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What are the electrical properties of insulating materials?

2019-05-27

The electrical properties of silicone waterproof sealing ring have dielectric coefficient, dielectric loss, breakdown strength and insulation resistivity, etc., are very important for the reliability and service life of motor winding operation. The basic requirements of insulating material performance include electrical performance, heat resistance and mechanical performance.


1, dielectric coefficient of insulating material


The relative dielectric coefficient of insulating material represents the movement of electric charge in insulating material under the action of electric field, that is, the degree of polarization. Generally, with the increase of electric field frequency, it decreases step by step and increases with the moisture absorption of the material; Because temperature affects polarization, there is a peak at a certain temperature.


2, dielectric loss of insulation materials


Under the action of electric field, insulation material will produce energy loss due to leakage and polarization. Generally, the loss power or loss Angle tangent is used to express the size of medium loss. Under the action of dc voltage, transient charging current, absorption current and leakage conduction current will be passed. When the AC voltage is applied, the instantaneous charging current is reactive current. Leakage conductive current is active current in phase with voltage. The absorbed current has both reactive and active components.


3, insulation breakdown strength


3.1 Thermal breakdown. Under the action of alternating electric field, heat is generated inside insulating material due to dielectric loss. If it cannot be dispersed in time, the internal temperature of the material will rise, resulting in the destruction of molecular structure and breakdown, which is called thermal breakdown. The thermal breakdown voltage decreases with the increase of the temperature of the surrounding medium, the thickness of the material increases, the heat dissipation condition becomes worse, and the breakdown strength decreases. As the frequency increases, the dielectric loss increases and the breakdown strength decreases.


3.2 Electrical breakdown. Under the action of strong electric field, the charged particles in the insulation move violently, collide and dissociate, destroy the molecular structure, so that the final breakdown, known as electrical breakdown. The electric breakdown voltage increases linearly with the thickness of the material. In a uniform electric field, the electric breakdown strength is generally independent of the time of voltage action unless the time of impulse voltage is less than 10 seconds.


3.3 Discharge breakdown. Under the action of strong electric field, the bubbles contained in the insulating material will discharge due to ionization. Impurities are also heated and vaporized by the electric field, producing bubbles, so that the bubble discharge is further developed, leading to the breakdown of the whole material, known as discharge breakdown.


The breakdown of insulation materials is often the existence of the above three forms at the same time, it is difficult to separate. Impregnating insulating materials with insulating paint or glue solution can improve the electric field distribution and increase the electric breakdown strength, and improve the heat dissipation condition and increase the thermal breakdown strength.


4, insulation resistivity


Insulation materials under the action of voltage, there will always be small leakage conductive flow. This current flows partly through the material and partly through its surface. Therefore, insulation resistivity can be divided into volume resistivity and surface resistivity.


In addition to the electrical properties of insulating materials introduced above, there are corona resistance, arc resistance, leakage traces and other electrical properties. Breakdown electric field strength and insulation resistance are the most important requirements for electrical performance of insulating materials. According to different motor types, other electrical performance requirements are not exactly the same.


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