DAWNCAP Tianming Special Capacitor for Electromagnetic Heating
Thin film capacitors are widely used in various household appliances/industrial equipment/military products due to their high voltage resistance, high ripple current, small stray inductance, low equivalent series resistance and long service life. The electromagnetic heating equipment is used frequently in our work and life. For example, household induction cooker/electromagnetic tea cooker, commercial induction cooker, high frequency quenching machine, sealing machine, industrial smelting furnace and so on. This paper takes three-phase high-power commercial electromagnetic cooker as an example to study how thin film capacitors are applied to electromagnetic heating equipment. 1 Business electromagnetic cooker three-phase full bridge circuit topology Fig. 2 C1-C6 Functional description C1/C
Thin film capacitors are widely used in various household appliances/industrial equipment/military products due to their high voltage resistance, high ripple current, small stray inductance, low equivalent series resistance and long service life. The electromagnetic heating equipment is used frequently in our work and life. For example, household induction cooker/electromagnetic tea cooker, commercial induction cooker, high frequency quenching machine, sealing machine, industrial smelting furnace and so on. This paper takes three-phase high-power commercial electromagnetic cooker as an example to study how thin film capacitors are applied to electromagnetic heating equipment.
Topology diagram of three-phase full-bridge circuit for commercial electromagnetic cooker
Functional description of 2C1-C6
C1/C2: Three-phase AC input filter and ripple absorption to improve the anti-interference ability of equipment
C1, C2 and three-phase common-mode inductors constitute Pi filter, which plays the role of electromagnetic interference suppression and absorption in the equipment. On the one hand, the circuit suppresses the electromagnetic interference caused by IGBT high-speed switch, which is transmitted to the three-phase power frequency network through the power line, affecting the normal use of other grid-connected devices. On the other hand, it prevents the electromagnetic interference signal generated by other equipment in the same power grid from being transmitted to the three-phase power frequency network through the power line, which affects the normal use of the electromagnetic heating equipment itself. EMC = EMI + EMS
In practical use, C1 can choose MKP-X2 (fixed capacitor for suppressing electromagnetic interference). Its capacity range is between 3?F-10?F, and its rated voltage is 275V.AC-300V.AC. It adopts Y-type connection, and the public end is hanging ungrounded. C2 can choose MKP metallized film capacitor with capacity ranging from 3?F-10?F and rated voltage 450V.AC-500V.AC. Triangular connection is adopted.
The larger the capacity of C1 and C2 in principle, the better the suppression and absorption effect of EMI. But the larger the capacitance, the greater the reactive current when the equipment is standby. In terms of withstanding voltage, it is necessary to reserve a reasonable margin according to the power grid situation in the area where the equipment is used, so as to prevent the voltage breakdown or life of capacitors from being affected by the overvoltage of the power grid when the power consumption at night is very small.
C3: Smooth filtering after rectification, DC-Link, absorbing ripple and completing circuit of AC component.
The LC circuit is composed of C3 and choke L. The pulsating DC current after three-phase bridge rectification is transformed into smooth DC current for the use of the back-stage inverter bridge and load. In the practical circuit of commercial electromagnetic cooker core, C3 is generally composed of dozens of micro-thin film capacitors. In fact, the function of the thin film capacitor in this position is DC-LINK, which is responsible for the absorption of ripple and the circuit of AC component, rather than what many people think (filtering). For tens of kilowatt loads, the capacitance of tens of micrometers plays a very small filtering role, and the voltage waveform of DC bus can not be very smooth at all. Because of the high-speed switch of IGBT, a large number of high-order harmonic currents and peak harmonic voltages will be generated. If there is no capacitor to absorb harmonic current and peak voltage, the DC bus circuit will produce a large number of self-excited oscillations, which will affect the safe use of IGBT and shorten the life time. Therefore, the use of thin film capacitors as the absorption of ripple voltage and ripple current of DC buses is one of the most commonly used methods at home and abroad.
In principle, the larger the capacity of C3, the better the absorption effect. However, it should be noted that the excessive capacitance can easily lead to overcurrent breakdown of rectifier bridge and fuse when the equipment is power-on just after closing, because the instantaneous charging current of capacitor is too large. In the core of commercial electromagnetic cooker, the general selection principle is: half bridge scheme (1.5? F / KW) full bridge scheme (1.2U/ KW). The configuration is inferred from the design process of 2A/? F that conventional thin film capacitors can withstand.
For example, for commercial electromagnetic cooker half-bridge 20KW model, the required C3 capacity is 20*1.5=30? The total ripple current of F C3 is 30*2=60A full-bridge 20KW model. The required C3 capacity is 20*1.2=24? F (actually 25-30?F). The total ripple current of C3 is 25*2=50A. The recommended actual selected capacity and the allowable ripple current of capacitor can not be lower than the above recommended value.
The C3 position must consider whether the actual ripple current value needed by the circuit is less than the total ripple current value that the selected thin film capacitor can withstand (and retain a certain current margin). Otherwise, if the circuit needs 60A ripple current and the selected capacitor can withstand only 40A total ripple current, it will lead to serious heating of the thin film capacitor, long-term overheating operation, greatly. The service life of thin film capacitors is reduced, which leads to the expansion and bulging of thin film capacitors and even the ignition and combustion of thin film capacitors. In terms of withstanding voltage, the rated voltage is usually 800-1000V.DC.
C4: Peak Voltage/Current Absorption, Buffering and Suppression of IGBT to Prevent IGBT Breakdown
As the peak absorption of IGBT on/off, C4 is usually connected to IGBT by C-type or RC-type connection.
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