Abstract: This paper introduces the application of Fengguang brand high-voltage frequency converters in coal mine belt conveyor systems, and provides detailed case studies of two control methods. Analysis of the application results demonstrates the success of the two control methods using Fengguang high-voltage frequency converters in coal mine belt conveyor systems.
Keywords: High-voltage frequency converter, belt conveyor system, master-slave control
1 Introduction
In coal mines, the transportation system mainly consists of belt conveyors, scraper conveyors, and hoists, with belt conveyors being the most numerous. Previously, to achieve soft starts for belt conveyors, hydraulic couplings or hydraulic soft starters were required. These hydraulic transmission devices involved significant maintenance, consumed a lot of energy, and generated severe impacts on the machinery, accelerating wear. Furthermore, the wear and maintenance of belts and hydraulic couplings resulted in substantial costs for enterprises, increasingly failing to meet user requirements. With the continuous advancement and improvement of high-voltage frequency conversion technology, its application scope is becoming increasingly widespread. Upgrading belt conveyors in coal mines to frequency conversion technology has significant economic and social implications for saving social energy and increasing the economic benefits of coal mining enterprises.
2. Basic Components of a Belt Conveyor System
Belt conveyors are widely used in mining, metallurgy, and coal industries. They are key equipment in coal mines, responsible for transporting loose materials or packaged goods up and down the mine shaft. Depending on the conveying process requirements, they can be used individually or in combination with other conveying equipment to form horizontal or inclined conveying systems to meet the needs of different layout types of work lines. Taking a belt conveyor system with two motors as an example, as shown in Figure 1: (1) shows the positions of the two motors above ground, and (2) shows one motor above ground and one motor underground.
Figure 1 Schematic diagram of mine conveyor belt system
The belt system mainly consists of the following parts:
(1) The head of the belt conveyor is the coal outlet of the mine. When the coal carried by the belt from the bottom of the mine passes the head position, it is automatically dumped into the mine coal yard.
(2) After unloading coal, the empty conveyor belt passes through a steering wheel, then through the driving drum driven by motor #1 and the driven drum driven by motor #2, and then passes through a guide wheel to the bottom of the well to complete one coal transportation process.
(3) At the bottom of the mine, there is a belt tensioning system. Its main function is to adjust the tightness of the belt, prevent the two drive wheels from slipping due to the belt being too loose or the car from slipping under heavy load, and prevent abnormal damage to the belt caused by the belt being too tight.
(4) Belt Conveyor Braking and Backstop Protection Devices. In addition to the inverter's protection system, belt conveyors also have their own protection measures, such as hydraulic braking systems and backstop devices. The on-site hydraulic braking system consists of one set installed on each of the two drive rollers. In a stopped or faulty state, both hydraulic brakes are in a braking state. During normal production, the brake pads are in a released state. The belt conveyor's backstop device is installed on the low-speed shaft of the reducer. In the event of a major belt conveyor malfunction and other protection measures fail, the backstop device uses mechanical force to prevent the heavy-load belt from sliding downwards.
3. Design and Analysis of Variable Frequency Speed Control System for Coal Mine Belt Conveyors
Belt conveyor systems typically use one or two motors to drive the belt, but depending on the actual operating conditions, there are also cases where three or even four motors drive the belt. This presents a multi-motor drive issue during variable frequency drive (VFD) retrofitting. According to motor principles, for a motor with a slip of 0.01 mm, a 0.2% difference in the inverter's output frequency will result in approximately 20% output torque imbalance. Under light loads, even a small difference in the inverter's output frequency can cause the inverter with the lower output frequency to enter energy feedback mode, leading to overvoltage faults. Therefore, effective control measures are generally required to balance the output of each motor. In practical applications, different VFD control schemes can be selected based on the specific on-site processes.
3.1 Direct "one-to-many" solution.
In this scheme, the stator windings of each motor are directly connected in parallel and driven by a single frequency converter. Because only one frequency converter is used, this scheme is characterized by low cost and small footprint.
