Units, conversions, standards and Preferred numbers

The modern SI system has been adopted in legislation by practically every country in the world. There are, however, extensive changeover problems and many different kinds of data are still only available in units which do not conform to the SI system. Chapter 22 attempts to bridge the difficulties experienced when dealing with quantities expressed in older units.

Almost without exception it is recommended that the quantities be converted directly to SI units; calculation can then be carried out using the coherent SI system. However, it is important that all results of calculations should be reviewed by someone fully conversant with the system of units and the practical values encountered.

The “Systиme International d’Unitйs” (SI) is therefore the universally accepted system of units. To undertake any calculations requires a knowledge of units and their correct usage. It is fortunate that the basic electrical units were defined in terms of the metric units from which the SI system evolved and for fans this has some unique advantages compared to other unit systems.

This Chapter outlines the basis of the SI system and the recognised ways of expressing quantities. Guidance is giving on how to check equations for the consistency of units, also a table of conversion factors is included for units likely to be met in drive systems calculations.

To ensure standardisation across national borders, most motors are manufactured to standard dimensions and sometimes standard output powers. This standardisation is based on a preferred number series which is explained in detail. The use of preferred numbers has, of recent years, been extended to the sizing of fans.

Special Note: Some American publications have great difficulties with SI units. Treat all SI

Values quoted with caution.

Conversion factors for hardness and toughness testing, which are not defined by SI are also given.

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