1 Introduction
Frequency converters are widely used in various industries. In various applications, frequency converters inevitably operate in humid environments, such as frequency converters operating by the sea, lakes or rivers, or frequency converters for wind turbines installed in humid mountainous areas and around lakes, or frequency converters installed in humid mines in southern regions.
Because of the high humidity in these areas, condensation may occur on the inverter when the ambient temperature changes. This can result in liquid water forming on the inverter's circuit boards, heat sinks of power devices, and other surfaces. This liquid water, mixed with dust, can damage electrical insulation and even form a direct circuit, causing the inverter to malfunction. For example, liquid water forming on the heat sinks of power devices can cause a short circuit between the drain and gate of the IGBT, thereby damaging the gate and causing the IGBT to fail. Liquid water forming on the circuit board can cause short circuits at certain terminals, resulting in pulse chaos and even short circuits between bridges.
It is evident that condensation has a detrimental effect on the normal and stable operation of frequency converters. When frequency converters operate in such environments, measures should be taken to eliminate condensation.
2. The formation of condensation and its harm to frequency converters
Air can be considered to consist of absolutely dry air, water vapor, and a small amount of dust. The higher the temperature, the more water vapor the air can hold. At a given pressure and temperature, the mass of water vapor contained in a unit of air is called the absolute humidity under those conditions; the maximum mass of water vapor it can contain is called the saturated humidity under those conditions; the ratio of the water vapor in the air to the saturated humidity under those conditions is called the relative humidity under those conditions; and the highest temperature at which condensation occurs in air of a certain humidity is called the dew point temperature under those conditions.
The lower the air temperature, the less water vapor it can hold. When the temperature is high, the air can hold less water vapor, and the excess water vapor will condense into liquid water and precipitate out. If high-temperature and high-humidity air encounters a low-temperature surface, and the surface temperature is lower than the dew point temperature of the air, condensation will occur, and liquid water will form on the low-temperature surface.
Condensation, forming liquid water, combines with dust on the surface of an object to create conductive channels, damaging electrical insulation and turning previously non-conductive areas into conductive ones. For example, condensation on the surface of IGBT power devices can combine with surface dust to form a conductive channel between the drain and gate, damaging the gate and ultimately causing the entire device to fail. Condensation on control circuit boards can form conductive channels, causing logic signal corruption, electronic component failure, and power supply short circuits. Although some circuit boards are coated, coatings always leave some blind spots, such as at circuit connections and on the bottom of certain components.
3. Methods to eliminate condensation
To eliminate condensation, the conditions for its formation must be disrupted: humidity and temperature difference. Condensation will not occur if either of these two conditions is disrupted.
Guided by this idea, two methods for eliminating condensation have been developed: humidity control and temperature control. Humidity control aims to reduce absolute humidity, while temperature control aims to reduce relative humidity.
3.1 Methods to reduce absolute humidity
The purpose of this method is to reduce the absolute humidity in the air, thereby reducing the water vapor content and preventing condensation. There are generally three methods: condensation dehumidification, adsorption and membrane dehumidification, and temperature difference dehumidification.
(1) Condensation Dehumidification Method
In a relatively enclosed space like a frequency converter cabinet, condensation will first occur where the temperature is lowest. If a consistently low-temperature point can be created inside the frequency converter cabinet, allowing moisture to condense there and escape outside, the humidity inside the cabinet will be reduced, thus maintaining a dry environment.
This measure requires special equipment, such as an air conditioning system with compressor cooling, or a semiconductor cooler based on the Peltier effect, as shown in Figure 1.
Figure 1 Peltier Semiconductor Refrigeration Dehumidifier
Air conditioning refrigeration utilizes the principle that compressed refrigerant absorbs heat when it expands through the evaporator, creating a lower temperature point on the evaporator. Semiconductor refrigeration, on the other hand, utilizes the Peltier effect of semiconductors; when current flows through a semiconductor, heat is generated on one side while cooling occurs on the other. An air conditioning dehumidification system is shown in Figure 2. Air conditioning systems are prone to compressor failures and refrigerant leaks, but offer high dehumidification and cooling efficiency. Semiconductor refrigeration, while less prone to failures, has lower dehumidification and cooling efficiency and higher energy consumption.