In this scheme, power balance is particularly important. Uneven output from the two motors will inevitably lead to one motor being overloaded and the other underloaded. In severe cases, the overloaded motor may burn out, and it could even damage the IGBT module in the inverter's main circuit, preventing the inverter from independently controlling the torque of each motor. The output of each motor is determined by its parameters and the belt system parameters. The main factors affecting motor power balance are differences in motor parameters, the diameter error of the motor's drive roller, and the difference in belt envelope angle. The greater the error, the greater the power difference between the motors in the system. Under normal circumstances, without human design variations, these errors are generally manufacturing errors.
The diameter error of the motor drive roller will cause motor power error in the initial production stage, but due to the physical characteristics of the belt system, this error will gradually decrease after a period of use and wear.
For applications with lower power, fewer motors (generally no more than 3), and lower cost, a one-to-many parallel operation scheme can be selected, which will greatly reduce the purchase price of the variable frequency speed control system.
3.2 Multi-frequency converter coordinated control scheme
For applications requiring active control to balance the output of each motor, a master-slave control scheme can be adopted.
Each motor on site is equipped with one frequency converter, and all high-voltage frequency converters in the system are uniformly coordinated and controlled by a "coordination control system." This coordination control system coordinates the operating commands of each frequency converter based on the motor operating status feedback. Each frequency converter independently controls its own motor according to the command, ensuring that all motors operate at the same speed and output. The control process is as follows:
Using one inverter as the master and the others as slaves, the output current of the inverters is sampled and compared. By changing the control signals of the slaves, they are made to always follow the master's changes. When the slave's current is greater than the master's, the slave's setpoint signal is decreased, thereby reducing the slave's output frequency, motor speed, load, and current. When the slave's current is less than the master's, the slave's setpoint signal is increased, thereby increasing the slave's output, motor speed, load, and current. Ultimately, this ensures that the motor load is basically consistent and the current is within the allowable range.
4. Working principle of the belt conveyor electrical control system
Belt conveyors are not used alone underground; multiple conveyors often work together. Therefore, the startup sequence must be such that the next level of equipment can only start after the previous level has started. After the preceding interlock is released, the status of each part of the belt conveyor's electrical control system is detected and judged. Only after the conditions for starting are met can the conveyor be started. The control principle of the belt conveyor's electrical control system is shown in Figure 2.
(1) When starting the machine, the belt control first judges the signals transmitted by the belt system, such as deviation, coal pile-up, longitudinal tearing, emergency stop lock-up, etc. Only if all signals are normal can the next procedure be entered; secondly, the brake and switch status are checked according to the signals returned by the sensors of the belt head; finally, the belt tension is judged to be within the allowable starting range.
(2) The start signal will be issued only when all parts of the belt conveyor are in normal condition and the conditions for starting are met. After the start command is issued, the frequency converter starts. The control panel issues a frequency conversion operation signal, and the frequency converter outputs power with varying frequency and voltage according to the operation signal given by the control panel, controlling the motor to start softly according to the given "S" curve.
(3) At the same time, the control panel issues a command to open the brake and checks whether the brake is fully open. If the brake is not open or not fully open, the running signal will be turned off, and a brake signal will be sent to the inverter to make the inverter stop urgently, so as to prevent the inverter from tripping due to overcurrent due to stall.
(4) When the belt reaches the rated speed, the electrical control system detects the belt speed and the roller speed in real time. When the speed difference between the two exceeds the specified value, the system will stop in an emergency and issue a slippage alarm signal.
(5) The system monitors the motor temperature, reducer temperature, bearing temperature, belt tension, motor current, brake status, frequency converter status, and PROMOS status in real time. When any indicator or status is abnormal, the system will stop in an emergency and trigger an alarm.
