[Abstract] This article introduces the application of thyristor soft starters in centrifugal fans, and elaborates on the classification and characteristics of thyristor soft starters, as well as the classification and working principle of fans.
[Keywords] Thyristor, fan, voltage, starting
1 Introduction
Currently, motors配套 with fans account for approximately 60% of the total installed motor capacity in various mechanical and electrical equipment across all industries in my country, consuming about one-third of the country's total electricity generation. It is particularly noteworthy that most fans suffer from under-engineering, and due to changes in production and processes, frequent adjustments to gas flow, pressure, and temperature are necessary. Currently, many units still use outdated methods such as adjusting dampers or valve openings to regulate gas flow, pressure, and temperature. This essentially increases resistance artificially, wasting energy and money to meet the demands of processes and operating conditions. This outdated adjustment method not only wastes valuable energy but also lacks precision, failing to meet the requirements of modern industrial production and services, resulting in significant negative effects.
With the rapid development of power electronics technology, thyristor soft starters have emerged. This has brought about a revolutionary change in the starting and stopping technology of three-phase asynchronous motors. Although thyristor soft starters have only been available for about 30 years, their main performance is significantly superior to traditional soft starting methods such as magnetic control soft starters and hydraulic resistance soft starters. They are small in size, compact in structure, virtually maintenance-free, feature-rich, have good starting repeatability, and offer comprehensive protection, making them a leader in the field of soft starters.
Based on the actual situation on site, this article details the application of soft starters in centrifugal fans.
2. Thyristor-based soft start
2.1 System Overall Structure
The thyristor soft-start system is shown in Figure 1. In Figure 1, QS is a high-voltage disconnector, QF1 and QF2 are vacuum circuit breakers, SCR is a (ordinary) thyristor, and M is a medium-voltage motor. QF1 is responsible for switching the main circuit on and off, and QF2 is responsible for bypassing the power devices.
In the SCR soft starter, there are a total of 6 SCRs, each containing m (the value of m is determined according to the voltage level and the withstand voltage of the thyristor) SCRs in series.
Figure 1. Schematic diagram of the main circuit of the thyristor soft starter
2.2 Basic Working Principle of the System
By utilizing the switching characteristics of thyristors, the conduction time of the thyristors is changed by adjusting their firing angle, thereby controlling the output voltage of the motor and thus controlling its starting characteristics. After the motor starting process is completed, the AC contactor is energized (as shown in Figure 1, i.e., QF2 is energized), short-circuiting all the thyristors and directly connecting the motor to the power grid.
2.3 Starting method
1. Voltage Ramp Soft Start: In voltage ramp soft start mode, the output voltage of the soft starter does not rise from 0 to the rated voltage Ue, but rather starts from a starting voltage U0. Because the starting torque of a motor is proportional to the square of the voltage, a low voltage will result in a low starting torque. If the starting torque is less than the load torque, the motor will not run and will instead overheat. Therefore, the output voltage of the soft starter in voltage ramp soft start mode starts from a certain starting voltage U0. The starting voltage U0 must be set according to the characteristics of the load to determine the minimum voltage value that will allow the motor to run.
2. Ramp-up constant current soft start: This starting method involves gradually increasing the starting current during the initial stage of motor startup. Once the current reaches a preset value, it remains constant (from t1 to t2) until startup is complete. During startup, the rate of current increase can be adjusted according to the motor load. A higher current increase rate results in a larger starting torque and a shorter starting time.
3. Step start-up: This refers to rapidly increasing the starting current to the set value in the shortest possible time. A rapid start-up effect can be achieved by adjusting the starting current setting.
4. Dual-slope voltage starting: During startup, the motor's output torque increases with the voltage. An initial starting voltage Us is provided, which is adjustable according to the load. Adjusting Us to be greater than the load's static friction torque allows the load to start rotating immediately. The output voltage then rises from Us at a certain slope (adjustable), and the motor continuously accelerates. When the output voltage reaches the speed-up voltage Ur, the motor has essentially reached its rated speed. The soft starter automatically detects the speed-up voltage during startup, and when the motor reaches its rated speed, it ensures the output voltage reaches the rated voltage.
5. Current-limiting starting is a soft-start method that limits the starting current of a motor to a certain set value (Im) during the starting process. Its output voltage rapidly increases from zero until the output current reaches the preset current limit Im, and then maintains the output current I. The advantages of this starting method are low starting current, which can be adjusted as needed, and minimal impact on the power grid. Its disadvantage is that it is difficult to know the starting voltage drop during startup, and the voltage drop space cannot be fully utilized.
6. Sudden start: In the initial stage of startup, the thyristor is fully turned on for a very short time and then drops back, and then rises linearly according to the original set value to enter constant current startup. This startup method is suitable for heavy load startup applications that need to overcome large static friction.
Among the various control methods mentioned above, voltage ramp and current-limiting soft start are the most representative.
3 ventilation fans
3.1 Overview A ventilator is a machine that uses input mechanical energy to increase gas pressure and discharge gas; it is a type of driven fluid machinery. Relationship between ventilator and fan: "Fan" is a common abbreviation in my country for gas compression and gas conveying machinery. The term "fan" usually includes ventilators, blowers, compressors, and Roots blowers, but excludes positive displacement blowers and compressors such as piston compressors. A ventilator is a type of fan, but people often simply call it a fan; ventilator is another name for fan. 3.2 Classification According to the different directions of gas flow, ventilators are mainly classified into centrifugal, axial, mixed-flow, and cross-flow types.