(2) Adsorption and membrane dehumidification methods
The air is kept dry by using adsorbent materials to absorb moisture from the air; or by using a membrane filter that blocks moisture to filter out moisture, allowing dry air to pass through while preventing moisture from passing through, thus obtaining dry air.
For adsorption dehumidification, a typical example is the rotary dehumidifier. A rotary dehumidifier typically consists of a motor driving the rotor, a rotor that adsorbs moisture, a fan, and a heater for rotor regeneration. Its working principle is shown in Figure 3: humid air, propelled by the fan, passes through the dehumidifying rotor, where moisture is adsorbed and the air is dried; the rotor, having adsorbed moisture, is then dehydrated by the high-temperature air, becoming dry again.
Rotary dehumidifiers are widely used due to their simple structure, high efficiency, and high reliability. A good quality rotary dehumidifier can last for 8 to 10 years.
Figure 2 Air Conditioning Cooling and Dehumidifying Unit
Figure 3 Working principle of rotary dehumidifier
Figure 4. Dehumidification principle of polymer membrane
The principle of membrane dehumidification is shown in Figure 4. It is mainly based on the dissolution-diffusion mechanism. First, water vapor comes into contact with the membrane and is then absorbed by the membrane surface, creating a concentration gradient on both sides of the membrane. This causes water molecules to diffuse forward within the membrane and precipitate on the other side, thus achieving dehumidification. Membrane dehumidification requires the use of an air compressor, and while the airflow is relatively low, the degree of air dryness can be very high. It is suitable for special applications with tightly sealed enclosures and harsh operating conditions of frequency converters.
(3) Cooling system temperature difference dehumidification method
Some frequency converter units are equipped with condenser for condensation. This condenser may be the same as the cooling condenser. The condensation on this condenser has no adverse effect on the frequency converter system. The condensate is discharged from the frequency converter cabinet through the outlet, thus keeping the inside of the frequency converter cabinet relatively dry.
Figure 5. Principle of temperature difference dehumidification method in cooling system
Figure 5 illustrates its working principle: The self-regulating thermostatic valve in Figure 5 maintains a relatively stable inlet water temperature for the frequency converter. When the ambient temperature is high, the opening of ab is larger, and the opening of bc is smaller, allowing more coolant to pass through the main air-water heat exchanger for cooling, thus appropriately lowering the inlet water temperature of the frequency converter. When the ambient temperature is low, the opening of ab decreases, and the opening of bc increases, reducing the amount of coolant flowing through the air-water heat exchanger, appropriately increasing the inlet water temperature of the frequency converter; thus, maintaining the relative stability of the inlet water temperature of the frequency converter. From this operating mechanism, it can be seen that the coolant temperature on pipe b is never higher than the coolant temperature on pipe a. If necessary, a heater can be activated to raise the temperature of the coolant flowing to the frequency converter via pipe a, further increasing the temperature difference between the two coolants. Therefore, a temperature difference exists between the radiator inside the frequency converter cabinet and the inlet water of the frequency converter. The inlet water temperature of the radiator is lower, so when condensation occurs, it will occur on the lower-temperature surface of the radiator, not on the frequency converter itself. The radiator can be installed on the cabinet door or at the bottom of the cabinet, so that the condensate can be easily drained. However, it is important to prevent the condensate from damaging other components inside the inverter cabinet.
3.2 Methods to reduce relative humidity and prevent condensation
Another way to prevent condensation is to disrupt the temperature difference, a condition necessary for condensation to form. For the relatively enclosed cabinet space of a frequency converter, if the temperature inside the cabinet can always be kept above the dew point temperature, condensation will not occur. This approach cannot reduce the absolute humidity of the air, but it can control the relative humidity and prevent condensation.
