Abstract: This paper introduces the application of VFD-BP series frequency converters in the drive system of a petroleum coke conveyor; it elaborates on the control strategy, principle, debugging process, and performance of the system. The application of frequency conversion speed regulation technology in equipment drive control enables functions such as low-current motor start-up and speed regulation; it improves production capacity and equipment operating efficiency, extends equipment service life, and saves energy.
Keywords: frequency converter, speed regulation, conveyor, energy saving
1 Introduction
After more than 20 years of continuous development and technological innovation, frequency conversion technology has become very mature and sophisticated, and its application is widespread and popular. Especially domestically produced frequency converters, in terms of technology, performance, and quality, are now completely comparable to imported products, and some even surpass them in functionality. However, due to outdated perceptions and awareness, people still hold negative views and reservations about domestically produced frequency converters. But with the passage of time and continued use, people will gradually re-evaluate the quality and performance of domestic products. As long as domestic companies continue to strive to improve their product performance and provide excellent after-sales service, they will gain the recognition and first choice of engineering and technical personnel in all domestic enterprises. In actual enterprise equipment renovation and upgrading, people generally prefer products from manufacturers they have used before. Once they have used domestically produced frequency converters, their past views will quickly change.
Delta frequency converters, with their high quality, superior performance, multiple functions, and high control precision, have been recognized and accepted by numerous enterprise engineering and technical personnel. The practical application results over the past few years have fully demonstrated the superior quality, performance, and functionality of the Delta VFD-BP series frequency converters. Their advantages include a wide speed range, high speed control accuracy, energy saving, ease of control and maintenance, reliable operation, convenient operation, comprehensive protection and monitoring functions, and simple and easy-to-implement control circuits, leading to their widespread application. Especially in more complex control systems, by properly setting the relevant parameters of the frequency converter, it can fully meet and realize the requirements of complex control systems.
2. Current Status of Production Technology and Equipment
In the production, forming, calcination, and assembly processes of prebaked carbon anodes for aluminum electrolysis, belt conveyors are widely used in the conveying of bulk materials such as petroleum coke, asphalt, and residual anodes. Due to the randomness of equipment failures, malfunctions, and changes in production processes, the actual conveying capacity of belt conveyors varies greatly: for example, the conveyor belt for calcined coke can sometimes reach 3.5-6.5 t/h (production of one rotary kiln); at other times it can reach over 18 t/h (production of three rotary kilns for calcination). However, ordinary belt conveyors are designed for a single speed (such as a belt speed of 0.82 m/s or 1 m/s), and their maximum conveying capacity is 12-13.5 t/h. Therefore, it is difficult to meet the actual production needs under special conditions (conveyor flow exceeding 14 t/h), and material accumulation or falling off often occurs (due to excessive material accumulation on the belt, some material will slide off to the sides of the conveyor during the belt's movement). Cleaning up spilled material requires significant manpower, and workers are easily injured by the conveyor belts, posing safety hazards. It also leads to raw material loss and a deteriorating production environment. Furthermore, due to equipment overhauls or planned maintenance, sometimes only one rotary kiln is in operation, resulting in a very small actual conveying capacity of the belt conveyors. The belts operate under extremely low load conditions for extended periods (24-hour operation). Additionally, planned maintenance (10-14 days of downtime) is scheduled every two months for the first and second phases of anode forming. Since the maximum conveying capacity of a single inclined belt conveyor in the two calcined coke raw material silos is only about 13.5 t/h, when three rotary kilns are in operation simultaneously, the calcined coke can only be transported and stored nearby using two inclined belts. The material is then balanced by vehicle transfer. Due to these problems, our plant proposed using variable frequency speed control technology to upgrade the control system of multiple belt conveyors, enabling belt speed adjustment to increase conveying capacity, reduce belt slippage, and extend belt life.
3 System Design
To better enhance the conveying capacity of our calcination belt conveyor, further reduce its energy consumption, and achieve belt speed regulation control requirements, we comprehensively considered the system design, configuration, and selection based on the current status of the existing equipment and in accordance with the principle of economy.
