The electrical properties of silicone waterproof seals are dielectric constant, dielectric loss, breakdown strength and insulation resistivity. They are important for the reliability and service life of motor windings. The basic requirements for insulation properties include electrical performance. Heat resistance and mechanical properties.
1. 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, the frequency decreases as the electric field frequency increases, and increases as the material absorbs moisture; due to the temperature affecting the polarization, a peak occurs at a certain temperature.
2. Dielectric loss of insulating materials
Under the action of an electric field, the insulating material generates energy loss due to leakage and polarization. The loss power or loss tangent is generally used to indicate the dielectric loss. Under the action of the DC voltage, the instantaneous charging current, the sinking current and the leakage current flow will pass. When an alternating voltage is applied, the instantaneous charging current is a reactive current; the leakage current is in phase with the voltage and is an active current; and the sinking current has both a reactive current component and an active current component.
3, insulation material breakdown strength
3.1 Thermal breakdown. Under the action of alternating electric field, heat is generated inside the insulating material due to dielectric loss. If it cannot be dissipated in time, the internal temperature of the material will rise, causing breakdown of the molecular structure and breakdown, which is called thermal breakdown. The thermal breakdown voltage decreases as the temperature of the surrounding medium increases, the thickness of the material increases, the heat dissipation condition becomes worse, and the breakdown strength decreases. When the frequency increases, the dielectric loss increases and the breakdown strength also decreases.
3.2 Electrical breakdown. Under the action of strong electric field, the charged particles inside the insulation move vigorously, and the collision is free, destroying the molecular structure, so that the final breakdown is called electric breakdown. The electrical breakdown voltage increases linearly with the thickness of the material. In a uniform electric field, unless the impulse voltage is shorter than 10 seconds, the electrical breakdown strength is generally independent of the voltage application time.
3.3 discharge breakdown. Under the action of a strong electric field, the bubbles contained in the insulating material are discharged by ionization; the impurities are also vaporized by the electric field to generate bubbles, so that the bubble discharge is further developed, resulting in breakdown of the entire material, which is called discharge breakdown.
The breakdown of insulating materials is often the same as the above three forms, and it is difficult to separate them. Impregnating the insulating material with insulating varnish or glue can improve the electric field distribution and increase the electric breakdown strength, and can also improve the heat dissipation condition to improve the thermal breakdown strength.
4, insulation resistivity
Under the action of voltage, there is always a small leakage current flowing through the insulating material. This current flows partially through the interior of the material and a portion flows through the surface of the material. Therefore, the insulation resistivity can be classified into volume resistivity and surface resistivity.
In addition to the electrical properties of the insulation materials described above, there are electrical properties such as corona resistance, arc resistance, and leakage resistance. The electrical performance requirements of the motor for the insulating material are the most important for breakdown electric field strength and insulation resistance. Depending on the type of motor, other electrical performance requirements are not exactly the same.