| Summary: | 碩士 === 國立臺北科技大學 === 電機工程系電力電子產業碩士專班 === 106 === Induction heating can be used in many different fields and the equipment for cooking food is called induction cooker. The induction cooker provides both the higher efficiency and more safety compared with the traditional heating methods. The objective of this thesis is focused on the design of induction cooker for the non-ferromagnetic material pan including the power converters and heating coil. Since the non-ferromagnetic pan is heated by eddy current which leads to low equivalent heating resistance, the pan needs larger current to fulfil enough power for heating. Moreover, there exists inherent problems such as low efficiency and that electric parameters between pan and heating coil are not easy to measure. Hence, the simulation software Ansys Maxwell 3D is adopted to assist design of heating coil for resonant circuit and a copper pan is used for design and test. To start with, the equivalent heating resistance of copper pan is estimated by Maxwell 3D. Then the optimal turns of heating coil is selected according to the heating power and efficiency. In the end, the equivalent inductance including heating coil and pan, which puts in heating coil, are estimated. In order to reduce the loss of heating coil caused by skin effect, the heating coil is made up of 0.05 mm/4800 strands Litz wire and the relation between resistance and frequency of coil is measured by impedance analyzer. Furthermore, the optimal resonant frequency of the converter is determined by maximizing heating power and heating efficiency. The electric parameters are obtained through above methods to construct the simulation environment. Hence, the characteristics of the resonant circuit are analyzed which can be used for the design of control strategy.
The proposed converter is cascaded by three stages, which are full-wave rectifier, Buck converter and half-bridge resonant converter. Due to the low heating equivalent resistance and high quality factor(Q) of the copper pan, it exists high slope ratio between resonant current and frequency. Hence the resonant circuit is operated with fixed switching frequency to achieve stable resonant current. Then the amplitude of resonant current flowing can be controlled by adjusting dc link voltage, which can prevent the noise caused by insufficient resolution of switching frequency. Moreover, the thesis proposed a resonant frequency estimated method to detect currently resonant frequency for different materials of pan, pan size and the position of pan on heating coil. Thereby, that suitable switching frequency for heating pan can be selected. In addition, the proposed method can be used for moving detection of pan during heating to reduce the safety concern. In order to control the heating power, this thesis also proposed a power control strategy with an estimated heating power as feedback signal, which is calculated by buck converter output power, resonant tank loss and resonant peak current.
Finally, a DSP-based controller TMS320F28075 is used to fulfil a half-bridge series resonant converter with 100Arms output current. Moreover, the heating coil is 15 turns which are made up of equivalent #4 AWG, 4800-stranded #AWG 44 Litz wire. Then, a copper pan is tested and the heating power is measured by Calorie meter according to the regulations of EN 60350-2:2013 and ASTM F1521-12 at 101kHz and 189kHz operating frequency. The measured results show that operating frequency do influence heating efficiency and the heating efficiency is better under 101kHz test condition (60.3%, 520W). In addition, the measured results by Calorie meter and impedance analyzer are compared with simulated results by Maxwell 3D, which show good match in values of resonant inductance and equivalent heating resistance for the induction cooker.
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