3.1 Equipment Configuration
① The driving equipment for the inclined belt conveyor is a three-phase asynchronous AC motor. Its main technical parameters and performance are as follows:
Model Y160M-4, power 11KW (3~), 23.6A, 380V, 50Hz, 1430r/min, belt speed 0.82m/s.
Model Y2200M2-4, power 15KW (3~), 29.6A, 380V, 50Hz, 1460r/min, belt speed 1.0m/s.
② Belt scale:
a. Model SM14 (three-roller suspension structure): It consists of SM14 type fully suspended weighing bridge, SM12C type speed sensor, SM2301-D type converter, and SM2301-D integrator (for metering of calcined coke after calcination in our factory's first and second phases).
b. Model SM20-1 (single idler): Composed of SM20 type lever-type weighing bridge, SM2012C type speed sensor, and SM2001 integrator (used in our factory's third calcination phase).
3.2 Inverter Selection
After comprehensively comparing the cost-effectiveness of frequency converters from numerous domestic and international manufacturers, and considering the control requirements of this system, the VFD-BP series (manufactured by Delta) frequency converter is the most suitable choice. This series of frequency converters features comprehensive functions, high speed regulation accuracy, good stability, and a wide range of applications. Furthermore, it also boasts high quality, high cost-effectiveness, strong anti-interference capabilities, suitability for harsh carbon environments, and multi-parameter control functions, effectively meeting the needs of multi-variable and complex control systems. Therefore, it fully meets the requirements of the inclined belt conveyor for automatically adjusting the belt speed according to the amount of material being fed onto the belt.
Based on practical experience, the selection of frequency converters should first clarify their purpose and application. According to the actual requirements such as the type of production machinery, speed range, control accuracy, and starting torque, comprehensive consideration should be given to aspects such as capacity, voltage, frequency, protection functions, and power grid stability. Secondly, based on the premise of meeting the basic conditions of process and production, the selection of the model and manufacturer should be systematically carried out, taking into account the principles of ease of use, economy, future maintenance convenience, and easy procurement of spare parts.
Theoretically, when the altitude of the application environment exceeds 1000m, the frequency converter should be derated. For every 1000m increase in altitude, the actual capacity of the frequency converter will decrease by 5%. Our factory is located at an altitude of over 2400m, so when selecting the capacity, load capacity, and heat dissipation capacity of the frequency converter, there should generally be a margin of more than 20%.
The VFD-BP series frequency converters are characterized by comprehensive functions, high speed regulation accuracy (speed regulation error of 0.01Hz), large overload capacity (up to 250%), good stability, and wide speed regulation range (maximum output frequency up to 400Hz). Furthermore, they feature high quality, high cost-effectiveness, strong anti-interference ability, suitability for harsh carbon environments, and multi-parameter control functions, making them suitable for multi-variable and complex control systems. Therefore, they fully meet the requirements of automatically adjusting the belt speed control of inclined belt conveyors based on the amount of material being fed onto the belt. Thus, the following capacity and model of frequency converter are selected.
a. Model VFD150B43P (drives Y160M-4, 11KW motor);
b. Model VFD185B43P (drives Y2200M2-4, 15KW motor);
Since the distance between the installation site of the conveyor and the electromagnetic station is more than 120 meters, it is necessary to add output reactors, namely TDL-4A001-0150 (adapted power 15KW) and TDL-4A001-0110 (adapted power 11KW).
3.3 Control Principle
Our plant's three molten petroleum coke storage silos are each equipped with a single inclined belt conveyor, driven by an 11KW (15KW for Phase III) ordinary three-phase AC asynchronous motor. The structure and principle of the inclined belt conveyor are shown in Figure 1. Since the belt conveyor line typically has multiple feeding ports, the raw materials from our plant's calcining rotary kilns (No. 1, No. 2, and No. 3, each with a capacity of 6t/h), calcined petroleum coke, one material balance feeding port, and one roasting filler recovery feeding port all supply materials for the first and second molding workshops. Although the discharge ports and two feeding ports of the three rotary kilns are connected to the two inclined belt conveyors via a bridge belt conveyor, the original design capacity of each inclined belt conveyor is 13.5t/h. If all three kilns simultaneously feed one molding raw material silo, the single inclined belt conveyor cannot transport the material in time. In practice, two inclined belts are usually activated simultaneously to transport materials to the two molding raw material storage silos, and then the materials are transferred by vehicle. Therefore, more than 50 vehicles per month need to be arranged to balance the materials (transfer petroleum coke in the raw material warehouse).