Figure 2 Control principle of belt conveyor system
5 Application Cases of "One-to-Many" Solutions
Shanxi Kangwei Group Nanshan Coal Mine Co., Ltd. is located approximately 32.5 km northwest of Qinyuan County and 2.5 km southeast of Lingkongshan Town, west of Wangzhuang Village, administratively under the jurisdiction of Lingkongshan Town. The mine has an approved production capacity of 900,000 tons/year, a mining area of 8.1554 km², and is approved to mine coal seams 1-11, with a service life of 28.7 years. The mine is designed using inclined shaft development, fully mechanized mining, and a caving roof management system. The entire mining area is divided into 17 mining zones, with the coal seams mined in a descending sequence: coal seam 1, coal seam 2 → coal seam 3 → coal seam 6 → coal seam 11.
Parameters for the conveyor belt system at Nanshan Coal Mine: Single-journey belt length 1200m, belt width 1m. Maximum belt speed is 2m/s when the motor operates at 50Hz. Mine inclination angle 180°. Two motors operate simultaneously at the mine entrance to drive the conveyor belt system; both motors have identical nameplate parameters.
The main parameters on the nameplates of the two motors at the wellhead are shown in Table 1.
The site uses a JD-BP38-710P high-voltage variable frequency speed control system manufactured by Shandong Xinfengguang Electronic Technology Development Co., Ltd., with a rated voltage of 10kV, a rated current of 51A, and a rated power of 710kW. It adopts a "one-to-two" control scheme, and the main circuit control wiring is shown in Figure 3.
Figure 3. Wiring diagram for a one-to-two control system.
On-site, a 710kW frequency converter was used to simultaneously drive two 315kW motors. The frequency converter monitored the current values of two parts: the total output current of the frequency converter and the operating current of motor #1. In this way, the frequency converter could calculate the operating current of motor #2, and by comparing the operating currents of motors #1 and #2, adjust the output of the frequency converter to balance the power of the two motors.
The frequency converter was commissioned on December 1, 2010. After the simulation test and the connection signal with the control panel were normal, the light-load linkage test was also successful. On December 2, during the heavy-load test of the equipment with coal loading, a slight current imbalance occurred between motors #1 and #2. After adjusting the belt envelope angle by adjusting the guide wheel of motor #1, the output current of the two motors was basically the same, and the equipment was successfully put into operation. The equipment has not experienced any failures to date.
6 Application Cases of Multi-Frequency Converter Coordinated Control Scheme
Daoqing Coal Mine of Tonghua Mining Group Co., Ltd. was established in 1967. It is located at the foot of Changbai Mountain and on the banks of the Hunjiang River in western Baishan City, bordering Tonghua City and adjacent to Daoqing Coal Preparation Plant. The north-south highway and the Tonghu Railway run side by side through the mining area. As of the end of 2006, the mining area had geological reserves of 45.804 million tons and an annual production capacity of 750,000 tons.
Parameters for the conveyor belt system at Daoqing Coal Mine: Single-journey belt length 1200m, belt width 1.2m. Maximum belt speed is 2m/s when the motor operates at 50Hz. Mine inclination angle 25 degrees. Two motors operate simultaneously at the mine entrance to drive the conveyor belt system; both motors have identical nameplate parameters.
The main parameters on the nameplates of the two motors at the wellhead are shown in Table 2.
Two sets of JD-BP37-450P high-voltage variable frequency speed control systems produced by Shandong Xinfengguang Electronic Technology Development Co., Ltd. were used on site. The rated voltage is 6kV, the rated current is 54A, and the rated power is 450kW. The "one-to-one" master-slave control scheme is adopted, and the main circuit control wiring is shown in Figure 4.
Figure 4. Wiring diagram for one-to-one control
For the frequency conversion control of a two-motor driven belt conveyor, in order to effectively achieve load balance control of the motors, the high-voltage frequency conversion speed regulation system is suitable for master-slave control. On-site, two 450kW frequency converters are used to control two 400kW high-voltage motors respectively, as shown in Figure 5. Frequency converter #1 is selected as the master frequency converter, and frequency converter #2 as the slave frequency converter. The control panel sends the set frequency value to the master frequency converter. The master and slave frequency converters communicate via ModBus bus. Frequency converter #1 acts as the master controller, detecting the operating current of motors #1 and #2, issuing the output torque set value, and controlling the #2 slave frequency conversion speed regulation system to operate synchronously.
