At rated voltages higher than 52 kV, the condenser or capacitance graded bushing principle is generally used. The insulating material for such a Applications in High Voltage Bushings is paper which is treated. There are three types of such paper, viz. resin bonded paper (RBP), oil impregnated paper (OIP) and resin impregnated paper (RIP). The paper is generally wound onto a central tube and conducting layers are then inserted to form a series of concentric capacitors between the tube and the mounting flange. The diameter and length of each layer is designed such that the partial capacitance give a uniform axial stress distribution and control the radial stress within the limits of the insulating material.
In resin bonded paper (RBP) bushings, the paper is first coated with phenolic or epoxy resin and wound into a cylindrical form under heat and pressure, after inserting conducting layers at appropriate intervals. They are designed to operate at a maximum radial stress of 20 kV/cm. On the other hand, oil impregnated paper bushings (OIP) are made by winding un-treated paper after inserting conducting layers at the appropriate positions and then impregnating with oil after vacuum drying.
The paper used is generally an unleached kraft paper and the oil used is a mineral oil. Prior to impregnation, processing is carried out to ensure that the moisture and gas contents are low. These Applications in High Voltage Bushings are designed to operate at a typical radial stress of 40 kV/cm. Resin impregnated paper bushings (RIP) are being used up to voltages of 520 kV. In its manufacturing process, creped paper tape or sheet is wound onto a conductor and dried in an autocleave under strictly controlled heat and vacuum conditions. Epoxy resin is then added to fill the winding. These Applications in High Voltage Bushings are designed to operate at a radial stress of about 30 kV/cm.
Application in Fractional Horse Power Motors:
Small generators and starters:
These are mass produced items and are used in automobile industry. Therefore the insulation system must guarantee reliability, combined with good processing capability, mechanical strength and tolerance to severe short thermal stress while in operation.
For this kind of application the following materials have been found to have excellent insulation properties, viz. presspaper reels, NMN multilayers (made of Nomex and polyester film) as well as saturated DMD (made from nonwoven polyester and polyester film). These materials are cured with high quality resin impregnation on both the sides.
Small size motors:
These motors are made in large numbers and are used in automobile industry and in household appliances. They should give reliable service in operation which involve vibrations. Manufacturers of this type of motors use pressboard as the main insulation.
In contrast to polyester film, pressboard has no melting point. This board has been widely used in small size motors over many decades. Further, in the case of electric power tools, the main insulation is the multi-layer material made of pressboard and polyester film. The greatest advantages of this material is that it can withstand short-term thermal stresses.
In recent years, many new high temperature polymers have been developed and are being widely used as insulating materials in small size electric motors and generators and for a variety of electronic applications, including manufacture of PCB, electronic components, etc.