Figure 1. Structure and schematic diagram of the inclined belt conveyor
In response to the aforementioned problems in production, our factory proposed a frequency conversion upgrade plan in 2009. After testing the control system of a ramp belt conveyor and operating it at 60Hz under different working conditions, the operating parameters such as motor temperature and current were normal. However, when running at 5Hz for an extended period of time, the temperature of the 11KW asynchronous motor was high (reaching approximately 58℃). Its operating parameters are shown in Table 1.
Table 1. Measurement of operating parameters of belt conveyor under frequency conversion control conditions (ambient temperature 19℃)
System Control Principle: Each inclined belt conveyor is driven by an AC asynchronous motor, controlled by a VFD150B43P frequency converter. The frequency converter adds the setpoint from an external potentiometer to the real-time measurement signal from the belt scale, serving as the primary setpoint signal. After filtering, PID control, and amplitude limiting, this signal becomes the actual frequency control setpoint signal, enabling automatic adjustment of the conveyor belt speed based on the actual material load. Furthermore, the system includes a constant-speed operation mode for the frequency converter (i.e., panel-set speed: 30Hz for one kiln, 45Hz for two kilns, and 60Hz for three kilns). If the belt scale malfunctions (affecting material conveying), simply changing the frequency converter's setpoint to the panel setting mode and setting the corresponding frequency will restore normal system operation. The system structure and control principle are shown in Figure 2; the frequency converter control wiring is shown in Figure 3.
Figure 2. Control principle of variable frequency speed control system for belt conveyor
Figure 3 Wiring diagram of main and control lines of frequency converter
The PID control function of the frequency converter effectively suppresses signal fluctuations in the weighing system caused by uneven material feeding and belt vibration. Filtering significantly reduces the problem of frequent acceleration and deceleration adjustments by the frequency converter. By setting a large integral time constant, the integral function is ineffective when a disturbance in the weighing system signal is introduced into the system (because integral regulation is lagging, the larger the integral time, the slower the regulation speed), which is beneficial to system stability. The lag also reduces fluctuations and increases the system's anti-interference capability. A small derivative time constant is beneficial for rapidly increasing belt speed control after a sudden increase in material load (equivalent to a step input). By appropriately selecting these parameters, accurate belt speed control can be achieved effectively.
By applying frequency converter control to regulate belt speed, different belt speed control methods are implemented based on the feed rate, achieving automatic adjustment of the conveyor's material delivery. For example, when three rotary kilns are operating simultaneously, the drive motor of one inclined belt conveyor can meet production requirements at around 58 Hz; when two kilns are operating, the motor operates at around 46 Hz; when one kiln is operating, the motor operates at around 31 Hz; and when the material flow rate is below 0.8 t/h or under no-load conditions, the motor operates at 10 Hz (this is the minimum operating frequency of the belt). Actual operation results demonstrate that after adopting frequency converter control, the conveying capacity of the inclined belt conveyor has increased by more than 36%, significantly increasing the material delivery rate while achieving more economical operation. It also reduces equipment idle time and wear, greatly minimizing the phenomenon of "over-powered equipment" in production. Furthermore, it significantly extends the equipment's service life. This significantly reduced the daily workload of frontline operators in tasks such as inspection, maintenance, and cleaning. The conveyor belt speed increased from 0.82 m/s to 1.27 m/s, and the material conveying capacity increased from 13.5 t/h to 18.6 t/h, improving the equipment's working capacity by nearly 40%. Monthly material balancing cart usage was reduced by 36 trips (costing 580 yuan per cart per trip); material cleaning frequency decreased to once every six shifts (previously once per shift); and the amount of material cleaned per trip was less than 0.1 t (previously around 0.3 t). One inclined conveyor belt can fully handle the coke conveying tasks of three rotary kilns. After more than a year of use, the results were excellent, so we subsequently modified two other conveyors, with equally satisfactory results.
4 System Parameter Settings and Debugging
After the variable frequency speed control system is installed, it needs to be debugged according to the actual production situation to maximize its control effect. The debugging of this system mainly consists of setting the belt scale system parameters and setting the frequency converter control parameters.
