Selection of heat dissipation method for mine flameproof VFD (Variable-frequency Drive)

1 Introduction

In recent years, the development of power electronics and automatic control technology has also promoted the upgrading of production equipment in coal mines. In order to meet the requirements of energy saving and safe production in coal mines, the use of flameproof VFD (Variable-frequency Drive) in coal mines is also increasing. , and its occasions are mainly used for load types such as underground belt conveyors, hoisting winches, fans, water pumps, and scraper conveyors, which require extremely high reliability. There are many reliability indicators that affect the VFD (Variable-frequency Drive), and its ventilation and heat dissipation are a crucial link.

When the VFD (Variable-frequency Drive) is working normally, components that generate high heat include isolation transformers, electronic power components, reactors and filters. The structure of the ordinary VFD (Variable-frequency Drive) is connected to the outside world. Through natural exchange with the air, it is easy to solve its heat dissipation problem. However, the flameproof VFD (Variable-frequency Drive) is in a closed cavity for a long time, and the heat Circulating in a fixed small space, heat exchange with the outside through the flameproof enclosure, the heat cannot be dissipated in time, so the failure rate increases with the temperature of the cavity, and the service life decreases exponentially with the increase of temperature. The most direct solution is to increase the volume of the explosion-proof chamber, but this also brings about an increase in production costs and restrictions on the use of occasions, which contradicts the optimal design of reducing costs, using less materials, and more output. In particular, with the increase in production output, the demand for large-capacity VFD (Variable-frequency Drive) has also increased, and the heat dissipation problem of VFD (Variable-frequency Drive) has become particularly prominent. This requires us to reasonably choose the heat dissipation method of the flameproof VFD (Variable-frequency Drive) according to the actual situation.

2 General heat dissipation method of VFD (Variable-frequency Drive)

The design of VFD (Variable-frequency Drive) cooling and heat dissipation system includes the design of radiator structure and the selection of cooling medium. The selection of radiator structure should consider the following factors: the energy consumption, volume and weight of auxiliary equipment; the complexity and ease of operation of the device, as well as the reliability, availability and maintainability of the device. The choice of cooling medium should consider electrical insulation, chemical stability, corrosiveness to materials, impact on the environment and flammability, and the cost of the medium.

VFD (Variable-frequency Drive) heat dissipation methods can be divided into air cooling and liquid cooling according to different cooling media.

2.1 Air cooling

The air cooling method is subdivided into two types: "natural cooling" and "forced air cooling".

(1) Natural cooling method for heat dissipation

It is mainly used for VFD (Variable-frequency Drive) where the power is relatively small, the heat dissipation area of components is large, and the fan is not allowed. Cooling takes heat away mainly through natural convection of air and radiation. Due to its simple mechanism, no noise, maintenance-free, high reliability, etc., it has a wide range of applications, especially suitable for impact loads and intermittent duty loads. The disadvantage is that it cannot be used for high-power long-term VFD (Variable-frequency Drive) superior.

(2) Forced air cooling to dissipate heat

It is mainly used in occasions where there is no special requirement and the power level is slightly larger. It is characterized by high heat dissipation efficiency, and the heat dissipation coefficient is 2 to 4 times that of natural cooling. The disadvantage is that it needs to be equipped with a fan, so it is noisy, easy to blow in coal dust, and its reliability is relatively low.

2.2 Liquid cooling

Liquid cooling methods are subdivided into two types: "water cooling" and "oil cooling".

(1) Water cooling and heat dissipation

The heat dissipation efficiency of the water-cooled radiator is extremely high, which can greatly increase the capacity of the power components. However, the electrical insulation performance of ordinary water is extremely poor, and the impurity ions in the water will cause electrical corrosion and leakage under high voltage. Hard, easy to form scale in the water channel and hinder the effect of heat dissipation, and may block the water channel, generally used for low-voltage VFD (Variable-frequency Drive). When used in high-voltage VFD (Variable-frequency Drive), two major problems of reliability and corrosion during operation must be considered and resolved.

(2) Oil cooling and heat dissipation

Although the heat dissipation efficiency of oil-cooled radiators is not as high as that of water-cooled methods, it has been widely used in ordinary high-power VFD (Variable-frequency Drive) due to its high electrical insulation and electromagnetic shielding effects, but its cost and environmental impact Requirements have gradually faded out these years, and according to coal mine safety regulations, oil-filled electrical equipment is not allowed to be used underground in coal mines, and oil cooling methods cannot be used underground in coal mines.

3 Selection of flameproof VFD (Variable-frequency Drive) radiator

Since igbts and rectifier tubes are high-density heating elements, this means that the heat sink must have sufficient instantaneous heat absorption capacity to absorb the heat as quickly as possible. Therefore, the material selected for the heat sink is not only directly linked to its volume and weight, but also affects the heat dissipation performance. The processing technology and production cost are important design links of the heat sink. The reason why metal has become the only choice for heat sinks today is that compared with other solid materials, metal has the advantages of better thermal conductivity, strong ductility, and relative stability at high temperatures.

For metals, thermal conductivity and specific heat are important basis for choosing heat dissipation materials.

The thermal conductivity represents the metal's ability to conduct heat. The unit is w/mk. The larger the value, the stronger the thermal conductivity, that is, the faster the heat conduction speed. It can be seen from Table 1 that the conductivity of copper is about 1.8 of that of aluminum. times, it can quickly take away heat, and although the thermal conductivity of silver is higher than that of copper, it is expensive and unrealistic to produce.

From the perspective of specific heat, specific heat is an inherent characteristic of metals, that is, the heat absorbed by 1 kg of this type of metal when the temperature rises by 1 °C. The specific heat of copper and aluminum is shown in Table 2. The specific heat of aluminum seems to be much higher than that of metal. Copper can better meet the requirements of heat storage fins and heat-absorbing bottoms. However, the density of copper is 3.3 times that of aluminum, that is, under the same volume, the weight of copper fins is 3.3 times that of aluminum fins, so the same volume The high-quality copper heat sink can absorb 40% more heat than the aluminum heat sink, and has greater heat storage capacity. Copper heat dissipation bottom plate has become the main material of VFD (Variable-frequency Drive) heat sink.

Table 1 Thermal conductivity table (w/mk)

Table 2 Specific heat coefficient table (j/(kg·℃)

According to the power of the heating element, in order to quickly dissipate the heat stored at the bottom of the heat sink, the heat must be conducted to each part of the fin to increase the heat transfer area. The heat conduction capacity between the bottom of the heat sink and the fin depends on Combination mode and connection area, for air cooling mode, more fins can be added on the heat dissipation plate to increase the heat dissipation exchange area. For large-capacity and high-power VFD (Variable-frequency Drive), a better heat dissipation method-heat pipe should be used. The heat pipe radiator is a boiling heat pipe that uses water or other thermal fluids as the cooling medium and is sealed in a copper tube with a capillary structure. The heat generated by the power device is transferred to the fluid through the radiator, and the fluid is vaporized and diffused to the entire copper tube, cooled by the heat sink to become a liquid, and then flows to the heat-absorbing surface. The heat transfer speed of the heat pipe is 10 times that of copper, so the heat pipe radiator has the advantages of strong heat transfer capacity, excellent temperature uniformity, variable heat density, no additional equipment, reliable operation, simple structure, light weight, and no maintenance.