There are generally two approaches. One approach includes a heater and ventilation openings. The ventilation openings are typically equipped with filters to ensure IP protection rating and prevent excessive dust ingress. The core of this system's normal operation lies in heating when humidity is too high and ventilation when temperature is too high. When humidity reaches a certain value, heating is activated to raise the air temperature, causing the relative humidity to decrease. When the temperature reaches a certain value, ventilation is activated, allowing fresh air from outside to enter the inverter cabinet, ensuring uniform absolute humidity inside and outside the cabinet while preventing the internal temperature from rising too high and causing adverse effects. Typically, the relative humidity for heater activation is around 80%, and the temperature for ventilation system activation is around 40℃.
Another approach is described below: The inverter's cooling capacity is controllable, maintaining a relatively constant temperature inside the inverter cabinet. When humidity is too high, the heat dissipation capacity is reduced, utilizing the inverter's own losses to raise the cabinet temperature, thus preventing condensation. When the temperature is high, heat dissipation is increased, preventing overheating. Using this approach, the inverter cabinet can be completely sealed, preventing the entry of dust, harmful gases, and salt spray, which is beneficial for the long-term operation of the inverter.
4. The inverter cooling type is the basis for selecting a dehumidification method.
Inverters typically have three cooling methods: air cooling, water cooling, and water jacket cooling (semi-water cooling). (Note: "water" here refers not only to water but also to other liquids used for cooling.) The attached table compares the three cooling methods.
Air cooling is widely used due to its simplicity and low cost. However, its drawback is the accumulation of dust, which covers the heating surface, affecting heat dissipation and potentially forming conductive strips that damage insulation. Water cooling, on the other hand, requires water-cooled heat sinks and water-cooled reactors for power devices, resulting in higher costs. However, it can prevent external dust from entering, keeping the cabinet clean and facilitating long-term operation. For water-jacketed/semi-water-cooled systems, power devices are typically water-cooled, while reactors and other components are cooled using a fan-water heat exchanger. This also achieves complete sealing, preventing external dust from entering and facilitating long-term operation.
If a water-cooling system uses a fresh air window in conjunction with a heater for dehumidification and condensation prevention, it is necessary to drill holes in the cabinet and install a fresh air window. This will cause air exchange between the inside and outside of the cabinet, allowing external dust to enter the cabinet, and a major advantage of the water-cooling system will be drastically reduced.
For air-cooled systems (unless there are special requirements), methods that reduce absolute humidity (such as using air conditioning dehumidification or rotary dehumidifier dehumidification) can also achieve the effect of dehumidification and condensation prevention, but they increase costs compared to traditional heating dehumidification. At the same time, the increased number of components affects reliability and maintainability.
It is evident that the type of cooling system is the basis for selecting a dehumidification and anti-condensation solution. The implementation of dehumidification and anti-condensation should, as far as possible, maintain the advantages of the cooling system and, ideally, combine it with the characteristics of the cooling system to minimize costs, maximize reliability, and minimize maintainability.
Therefore, while achieving dehumidification and anti-condensation, the characteristics of the inverter's own cooling system should be fully considered, along with the environmental characteristics of the actual usage area, taking into account factors such as cost, reliability, and maintainability, to select the most suitable dehumidification and anti-condensation solution for its own characteristics.
The attached table compares the performance of three cooling types.
5. Conclusion
Moisture is an objective reality. When a frequency converter operates in a high-humidity environment, it is essential to consider its ability to resist moisture in order to ensure that the frequency converter can operate reliably and stably, and to reduce or even avoid various electrical problems caused by moisture.
Regardless of the chosen solution, dehumidification and anti-condensation are all based on humidity and temperature difference. As long as either of the two conditions for condensation formation is eliminated, condensation can be prevented and the inverter system can be protected from damage.
When the operating environment is in a high-humidity environment for a long period of time, it is also necessary to consider the corrosion and aging rate of the components inside the inverter cabinet. In addition to just preventing condensation, relevant measures should be taken to control the humidity within a reasonable range so that the corrosion and aging rate is kept within an acceptable range for long-term operation.