4.1 Belt weigher adjustment
The SM20-1 is a single-roller belt scale (the SM14 is a three-roller suspended structure). The SM2001 (SM2301-D) integrator has calculation functions such as metering, instantaneous belt feed rate, belt speed, and material accumulation. It is equipped with a 4-20mA signal output interface. By setting the output ratio parameter of the integrator, the signal value at different feed rates can be obtained.
4.2 Inverter Control Parameter Settings
Because the VFD-BP series frequency converter has user parameters (00-00┅00-10), basic parameters (01-00┅01-23), operation mode parameters (02-00┅020-15), auxiliary function parameters (03-00┅03-12), input function parameters (04-00┅04-25), multi-speed and automatic program operation parameters (05-00┅05-34), protection parameters (06-00┅06-18), motor parameters (07-00┅07-15), special parameters (08-00┅08-22), communication parameters (09-00┅09-07), feedback control parameters (10-00┅10-16), and multiple sets of motor control parameters (11-00┅11-07), correctly setting these parameters is essential to ensure that the frequency converter operates at its optimal working state. Under normal circumstances, the control parameters of the frequency converter can only be modified when it is stopped; during operation, only a few monitoring and viewing parameters (such as frequency, current, voltage, temperature, etc.) can be modified.
(1) Input settings and other main parameters are determined.
Set AVI and AIC1 as signal input terminals: AVI—main speed command input; AIC1—auxiliary command (belt weighing signal input). The main and auxiliary commands are summed: the AVI and AIC1 signals are proportionally amplified and processed to serve as the actual control signal input for the frequency converter. The potentiometer input serves as the control signal from the frequency converter's AVI input; the instantaneous flow signal (4-20mA) of the belt scale serves as the control signal from the frequency converter's AIC1 input. Through the frequency converter's program processing (proportional and synthetic), this becomes the final control signal for the frequency converter. The main setup method is as follows:
① Determine the range of the given AVI input signal. First, disconnect the belt scale signal (or set the ACI current to 4 mA), adjust the main speed command setting potentiometer to its minimum (generally not exceeding 100Ω), start the frequency converter (under load), and slowly increase the setting potentiometer resistance until the frequency converter operates at 60Hz. During the commissioning process, several operating points should be selected to accurately obtain the system control curve. In actual verification, tests should be conducted under different feeding conditions. The maximum conveying capacity is determined based on the principle that it can promptly convey materials under the actual maximum load (e.g., when the belt feed rate reaches 18-19 t/h), and relevant system data are measured. Some of the measured data are shown in Table 1 (including the frequency converter output frequency and the instantaneous flow rate of the belt scale).
Note: The data in this table are the operating parameters of an 11KW inclined belt conveyor.
② Determine the proportional relationship of the belt scale signal: First, run the frequency converter at 10Hz, connect the belt scale output signal to the frequency converter, and feed continuously and uniformly (the belt scale signal increases from 4mA; if the change is less than 0.1mA, the feeding can be considered constant). Record the feeding amount, frequency converter output (frequency), motor temperature, and other parameter values (see Table 2). Analyzing the data can verify whether the set value is reasonable. Based on the data change trend, corresponding countermeasures can be taken (if necessary, the slope of the weighing system signal output can also be modified) to adjust the proportional coefficient. The proportional coefficient of the belt scale signal in this system was finally determined to be 82%. In addition, if the set value correction is not ideal, the matching parameter correction value can also be optimized by changing the PID parameters of the belt scale (generally changing the proportional constant).
Table 2 Measurement Record of Belt Scale Signal Technical Parameters
③ Determination of the maximum frequency: When all three rotary kilns are operating at full load, the conveyor capacity of one of their inclined belt conveyors must reach at least 18.6 t/h (±0.2) to ensure that the belt does not experience material compression. At this point, the inverter output must exceed 58 Hz. Since this frequency only slightly exceeds the mains frequency, and the motor's rated speed is 1460 r/min (4 poles), its speed does not exceed 1740 r/min when operating at 58 Hz. According to motor design principles, a slight overfrequency operation is perfectly acceptable. After a relatively long period of overfrequency (60 Hz) operation, all motor operating parameters remained normal.
