Reburned magnesite What is the relationship between the operating temperature of components and the insulation resistance value?
For almost all insulating materials, the higher the temperature is, the smaller the insulation resistance value will be. The insulation performance of magnesium oxide powder conforms to this rule.
What effect does the minimum insulation resistance value have on the overall design?
During product design, we should make clear the minimum value of insulation resistance of components at the highest operating temperature, and whether it can meet the requirements of appliances for leakage current.
Give an example to illustrate:
We conducted the following tests (using some inorganic powder, the test voltage is the rated voltage): a heating element: 220V/500W, under normal conditions, the maximum surface temperature is 770~780 ℃, and the insulation resistance is 5m Ω
Leakage current:==0.000044A=0.044mA
When the electric heating tube is used alone, the leakage current is safe and reliable.
However, as a part of the whole machine product, such as the superheated air duct, according to the technical requirements of the whole machine product, the total power is not less than 50kW, and its thermal leakage current I ≤ 5mA.
When the above electric heating elements are used, as the hot leakage current is 0.044 mA, the maximum number of electric heating elements for the whole machine is 5 mA/(0.044 mA/piece)=113 pieces.
For three-phase balance, the number of branches per phase is 113/3 ≈ 37, and the total power of the system is about 500W/branch × 37 × 3=55.5W, which can meet the product requirements.
However, the above design method is only theoretical analysis, and its safety reliability coefficient is only 1. In fact, there is no safety margin, and there is only reference value in the design. Considering the actual situation, the safety factor should be at least 1.5, that is, the total power should not exceed: W=55.5kW/1.5 ≈ 37kW. Then, the previous data cannot meet the performance requirements of the whole machine, and effective measures must be taken to increase the minimum insulation resistance (such as using better magnesia powder, adding other insulation measures to the heating element during the design of the whole machine, to improve the overall insulation resistance, increase the maximum number of heating elements available for the whole machine, and meet the power requirements of the whole machine).
