Polyester film has become the accepted ground and coil to coil insulation t material in most hermetic motors today. This paper deals with why the , properties of polyester film have made this development possible and how the recognition and control of key properties assure a consistently functional product.

The discovery and development of oriented polyester film is briefly presented along with a general description of its properties. The manufacture of the film is described with particular emphasis on those steps which have influence on properties important in motor applications. Specific applications of polyester film in hermetic motors are shown and explained and film properties for this end use are enumerated and discussed in detail. Typical methods for controlling these properties are presented. Potential future product development is discussed.