30 May How Energy Efficient is Induction Cooking?
In order to improve the energy efficient induction cooking, the principle of the main circuit of the induction cooker is analyzed, and the main circuit parameters affecting the energy efficiency are analyzed and tested. Through the IGBT conduction waveform analysis and energy efficiency test, the optimized circuit parameters are obtained. The experimental data show that the energy efficiency of induction cooker can be greatly improved by optimizing the circuit parameters of the main circuit.
The usage of induction cooktops in the country is very large. In order to improve the energy efficiency of induction cookers, guide the progress of energy-saving technology of production enterprises, and encourage enterprises to produce high-efficiency and energy-saving products, the state made the energy efficiency requirements and standards of induction cookers [1-2] . In September 2008, the first edition of “Limited Values and Energy Efficiency Grades of Household Induction Cookers” (GB 21456-2008) was implemented, and in January 2015, the “Limited Values and Energy Efficiency Grades of Household Induction Cookers” (GB 21456-2014 ) was revised. In the national compulsory certification standard, the energy efficiency of induction cooktops is also a mandatory requirement, and the research on improving the energy utilization efficiency of induction cooktops will be in accordance with “Made in China 2025” strategy on low carbonization.
Currently, household induction cooktop products are low-power electromagnetic heating systems, most of which are below 2200W, and their energy efficiency is generally three-level (thermal efficiency 86% to 88%), and the second-level energy efficiency (thermal efficiency above 88%) on the market is very few. Most of the methods for improving energy efficiency of induction cooktop manufacturers focus on the coil plate, and there are many factors affecting the energy efficiency of the induction cooker, which are not only determined by a certain component. If the research direction is limited to the coil plate, it is often difficult to have huge improvement on energy efficient induction cooking.
In this paper, the system engineering concept is used to study the measures to improve the thermal efficiency of the induction cooker. From the structure of the coil plate, the structure of the coil is improved to avoid problems such as magnetic leakage and excessive temperature concentration, and the circuit principle parameters are optimized to avoid the IGBT waveform discount and the back pressure state. Thereby reducing the internal consumption of the circuit to improve the thermal efficiency [1-2]. Finally, the best state of the thermal efficiency of the induction cooktop is obtained by the method of the whole machine test.
1 Main Circuit Analysis
The main circuit of the induction cooker is composed of the resonance unit, the synchronous oscillation control unit, the IGBT high voltage protection unit and the current sampling unit [3-4], as shown in Figure 1. Among them, the main factors that affect the thermal efficiency of the induction cooker are the resonance unit, the synchronous oscillation control unit, and the IGBT high-voltage protection unit, the main circuit composed of these 3 circuits.
Determine the resonant waveform of the spool. If the resonant waveform is discounted, the back pressure is limited, etc., it will affect the thermal efficiency.
The parameters of the control circuit will affect the heating of the components; especially the circuit parameters of the main circuit, such as the parameters of the choke coil, capacitor, bridge stack and the IGBT of the main working circuit will affect the heating of the components. For different capacitance parameters, the parameters of the resonant circuit composed of the key components of the main circuit of the induction cooker change, resulting in the change of the working waveform, and it cannot work under the normal waveform. If the synchronous circuit cannot ensure that the IGBT drive circuit conducts at the zero point of the resonance waveform. The IGBT drive waveform will be discounted, and the discount of the drive waveform will reduce the working thermal efficiency or the power of the induction cooker.
Figure 1. Schematic diagram of the main circuit of the induction cooker
2 Optimization of Key Component Parameters
Using the concept of system engineering, the circuit composition parameters of the induction cooker, the structure of the coil plate and the radiator, and the temperature rise of the components are systematically analyzed, and the factors that may affect the thermal efficiency of the induction cooker are improved. The whole machine test is verified, and the improvement scheme to improve the thermal efficiency is obtained [5-7]. Circuit parameters determining is mainly to determine the inductance of the coil, the change of the working inductance (load change) and the fluctuation of the grid voltage will change the size of the resonance frequency and the size of the power.
