Contacted Heat Transfer System Of Piezoelectric Transformers

Piezoelectric transformers, which adjust the state of an electric power supply through piezoelectric direct and converse effects and vibration propagation, have potential to meet the requirements of next-generation transformers because of their desirable advantages of miniaturization, high power density, flexible transforming ratio, electromagnetic immunity, incombustibility, and good isolation. In theory, power density can achieve as high as400W/cm3. However, the actual maximum output power density is typically less than30W/cm3for current designs with piezoceramic materials. The main reason for such large decrease of performance is the inevitable increase of their working temperature generated from dielectric, piezoelectric, and mechanical losses. For efficient and reliable operation, it is very important to keep a proper working temperature for the piezoelectric transformers. The conventional piezoelectric transformers were fixed by node support way, which only depend on convection with air for cooling. Therefore, piezoelectric transformers owned shortcomings of low thermal efficiency, poor stability, low power density, which greatly restricted the popularity of the piezoelectric transformer. In the thesis, an innovative design for the piezoelectric transformers heat transfer system(HTS) to fill the blank of a piezoelectric transformer cooling device, which was analyzed in-depth theory and taken experimental studies. In addition, the cooling methods also have potential to other piezoelectric materials application. Main work and conclusions as follow:According to the piezoelectric effect and the linear piezoelectric equations of the piezoelectric physical basis, a circular piezoelectric transformer was analyzed through equivalent circuit theoretical analysis for basic parameters. For rectangular extension vibration mode of piezoelectric transformer, three-dimensional finite element analysis was used to calculate the ideal case, with damping and with damping&load situations separately. Displacement distribution, input admittance, output voltage ratio and stress distribution were got from ANSYS. The piezoelectric material loss is generated by the hysteresis. We had the reason analysis of hot produced and found its calculation method. At the same time, we have made experimental results compare to the simulation result for confirming the simulation reliability and accuracy; We concluded the calculation method of dielectric loss, piezoelectric loss and mechanical loss. According to the characteristics of piezoelectric transformers, we obtained loss calculation formula for analytical method and finite element method separately.For piezoelectric transformer in a resonant state, the design of heat transfer structure is the most critical core components. We designed the different heat transfer structure for different vibration mode piezoelectric transformer type. In order to more clearly understand the role of the heat transfer system, the lumped parameter model is used to analyze heat influence into piezoelectric transformer theoretically. Three-dimensional finite element method offered a detailed analysis of the temperature distribution of a single piezoelectric transformer temperature, thermal gradient, thermal stress. We measured temperature distribution by experimental to verify accuracy of the analysis of the temperature; We analyzed and compared the ideal situation and with the thermal contact resistance of heat coupling component on the piezoelectric transformer.In order to verify the actual effect of the heat transfer system, the surface contacted heat transfer system was designed to test the characteristics of piezoelectric transformer. The contour vibration’s thermal structure of the rectangular piezoelectric transformer was selected as prototype. The experimental test circuit was set up. Comparison and discussion between the prototype with and without the heat transfer system are implemented by comparing the input admittance curve, no-load and matching the impedance of the output voltage ratio, input and output power density, efficiency, temperature, and thermal conductivity. The experiment showed5.5times improvement of heat dissipation ability of the PT with HTS, the output power density increases more than2times (from41W/cm3to135W/cm3), with a temperature rise less than10℃and efficiency greater than90%. The results verified that the heat transfer system suppressed the temperature rise excellently and improved power density extraordinary. We also tested a separate contact thermal resistance of the piezoelectric transformer and heat couple component. Under several working hours, the wear of the electrode surface was recorded, then we found the usual decrease wear method in mechanical system, which can be used for future optimization.In order to reduce the wear influence and improve characteristic of piezoelectric transformer, a layer of polypropylene membrane was glued on thermal coupling component and a simple optimization of thermal structure was taken. The experimental tested the impact on the efficiency of the thickness of the membrane. The thickness of polypropylene film layer was chosen as58μm. Up to110days of cumulative work, the testing result had shown the optimized wear ratio had decreased to approximately1%, with the efficiency and output power stability. The result certificated the optimize structure was suitable for practical applications.