John C. Botts

Development of thermalastic epoxy for large high voltage generators

During the last fifteen years, the emphasis in stator insulation for central station generators has shifted from electrical to physical properties. To meet this requirement change, the Westinghouse Thermalastic Epoxy insulation system was developed, in which the previously used polyester resins were replaced with epoxy resins. The improved physical properties of this new resin system resulted in a mechanically stronger coil insulation. The insulation was also found to be able to better withstand thermal cycling forces, reflecting both its improved mechanical and thermal characteristics. In addition, its superior resistance to water diffusion ïndicated a great safety margin in the unlikely case of internal leaks in water-cooled windings.

Water Immersion Testing of Form-Wound Coil Insulation

Motors are today required to operate in more severe environments than formerly. This paper reports a study of suitable test methods for evaluating the ability of insulation systems to withstand moisture exposure as thermal aging progresses. A new motorette exposure cycle has been recommended to the IEEE Working Group on Sealed Insulation Systems of the Rotating Machinery Committee.

High voltage generator insulation

The insulation of high voltage stator windings for electric utility generators must have a very high order of reliability. Machines of this category have always been of extreme importance because of their size with the accompanying high capital investment and the dependence of so many things in our society upon their continuous and uninterrupted operation. Lately, the size of these machines, particularly steam driven turbine generators, has increased tremendously, so that units are now being produced with ratings of 500 MVA and more on single shafts. Waterwheel generators are approaching a size of 300 MVA. These sizes will continue to increase in the future and it is almost a certainty that turbine generators will soon approach 1000 MVA, possibly within 5 years. This means that extremely large blocks of power are dependent upon the insulation. Figure 1 is a view in a winding aisle showing parts of a typical waterwheel generator and a turbine generator of the types employed by electric utilities.

Evaluation of AC Motor Insualtion Systems by IEEE 275

Two decades of experience at Westinghouse Large Rotating Apparatus Division evaluating motor insulation systems according to IEEE 275 and its precursor standards show good correlation between service experience on the systems tested and the conclusions drawn from the procedure. Despite its drawbacks and abuses, the test procedure has proved itself a practical and very important technique for thermal classification of insulation systems.

Further Experience with Form-Wound Motorettes

Nineteen insulation systems for form-wound motor coils were tested according to IEEE No. 275, and were successfully designated Class A, B, F, or H. Systems evaluated included mica splittings, mica papers, and non-micaceous materials in combination with resinous treatments of various kinds. It was found that not only materials, but processing procedures, have a great effect upon the thermal capability of an insulation system. Statistical techniques were used to analyze data, and it was found that excessive extrapolation can be misleading. The data presented confirm the desirability of testing at a minimum of three temperatures, one of which is within 20°C of the anticipated hot spot temperature rating. It is evident that micaceous products can be built into all temperature classes. However, the system classification of mica containing systems is dependent upon the resinous binders and processing techniques in the same manner as the non-micaceous systems.