Motor insulation material performance requirements

Motor insulation material performance requirements

2, insulation resistivity

Under the action of voltage, there is always a small leakage current flowing through the insulating material. A portion of this current flows through the interior of the material; a portion flows through the surface of the material. Therefore, the insulation resistivity can be classified into volume resistivity and surface resistivity. The volume resistivity characterizes the internal electrical conductivity of the material in ohm-meters; the surface resistivity characterizes the conductance characteristics of the material surface in ohms. The volume resistivity of the insulating material is usually in the range of 107 to 1019 m•m. The electrical resistivity of insulating materials is generally related to the following factors.

● As the temperature increases, the resistivity decreases exponentially.

● Water can promote the dissociation of polar molecules, so the insulation resistivity decreases with increasing humidity, and it is more sensitive to porous materials (such as insulating paper). A hydrophilic substance such as a polar material easily forms a continuous water layer on the surface to lower the surface resistance; a non-polar material such as ceramics, polytetrafluoroethylene or the like does not easily form a continuous water layer on the surface, and thus has little influence on the surface resistance thereof.

● The impurities in the insulating material mostly produce conductive ions, which can promote the dissociation of polar molecules and cause the resistivity to drop rapidly.

● Under the action of high electric field strength, the migration force of ions increases, and thus the resistivity decreases.

3. Dielectric coefficient of insulating material

The relative dielectric constant of the insulating material indicates the state in which the internal charge of the insulating material moves under the action of an electric field, that is, the degree of polarization. Generally, it decreases step by step as the frequency of the electric field increases; it increases as the material absorbs moisture; and because of the influence of temperature on polarization, a peak occurs at a certain temperature.

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