Applications of Rotating Machines are normally divided into two categories: those with voltage ratings less than 6,600 V are called low voltage machines, and the others are high voltage machines. Because of the difficulty of insulating high voltages, machines above 22 kV rating are not built except under special conditions. Classes Y and C insulation find no Applications of Rotating Machines. Class E which was widely used in low voltage machines for over 20 years is now being replaced by class F which is meant for the high voltage machines. Also, class F is being increasingly used in place of class B. Thus class F appears to be the insulation of the future. Considerable progress has been made in recent years, in reducing the size of the machines for a given rating by use of class H materials, particularly, for small machines. However, the cost of class H materials (silicones, teflon) is very high, and hence they are used only under special conditions like severe overloads in traction motors and mill motors.

Mica has been used in the electrical industry since its inception. Normally, mica is available in the form of very thin splittings. Hence it is bound to a supporting sheet of electrical grade paper or glass cloth with a suitable binding agent. The resulting mica sheets are known as micanite. Since mica splittings of fairly large surface area were not available, methods were evolved to make mica paper using mica of any size. The mica paper so obtained is not sufficiently strong or self-supporting. Hence, it has to be given a backing of glass cloth or other binding material such as epoxy resin. Epoxy resin bounded mica paper is extensively used in both low and high voltage machines. In modern insulation practice, there are various requirements of the insulating material for use in electric generators and motors. The selection of the right material depends on the power rating of the motor and the conditions under which it operates. Today, multilayer insulation is made of pressboard and polyester film or Nomex and polyester film, or non-woven polyester and polyester film, with or without impregnation. These are being used for slot insulation. Multi-layers made of Nomex or Kapton polymide film are used for very high temperature applications.

In the case of generators, a particularly important parameter is the insulation of rotors and stators and its reliability. Individual manufacturers have developed their own insulation systems and changes, if any, the insulation systems are done very carefully. For many decades rotors have been insulated with single layer glass or hard fabrics or with glass fabrics and/or Nomex. These are also used as winding insulation as well as slot insulation. The main insulation of the stators have always been mica based. In addition, conducting non-woven polyesters are used for corona protection both inside and at the edges. Particularly important parameter of an insulating system is the support between the winding bars, slots and the core laminations. Glass fiber reinforced epoxy wedge profiles which allows long service life are used for this purpose. The maintenance of good mechanical properties is also equally important for the reliable operation of machines. The insulation should withstand the expansion and contraction during temperature cycles in large machines. These effects become very severe at the high temperatures observed in power generators of a very large size. Maintenance of good mechanical properties and thermal endurance are very essential in low voltage machines also.