3.3 Centrifugal Fan Structure Centrifugal fans mainly consist of an impeller and a casing. In small fans, the impeller is directly mounted on the motor; in medium and large fans, it is connected to the motor via a coupling or pulley. Centrifugal fans generally have single-sided air intake and use a single-stage impeller; those with large flow rates can have double-sided air intake, using two back-to-back impellers, also known as double-suction centrifugal fans. The impeller is the main component of the fan, and its geometry, size, number of blades, and manufacturing precision have a significant impact on performance. Static or dynamic balancing of the impeller is necessary to ensure smooth fan rotation. Based on the direction of the blade outlet, impellers are classified into three types: forward-curved, radial, and backward-curved. In forward-curved impellers, the blade tips are inclined in the direction of impeller rotation; in radial impellers, the blade tips are radial, and they are further divided into straight-blade and curved-blade types; in backward-curved impellers, the blade tips are inclined in the opposite direction of impeller rotation. Forward-curved impellers generate the highest pressure and require the smallest impeller diameter for a given flow rate and speed, but their efficiency is generally lower. Backward-curved impellers, on the other hand, generate the lowest pressure, require the largest impeller diameter, and generally have higher efficiency. Radial impellers fall somewhere in between. Blade profiles are simplest with straight blades and most complex with airfoil blades.
To achieve a suitable velocity distribution on the blade surface, curved blades, such as uniformly thick circular arc blades, are generally used. Impellers usually have a cover plate to increase impeller strength and reduce gas leakage between the blades and the casing. The connection between the blades and the cover plate is achieved by welding or riveting. Welded impellers are lighter and have smoother flow channels. Impellers for low- and medium-pressure small centrifugal fans are also sometimes made of cast aluminum alloy.
3.4 Working Principle of Centrifugal Fans When a centrifugal fan is working, the power unit (mainly an electric motor) drives the impeller to rotate within a volute casing. Air is drawn in through the intake port from the center of the impeller. Due to the dynamic action of the blades on the gas, the gas pressure and velocity are increased, and under the action of centrifugal force, the gas is thrown along the impeller passage towards the casing and discharged from the exhaust port. Because the gas flow within the impeller is mainly in the radial plane, it is also called a radial flow fan.
4. Practical Applications
4.1 Operating Conditions: The power grid of a certain mine is responsible for supplying power to all electrical equipment underground and above ground. Therefore, the power grid has a relatively small margin. To ensure that the operation of other equipment is not affected during startup, the starting current requirements for the ventilation fans are very strict. 4.2 Main Technical Parameters of the Equipment
1. Main technical parameters of the fan:
Name: Mine Centrifugal Ventilation Fan
Model: DFK32—C4A
Flow rate: 28000 m³/min
Total pressure: 6024 kPa
Spindle speed: 480 r/min
Shaft power: 3730KW
Wind turbine diameter: 3200mm
Total weight of the fan: 28000Kg
Rotor assembly weight: 12260Kg
Air intake direction: 210°
Exhaust direction: 0°
2. Main technical parameters of the electric motor:
Model: Y800-8
Power: 4000KW
Power factor: 0.857
Weight: 17.7t
Insulation class: F
Rotational speed: 746 r/min
Current: 465.9A
4.3 Working Principle
When a fan starts from a standstill, it must overcome the bearing friction torque (30% to 50% of the rated torque). The opening and closing of control dampers in the fan's fluid transport pipeline creates static pressure, affecting the starting torque. Therefore, regardless of whether the fan's outlet damper is closed or open, it is considered a load-bearing start. In other words, at zero speed, its torque already has a fixed value.
Centrifugal fans are generally square torque loads. Due to their large inertia and the fact that starting torque increases with speed, they do not require much torque at the start. Therefore, soft starting is used to start them, with torque-based starting methods preferred. Since the starting torque of a motor is proportional to the square of the voltage, voltage ramp starting is used in this case, which makes it easier to control the starting torque and better utilize the soft start function.
4.4 Start-up and Operation Records
Table 1 Soft starter operation record
5. Conclusion
In different applications, the starting method of a soft starter should be selected according to the type of load to ensure that the soft starter performs better.
References
[1] Liu Li, Wang Dong. Practical Technology of Motor Soft Starter [M]. China Electric Power Press, 2009.
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[3] Xu Furong. A review of energy-saving protection control for starting and speed regulation of AC asynchronous motors [J]. Journal of Electrical Engineering, 2003(4):1-5.
[4] Wang Zhaoan, Huang Jun. Power Electronics Technology [M]. Beijing: Machinery Industry Press, 2000.
[5] Xu Honggang. Principle and application of soft starters [J]. Energy Technology, 2002(6):132-135.
[6] Wang Yufeng, Ma Guangcheng, Wang Changhong, et al. Study on oscillation phenomenon during the starting process of thyristor-controlled induction motor [J]. Journal of Electrical Machines and Control, 2002, 6(3): 186-190.
About the Author
He Xiaoping is a male electronics engineer currently working in the soft starter project group at Harbin Jiuzhou Electric Co., Ltd. His research interests include power electronics and motor drives.