2.1 Confirm the Inductance of the Reel
The inductance L of the reel is determined according to the voltage and power values to be realized by the product. When working, the maximum back-voltage value of the resonant tank cannot exceed the maximum voltage value that the IGBT can withstand. When the required power increases, the back pressure generated by the resonance increases. To reduce the back pressure, the required coil inductance L should be reduced. When the required power decreases, the back pressure generated by the resonance decreases. At this time, the non-fume cooker and the double bottom cooker such as 304 are not limited by the back pressure protection, and it is easy to generate large power, resulting in a high temperature rise. To increase the back pressure, the required coil inductance L increases.
2.2 Determining the Number of Turns of the Reel
As long as the inductance, the type of magnetic stripe and the number of magnetic strips are determined, the number of turns of the reel can be determined [5-6]. At present, the most common number of magnetic strips used in the industry is 6 to 8. If the number of magnetic strips is too small, the phenomenon of magnetic leakage of the coil will easily occur, and too many magnetic strips will directly lead to an increase in cost. The types of magnetic strips are now common magnetic strips (high-density magnetic strips cost a lot), and the wire reel is 8 magnetic strips, because there is no way for 6 magnetic strips to meet the inductance requirements, which will affect the thermal efficiency.
2.3 Determining the Number of Strands in the Reel
According to the maximum current value during operation (I=P/ U), the reel can be preliminarily judged whether the number of shares needs to be increased over other line reels. After the preliminary parameters of the wire reel are confirmed, make a sample of the wire reel, and test it in combination with the height of the base column, the base air duct and the circuit board, then make adjustments.
2.4 IGBT Back-Pressure Control
What a direct impact has on the induction cooker frying IGBT is the back-pressure generated by resonance, which is not directly related to the inductance (basic inductance and working inductance), resonant frequency changes and power fluctuations. Changes in inductance, frequency and power will suddenly increase the resonant back pressure, thereby burning the IGBT. If the IGBT waveform is discounted, as shown in Figure 2, that is, the IGBT is not turned on at the zero point, and a hard turn-on occurs. At this time, IGBT breakdown may occur, which will also affect the thermal efficiency of the induction cooker or the energy efficient induction cooking.
Figure 1. The IGBT waveform appears to be discounted
After repeated adjustment, the optimized parameters are determined, namely wire diameter × strands × turns = 0.37mm × 28 strands × 28 turns, the number of magnetic strips is 8, and the inductance L= (100±5) μH.
3 Experimental methods and results
3.1 Experimental methods
The test is carried out according to the test standard of “Limited Values and Energy Efficiency Levels of Household Induction Cookers” (GB 21456-2014). The standard test pot (see Appendix A in the standard) is used, and the test conditions, test instruments and test methods for heating efficiency are carried out in accordance with the requirements of the standard [5-7]. The standby power is statistically calculated by the average power method, as shown in formula (1).
In formula (1), P is the average power, the unit is W, accurate to 0.1; E is the total power consumption, the unit is W·h, and t is the heating duration, the unit is h.
3.2 Analysis of Working Waveforms
Record the driving waveform state and resonant waveform when the maximum working power (2000W) is working, as shown in Figure 3(a) and Figure 3(b) respectively. There is no discount in this waveform, that is, the IGBT does not turn on at a non-zero point. At this time, the temperature rise of the IGBT will not be abnormally high, which improves the previous IGBT discount problem.
(a) Drive waveform
(b) Resonant waveform
3.3 Analysis of results
Comparing the thermal efficiency before and after optimizing the parameters, there is no discount in the waveform diagram. Due to the use of ordinary magnetic strips and the reduction of the number of strands, the cost has decreased. Through the optimization of circuit parameters, the thermal efficiency is improved, as shown in Table 1, which is consistent with the analysis results of the working waveforms.
In order to improve energy efficient induction cooking, the parameters and structure of the coil are carried out. There is improvement after the energy efficiency test of the optimized IGBT has been carried out. The energy efficiency of the now discounted waveform graph has improved by nearly 1%. After optimization, the inductance of the reel remains unchanged, changing the material of the magnetic strip and the number of strands of the reel, you can reduce costs while maintaining quality.