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I. Introduction
With the continuous development of power electronics technology, frequency control technology has become more and more mature, and general-purpose inverters have been rapidly developed. Various brands of inverters have been widely used in various fields of automation. However, in some places where explosive gas and dust are relatively large (coal, coking, partial chemical plants), frequency converters have not been fully used. The main reason for this is that the inverters in these places need to be explosion-proof, and such converters are now There are no mature products on the market yet. Based on the above, Hualong Explosion-proof Electric Co., Ltd. specially developed a new type of explosion-proof inverter for use by the required manufacturers.
Second, the technical requirements of the inverter
1, the working environment has explosive gas or dust more environment (explosion-proof);
2, the input voltage AC 660V ± 10%;
3, speed range 2 - 50HZ;
4, the protection function has a short circuit, over current, over voltage, under voltage, over temperature, power lockout and other functions;
5, working methods 24 hours continuous work system;
6, other technical indicators and common inverter.
Third, the inverter control principle
1, the main loop part
Main circuit of frequency converter includes air circuit breaker, three-phase rectifier, filter capacitor, IGBT inverter
Inverter main circuit electrical block diagram
Figure 1: Main circuit electrical block diagram
2. Control loop section
The inverter control part includes a CPU controller, an IGBT drive circuit, a current detection circuit, a voltage detection circuit, a protection circuit, and an operation display circuit. The control block diagram is as follows:
Inverter control schematic
Figure 2: Control Block Diagram
3, the main components used in the inverter are selected world-renowned company's brand-name products, excellent performance and reliable quality.
(1) The IGBT module of Germany's Siemens Company is selected as the inverter. The saturation voltage of this module will be low, the busbar inductance is small, the switching frequency is high, the structure is simple, and the installation is convenient.
(2) The CPU controller adopts the 87C196MC MCU from Intel Corporation of the United States. This chip has complete functions and fast operation speed. It is a high-performance CHMOS 16-bit MCU. It has low power consumption and can work in two kinds of sections in addition to normal work. Electrical mode: standby mode and power-down mode. It has a waveform generator inside it that can output 2 sets of complementary 3-phase PWM signals, especially for motor control systems.
(3) The drive circuit adopts the M57959L dedicated drive module from Mitsubishi, Japan. The module has large drive power, strong signal isolation, high degree of integration, excellent performance, and easy to use.
Fourth, explosion-proof inverter development problems and countermeasures
1, explosion-proof problem
Because the environment used by this inverter has more explosive gas or dust, this requires the inverter to be sealed and explosion proof, so its shell can not use the ordinary shell, it must use the standard explosion-proof cavity, and install all the components of the inverter. In the explosion-proof cavity. Open an observation window on the explosion-proof chamber door, install the display part on it, and install start, stop and speed control on the explosion-proof chamber door.
2, heat problem
Because all the components of the inverter are installed in the explosion-proof cavity, the air cannot flow, and the heat dissipation problem becomes the key problem that the inverter must solve. Here we use a new cooling technology - heat pipe cooling technology.
(1) Principle of heat pipe technology
The heat pipe is a heat transfer element with extremely high thermal conductivity. It transfers heat through the evaporation and condensation of the working medium in a totally enclosed vacuum tube. It has extremely high thermal conductivity, good isothermality, and heat transfer on both sides. The area can be arbitrarily changed, long-distance heat transfer, temperature control, and other advantages. The heat exchanger composed of heat pipes has the advantages of high heat transfer efficiency, compact structure, and low fluid loss.
(2) Inverter structure layout
We design the main circuit as a large unit and install it in the rectangular explosion-proof cavity inside the back wall. The back wall is connected to the heating element such as the IGBT module and the rectifier module through an overheating radiator. The outer wall of the explosion-proof housing is welded with a slotted heat sink. The heat sink is connected to the trough radiator. The heat generated inside the inverter is dissipated through the heat sink slot heat sink of the radiator in the back wall of the explosion-proof chamber.
3, the main circuit structure and the general inverter
(1) No delay loop
Avoid the unsafe factors caused by the spark when the time delay relay operates, and increase the safety and reliability of the inverter.
(2) Rectifier capacity selection is double that of general inverter
The purpose is to withstand the impact of capacitor charging current at the instant of inverter startup.
(3) Filter capacitor uses multiple non-inductive capacitors in parallel
Electrolytic capacitors are bulky, easily explosive in high temperature environments, and unsafe; while non-inductive capacitors are small in size and resistant to high temperatures and pressures, and are very safe for use in such environments.
V. Conclusion
The explosion-proof inverter has been successfully developed, filling a gap in the inverter application industry. Of course, we will further improve the structure, process, and functions, and make the explosion-proof inverter more perfect, more reliable, and more complete. It is believed that explosion-proof inverters in the near future will have a bright sky in the field of automation.