Electrical performance requirements for motor insulation
The reliability and service life of the winding operation depend to a large extent on the properties of the insulating material. The basic requirements for the performance of insulating materials include electrical properties, heat resistance and mechanical properties. The electrical properties of insulating materials are briefly described in this paper. Electrical properties of insulating materials include breakdown strength, insulation resistivity, dielectric constant, and dielectric loss.
1. Insulation material breakdown strength
The thickness of the breakdown at the insulation material is divided by the breakdown voltage, expressed in kilovolts per millimeter. The breakdown of insulating materials can be roughly divided into three types: electrical breakdown, thermal breakdown and discharge breakdown.
● 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 action time.
● 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 conditions become worse, and the breakdown strength decreases. When the frequency increases, the dielectric loss increases and the breakdown strength also decreases.
● 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 electrical breakdown strength, and can also improve the heat dissipation conditions to improve the thermal breakdown strength.
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