Figure 5. Simplified diagram of master-slave control of belt conveyor frequency converter
Both the master-controlled and slave-controlled high-voltage variable frequency speed control systems have external signal interlocking control and status/alarm logic signal outputs. The master-controlled system receives local or remote "start," "stop," and "emergency stop" commands, as well as an "emergency stop" command from the slave system. The slave system receives "start" and "emergency stop" commands from the master system, and also receives local or remote "emergency stop" commands from its own system. Logic interlocking is implemented in the PLC logic programs of both systems, allowing them to start and stop simultaneously.
The system was successfully put into operation on July 12, 2010, and has been running normally without any malfunctions.
7. Advantages of using high-voltage frequency converters to drive belt conveyors
(1) Truly achieve soft start of belt conveyor. By starting the belt conveyor slowly with the motor, the energy stored inside the belt is released slowly, which can minimize the impact generated when the conveyor starts and stops, and hardly cause any damage to the belt;
(2) Reduce belt strength. Since the start-up time of the frequency converter can be adjusted from 1 to 3600s, the start-up time of the belt conveyor is usually set from 60 to 120s depending on the site conditions. The extended start-up time greatly reduces the requirements for belt strength and reduces the initial investment in equipment. In practical applications, due to the reduction in start-up impact, the wear and tear of the mechanical system is also reduced, especially the life of the idler rollers and drums is greatly extended;
(3) Achieve torque balance when the belt conveyor is driven by multiple motors. When driving multiple motors, a master-slave or coordination cabinet control method is adopted to achieve torque balance;
(4) Belt inspection function. The low-speed belt inspection function is a requirement for belt conveyor maintenance. The frequency conversion speed control system is an AC drive system with stepless speed regulation, which can adjust any belt speed within the range of 0 to 100% of the rated belt speed under no-load belt inspection conditions;
(5) Smooth heavy-load start-up. The frequency converter can output 2.2 times the rated torque when operating at low frequency, which is suitable for heavy-load start-up;
(6) Automatic speed regulation. The frequency converter, in conjunction with the coal flow sensor, can automatically adjust the conveyor belt speed according to the load, saving electricity and reducing belt wear;
(7) Energy saving. Due to the special production conditions of coal mines, the coal output is sometimes extremely uneven, and the coal transport volume of the belt conveyor system is also uneven. When the load is light or there is no load, the high-speed operation of the belt conveyor system causes serious wear and waste of the mechanical transmission system, and the power consumption is much greater than that of the low-speed operation. However, due to production needs, the belt conveyor system cannot be stopped at any time. A separate control system is used to measure the load of the upstream transport system and the load of the machine transport system separately. This allows the frequency converter to be controlled to reduce speed or increase speed in advance. For belt conveyor systems with uneven loads, power can be saved and belt wear can be reduced.
8. Conclusion
The successful application of the high-voltage variable frequency speed control system for belt conveyors produced by Shandong Xinfengguang Electronic Technology Development Co., Ltd. in Baishan Daoqing Coal Mine and Changzhi Nanshan Coal Mine fully verifies that the Fengguang brand high-voltage variable frequency speed control system can realize the "one-to-many control" and "master-slave control" technologies of belt conveyor systems. Practice has proven that utilizing the advanced, mature, and safe high-voltage variable frequency speed control system can comprehensively and efficiently adjust on-site operating conditions, improve system automation, and save energy. Using high-voltage frequency converters can significantly reduce on-site maintenance, bringing considerable benefits and effectively responding to the national call for energy conservation and emission reduction, making it worthy of widespread promotion.
