Abstract: This paper introduces the energy-saving application and retrofitting scheme of the Fukagawa SVF-EV series high-performance vector general-purpose frequency converter in fans and water pumps. It elaborates on the functions and characteristics of the variable frequency speed control system, the necessity of the retrofitting, and analyzes the energy-saving effects. A comparative analysis is also provided from the perspective of practical application.
I. Overview
In recent years, my country's economy has continued to develop rapidly, and energy issues have become an increasingly important constraint on industry development. With the rapid rise in energy prices and fierce competition in the domestic market, energy conservation has become a major issue for many industries, especially those with high energy consumption such as petroleum, chemical, pharmaceutical, metallurgy, manufacturing, environmental protection, and municipal industries. According to data, the total capacity of high and low voltage motors in my country is over 3,500 MW, most of which are fan and pump loads, and most of them are still in a high-energy-consuming and low-efficiency state.
Most fan and pump systems rely on valves to regulate water flow or pressure. This method of regulation, using baffles, increases pipe network losses and consumes a large amount of energy, inevitably leading to energy waste. Moreover, since the system is designed for maximum load, it is unlikely to operate at full load most of the time in actual operation, leaving a significant margin and thus presenting considerable energy-saving potential.
By employing a frequency converter control device, the fan speed is changed, thereby altering the fan airflow to meet the needs of the production process. This method offers the lowest energy consumption and the highest overall efficiency. Therefore, frequency converter speed regulation is the most efficient and optimal speed control solution. It enables stepless speed regulation of the fan and can be easily integrated into a closed-loop control system to achieve constant pressure or constant flow control.
II. Skills Analysis for Variable Frequency Drive Retrofitting (Taking Fans as an Example)
2.1 Energy-saving principle of variable frequency speed control
According to the principles of fluid mechanics, the relationship between shaft power P, air volume Q, and air pressure H of a fan driven by an induction motor is as follows:
When the motor speed changes from n1 to n2, the relationship between Q, H, P and the speed is as follows:
It is evident that the air volume Q is directly proportional to the motor speed n, while the required shaft power P is directly proportional to the cube of the speed. Therefore, when 80% of the rated air volume is required, the motor speed can be adjusted to 80% of the rated speed, i.e., the frequency can be adjusted to 40.00Hz. At this time, the required power will only be 51.2% of the original.
As shown in Figure (1), the energy-saving effect of using a frequency converter for speed regulation is analyzed from the operating curve of the fan.
Figure (1) Operating curve of the fan
When the required air volume changes from Q1 to Q2, if the damper is adjusted, the pipeline resistance will increase, the pipeline characteristic curve will shift upward, and the system's operating point will change from point A to a new operating point B. The required shaft power P2 is proportional to the area H2 × Q2. If speed control is used, the fan speed will decrease from n1 to n2, and the pipeline characteristics will not change, but the fan's characteristic curve will shift downward. Therefore, its operating point will shift from point A to point C. At this time, the required shaft power P3 is proportional to the area HB × Q2. Theoretically, the saved shaft power delt(P) is proportional to the area (H2-HB) × (CB).
Considering the efficiency reduction after deceleration and the wear and tear of accessories in the speed control device, statistical data from practice show that fans can save 20%-50% of energy through speed control.
2.2 Energy Saving Analysis of Variable Frequency Drive Retrofit
Before the modification, the formula for calculating the power frequency operating power was P1 = U × I × 1.732 × cosΦ, where:
U------ Motor voltage, kV;
I-------Motor current, A;
P1-----Power output at power frequency under single load, in KW;
cosΦ-----Power factor under single load, less than the rated power factor.
C1 = T × Σ (P1 × δ), where:
T-------Average annual operating time, in hours;
P1------Operating power under single load, KW;
δ------Proportion of annual operating time under this load;
C1-----Total power consumption before the upgrade, kWh.
Formula for calculating the expected power of the variable frequency drive after modification:
Using the formula: η
P2 is calculated to be the expected operating power after the frequency converter retrofit, and η is the efficiency of the frequency converter.
C2 = T × Σ (P2 × δ), where:
C2-----Total power consumption after modification, kWh.
1.3 Energy Saving Comparison Table: Energy Consumption Comparison of Fans and Pumps under Variable Frequency Speed Regulation
All figures above are percentages. 100% refers to the fan's rated flow rate, and 100% refers to the power consumption during operation at industrial frequency (i.e., motor input power = fan rated shaft power / motor efficiency; motor efficiency is generally 93-96%, with higher rated power resulting in higher efficiency). The energy saving during variable frequency speed control is the energy consumption difference between the two control methods (percentage multiplied by rated power consumption).
III. Other Additional Benefits of Variable Frequency Speed Control
(1) Improved internal power factor: When the original motor is directly driven by the power frequency, the power factor is about 0.85 when fully loaded, and it is far lower than 0.8 when actually running. After adopting the variable frequency speed control system, the power factor on the power supply side can be improved to more than 0.9. The reactive power can be greatly reduced without the need for reactive power compensation device, meeting the grid requirements and further saving the operating costs of upstream equipment.
(2) Reduced equipment operation and maintenance costs: After adopting frequency conversion regulation, energy saving is achieved by adjusting the motor speed. When the load rate is low, the motor speed also decreases, reducing wear on the main equipment and corresponding auxiliary equipment such as bearings. This allows for longer maintenance cycles and extended equipment lifespan. Furthermore, after frequency conversion modification, the damper opening can reach 100%, and it does not bear pressure during operation, significantly reducing damper maintenance. During frequency converter operation, only periodic dust removal is required, eliminating the need for shutdown and ensuring production continuity. As production needs dictate, the fan speed is adjusted, thereby adjusting the fan airflow, which not only meets the requirements of the production process but also greatly reduces workload. After adopting frequency conversion technology for speed regulation, mechanical wear is reduced, maintenance workload is decreased, and repair costs are lowered.
(3) With the use of a variable frequency speed control device, the motor can be soft-started. The starting current does not exceed 1.2 times the rated current of the motor, and there is no impact on the power grid, thus extending the service life of the motor. Throughout the entire operating range, the motor can ensure smooth operation, reduced losses, and normal temperature rise. The noise and starting current of the fan during startup are very low, with no abnormal vibration or noise.
(4) Compared with the original old system, the frequency converter has multiple protection functions such as overcurrent, short circuit, overvoltage, undervoltage, phase loss, and temperature rise, which provides more comprehensive protection for the motor.
(5) Simple operation and convenient running. Parameters such as air volume or pressure can be remotely set via computer to achieve intelligent adjustment.
(6) It has a strong ability to adapt to grid voltage fluctuations and a wide voltage operating range. The system can operate normally when the grid voltage fluctuates between -15% and +10%.
IV. Conclusion
In summary, to save energy and costs, it is necessary to retrofit the fan system with frequency conversion technology to achieve energy savings and extend the service life of motors, contactors, mechanical parts, bearings, valves, and pipelines. Generally speaking, for continuously operating air supply systems, the motor operating frequency can be dynamically adjusted between 35.00 and 50.00 Hz as the process changes, resulting in a power saving rate of 20%-35%.
