Abstract: The motors of three 1120kW high-pressure circulating water pumps in our factory have non-adjustable speeds, resulting in significant adjustment losses and wasted electricity.
1. Introduction
Our factory has three 1120kW high-pressure circulating water pump motors with non-adjustable speeds, resulting in significant adjustment losses and wasted electricity. To address this, we adopted a 1250kW ZNR-type intelligent high-voltage frequency converter for variable frequency speed control of the circulating water pump motors. This significantly improved the working conditions for process operators, enhanced equipment starting performance, achieved stepless speed regulation, and saved energy, thus achieving energy conservation, consumption reduction, and reduced equipment noise pollution. Because the internal components of the high-voltage frequency converter have strict environmental requirements, a high proportion of equipment failures are caused by excessively high temperatures. Therefore, it is essential to ensure the frequency converter's safe and long-term operation by providing adequate cooling.
2. Components of the frequency converter
The high-voltage variable frequency speed control system mainly consists of the following three parts: bypass cabinet, transformer cabinet, and power control cabinet.
To ensure effective heat dissipation of the high-voltage variable frequency speed control system, the manufacturer uses four three-phase centrifugal fans: two on top of the power control cabinet and two on top of the transformer cabinet. The rated air volume of the three-phase fans is 3300 m³/h.
When the frequency converter operates at full load, its total loss (converted into heat) is approximately 4% of the system's rated power. The maximum heat dissipation capacity of a 1250kW high-voltage frequency converter is 1250 × 4% = 50kW. If such a large amount of heat is dissipated into the room where the frequency converter is installed, the room temperature will rise rapidly, seriously affecting the normal operation of the frequency converter.
3. Solving the problem of inverter heat dissipation
Inverter cooling can be achieved by installing air conditioning in the inverter room, adding air ducts, or using a water-air cooling device.
If air conditioning is added, the cooling capacity of a 1-horsepower air conditioner is approximately 2000 kcal. Converting to international units and multiplying by 1.162 , a 5-horsepower air conditioner would have a cooling capacity of 11.6 kW . To meet the system's needs, at least six 5-horsepower air conditioners would be required, significantly increasing investment costs. Adding a water-air cooling system to the high-voltage inverter is also difficult to install. Therefore, we chose to address the issue of rising temperatures inside the high-voltage inverter room by adding ventilation ducts.
The specific method involves installing air ducts on the cabinet to directly exhaust the heat generated by the frequency converter to the outside through the air ducts, while cold air is continuously supplied from the air inlet of the frequency converter room to cool the system.
The system's ventilation volume is qf. Assuming the air velocity at the inlet is no more than 3 m/s, and given qf = s × v, the inlet area s ≈ qf/v. Therefore, the inlet area is calculated to be approximately 4 m². An air filter with a mesh size of 5 × 5 mm is installed at the inlet.
Based on the heat dissipation principle of air-cooled systems
△q=△t×qf×cp×ρ
In the formula:
△q—Total power loss of the system;
△t—The temperature difference between the air inlet and outlet;
qf—Total ventilation volume;
cp—Specific heat of air: 1005 J/kg℃;
ρ—the density of air: 1.165 kg/m³.
Based on the above conditions, Δt = 50000 / (3300 × 4 × 1.005 × 1.165 ) = 3.24 ℃ (the temperature difference between the air inlet and outlet).
Utilizing an open-duct cooling system, when the outdoor ambient temperature is below 28℃, the equipment operating temperature can be maintained below 76℃ for the transformer and 33℃ for the power cabinet, fully meeting the operating environment requirements of less than 95℃ for the transformer cabinet and less than 40℃ for the power cabinet of the high-voltage frequency converter. After heat dissipation through the duct, the ambient temperature inside the high-voltage frequency converter room can be controlled below 40℃, meeting the operating temperature requirements of the high-voltage frequency converter and thus ensuring a good operating environment inside the high-voltage frequency converter room.
4. Conclusion
As can be seen from the above brief introduction, using air ducts for heat dissipation is low-cost, highly reliable, and provides excellent heat dissipation. Our factory's high-voltage frequency converters have consistently operated stably, improving equipment efficiency, meeting production process requirements, significantly reducing equipment maintenance and repair costs, and minimizing downtime.
