions. In 1961, a proposed standard (AIEE No. 86) was published for trial use. A critical review of that proposed standard has resulted ...

Since rotating machines convert mechanical energy to electrical energy, or electrical energy to mechanical energy, base values for each kind of energy appear desirable. However, the relationships between these energies involve power factor and efficiency. These are not fundamental quantities, and vary considerably between different designs. While generator designers consider base power to be output power, synchronous-motor designers consider base power to be input power. Other motor designers consider it to be output power. The consensus of large synchronous motor designers favors using the rated apparent input power as the base even though it is necessary to know power factor and efficiency. This permits the use of the same design equations for both synchronous generators and synchronous motors. For induction motors, however, a more practical approach is to use the rated output power as a base, thereby eliminating assumptions of power factor and efficiency. This is particularly desira...