④ Minimum frequency determination: Since the belt conveyor is driven by a common asynchronous motor, the motor temperature rises rapidly when its operating frequency is below 10Hz (see Table 3). Although such ultra-low speed operation is rare in practice, it should be avoided as much as possible for the safety and reliability requirements of the drive system. Therefore, the minimum frequency of the system is set at 10Hz.
Table 3 Operating parameters of frequency converter driving ordinary asynchronous motors
After the system is put into operation, to reduce the frequent acceleration and deceleration adjustments of the frequency converter caused by signal fluctuations from sensors, integrators, and other controllers (which can lead to high operating temperatures of the motor and frequency converter), measures such as extending the integrator signal sampling period and increasing the PI integral constant can significantly improve the stability of the frequency converter. Additionally, adding a high-frequency filter to the ACI terminal of the frequency converter can also improve system fluctuations. An acceleration time of 4-7 seconds is suitable: if the acceleration time is too long, the belt cannot accelerate in time when the material load suddenly increases significantly, causing material compression; if the acceleration time is too short, the belt will slip due to its high inertia under load. While this setting reduces the system's dynamic responsiveness, it improves the normal operating stability of the belt conveyor.
(2) Operating parameters set for the frequency converter
The final operating parameters of the VFD110B43P frequency converter in the control system of the incised coke conveyor belt (Y160M-4, 11KW) in the first and second phases of our plant are shown in Table 1. The remaining parameters are the factory default values (i.e., the values set when the frequency converter was manufactured).
Table 1. Main Parameter Settings for VFD110B43P Frequency Inverter
5. System Performance
The petroleum coke belt conveyor uses VFD110B43P or VFD150B43P frequency converters for speed control, enabling a wide range of belt speed adjustment (0.3m/s to 1.24m/s). This significantly increases conveying capacity. From 2009 to 2010, our plant implemented frequency conversion speed control upgrades on the control systems of three belt conveyors in the petroleum coke calcination system. Three years of practice have proven that this small-range increase in motor operating frequency is entirely feasible, and the upgrade has yielded excellent results: belt conveying capacity increased by 38% (e.g., the conveying capacity of the inclined belt conveyor increased from 13.5t/h to 18.6t/h); belt service life was significantly extended. Due to the substantial reduction in belt slippage, the monthly power consumption of the conveyor decreased by approximately 26%; the average belt life was extended by more than 100%, and each conveyor reduced direct maintenance costs (such as the purchase of belts, rollers, lubricating oil, and other mechanical spare parts) by more than 8,000 yuan per year. Secondly, the application of VFD-BP series frequency converters for retrofitting also has the following advantages:
① The modification is easy to implement. It is suitable for enterprises to carry out the modification themselves, without complicated external control circuit design, easy to debug, and the system is easy to implement.
② Optimal control of the system and optimization adjustment of various parameters can be achieved by setting the relevant parameters of the frequency converter.
③ Low retrofit cost: Equipment retrofitting can be achieved simply by purchasing a frequency converter. Many existing electrical components are fully utilized, such as motors, belt scales (including controllers), control cabinets, and power cables.
④ The frequency converter operates stably and requires minimal maintenance (dust cleaning and inspection can be arranged appropriately according to the characteristics of the installation site environment).
⑤ The equipment operates efficiently. In the three years since the upgrade, the equipment has operated smoothly, and the belts have not torn or broken.
⑤ Saves energy. Under the same material conveying conditions, the motor current is reduced by an average of 4.2A.
6 Conclusion
The control system of the petroleum coke belt conveyor was upgraded using VFD-BP series frequency converters, which enabled automatic control of belt speed based on the actual feed rate. This improved the conveying capacity, reduced belt running, and significantly reduced energy consumption during material transport, saving electricity. Based on the actual current of the belt conveyor before and after the upgrade, it was preliminarily calculated that each conveyor can save more than 1,100 kWh of electricity per month, and more than 13,000 kWh of electricity per year.
About the Author
Han Min (1967-) is a male senior engineer who has worked in equipment technical management and maintenance in the electrolysis and carbon industries. He is currently engaged in equipment maintenance, technical upgrading and overhaul.
