Design of Rotating Electrical Machines, 2nd Edition

8

Insulation of Electrical Machines

Here, an insulator refers to a nonconducting material or an insulating material with a very low conductivity. An insulation system comprises insulating materials and insulation distances. The main function of insulation is to separate components of different electric potentials or of different electric circuits. Further, insulators improve the strength of the winding structures; they also have to act as heat conductors between the winding and the surroundings, and they have to protect the winding from external stresses such as dirt, moisture and chemicals.

In electrical machines, there are typically insulation distances of three types. First, a clearance in air is simply an air gap between the objects, the insulating strength of which is determined by the distance between the objects, their shape and the state of the air. A clearance in air can be defined by either insulated or noninsulated surfaces. In certain cases, the medium of the insulation distance, which also simultaneously acts as a coolant, can be a gas other than air. In the case of a homogeneous electric field, the breakdown voltage depends on the width of the air gap according to the Paschen curve, Figure 8.1.

Figure 8.1 Paschen curve illustrating the voltage strength of an insulation distance formed from a homogeneous air gap at a pressure of 101 kPa. l is the gap spacing between the electrodes.

The Paschen law essentially states that the breakdown characteristics of a gap are ...