Abstract: High-power belt conveyors are the main equipment for mine transportation. The balanced distribution of power driven by multiple motors is a major issue. This article mainly introduces the application of Hekang HIVERT series general-purpose frequency converters in mine high-power belt conveyors.
Keywords: frequency converter, power balance, master-slave control
introduction:
A belt conveyor is a continuous transportation device that uses the friction between the drive drum and the conveyor belt to drive the belt's movement. The conveyor belt serves as both the traction component and the load-bearing component.
Most domestic coal mine belt conveyors currently use power frequency drives, with frequency converters being less common. Due to the long-term operation of the motor at power frequency and issues with the efficiency of the hydraulic coupling, belt conveyor operation is highly uneconomical. Furthermore, the inability to use soft start and stop mechanisms results in severe mechanical impacts, accelerating wear. Wear and maintenance of the belt and hydraulic coupling also incur significant costs for enterprises. This is incompatible with the current goal of creating an energy-saving society. Upgrading coal mine belt conveyors to frequency converters has substantial economic and social significance for saving social energy and increasing the economic benefits of coal mines. With the continuous advancement and improvement of high-voltage frequency converter technology, its application scope is becoming increasingly wide. This article mainly focuses on the practical application of Beijing Hekang Frequency Converter's HIVERT series general-purpose high-voltage frequency converter on the belt conveyor of Taiye Coal Mine in Sanjiao Town, Linxian County, Lüliang City, Shanxi Province, to briefly introduce the application characteristics and precautions of high-voltage frequency converters in belt drive applications.
I. Basic Situation of Application Site
Taiye Coal Mine in Sanjiao Town, Linxian County, Lüliang City, Shanxi Province belongs to Shanxi Loujun Mining Group Co., Ltd., as shown in Figure (1); the mine structure diagram of Taiye Coal Mine is shown in Figure (2); the data of the high-strength belt conveyor is shown in Table (1), and the data of the two drive motors are consistent, as shown in Table (2).
Figure (1) Exterior view of Taiye Coal Mine
Figure (2) Structure diagram of the high-strength belt conveyor in Taiye Coal Mine
belt width | tilt angle | Belt speed | Conveying length | Conveying capacity |
1000mm | 25° | 2.5m/s | 665m | 600T/H |
Table (1) Data of high-strength belt conveyor
power | Rated voltage | Rated current | Power factor | Rated speed |
450kW | 10000V | 32.2A | 0.86 | 1485r/min |
Table (2) Data of dual drive motors
II. Composition of Belt Conveyors
- Drive motor 1 and motor 2, as shown in Figure (3).
Figure (3) Motors M1 and M2
2. Drag the roller and the backstop, as shown in Figure (4).
Figure (4) Roller and backstop
3. Disc brake, as shown in Figure (5)
Figure (5) Disc brake
4. Transport with a high-strength conveyor belt, as shown in Figure (6).
Figure (6) High-strength belt
III. Design of Variable Frequency Speed Control System for Coal Mine Belt Conveyors
The core issue in multi-motor variable frequency speed control systems for belt conveyors is balancing the speed and torque of each motor within the system. In practical applications, different variable frequency control schemes can be selected based on varying on-site processes.
1. "One-to-two" solution
This solution uses one frequency converter to drive two motors simultaneously, and connects the stator windings of each motor directly in parallel to the output of one frequency converter.
In the "one-to-two" configuration, the frequency converter cannot independently control the motor torque. Therefore, the output of each motor is determined by the motor parameters and belt parameters. The main factors affecting motor power balance are the differences in motor parameters and the differences in the envelope angles of the drive drum and belt. Under normal circumstances, the size of the envelope angle only exists in design and manufacturing differences. However, when the system is running normally, because the lifted goods are wet, some coal slag remains on the belt after unloading. When this slag is transferred to the drive drum, the diameter of the drive drum will increase. The greater the error, the greater the power difference between the motors in the system.
The "one-to-two" scheme has three drawbacks: first, the frequency converter cannot effectively distribute the power of the two motors; second, users need to frequently clean the slag and debris from the driven rollers; and third, the frequency converter cannot effectively protect each motor.
2. "Dual-machine linkage" scheme
This scheme uses two frequency converters to drive two motors respectively, and the stator windings of each motor are directly connected to the output of the two frequency converters.
The dual-motor synchronous operation inverter system consists of two completely independent inverters that maintain speed and power balance between the two motors through synchronous communication between the master and slave. Either motor can act as the master, and the other as the slave.
In the dual-motor drive scheme, the frequency converter can independently control the torque of the motor. The communication between the master and slave frequency converters adopts fiber optic communication, which has strong anti-interference ability and fast communication speed. Users do not need to clean the slag and debris on the driven roller. The master and slave frequency converters can automatically adjust the output speed and power of the frequency converters to be consistent.
IV. Field Application of HIVERT Series General-Purpose High-Voltage Frequency Converters
1. Selection of frequency converter
Considering the overall on-site process requirements, and since the heavy-duty belt conveyor on-site has no load, two HIVERT series general-purpose high-voltage frequency converters are used. The frequency converter data is shown in Table (3).
Inverter power | Rated voltage | Rated current | Output frequency |
630kW | 10000V | 48A | 0～50Hz |
Table (3) Inverter Data
2. Hekang frequency converter dual-machine linkage control technology
The dual-motor linkage (master-slave control) synchronous operation frequency converter system uses two or more completely independent frequency converters to ensure the speed and power balance of the two motors through synchronous communication between the master and slave. Either motor can act as the master, and the other as the slave. The synchronous control between the master and slave is achieved by our company's self-developed and manufactured "embedded synchronous controller". The transmission medium for synchronous control is fiber optic communication. This dual-motor communication method does not require the addition of additional complex synchronization equipment, and it is already integrated into the frequency converter. Only simple installation, operation, and settings are needed to achieve synchronous operation of two (or more) motors.
During operation, the system may experience power asymmetry due to operating conditions, where one motor's output significantly exceeds its rated power, while the other operates under excessively light load. Asynchronous operation of the master and slave motors can easily damage the equipment. Furthermore, prolonged operation can lead to overheating, overload protection failures, overcurrent issues, or even shutdowns. To prevent power asymmetry, the HIVERT high-voltage frequency converter, through its built-in high-speed synchronous communication, can promptly balance the overload power of the motor with higher output power during asynchronous operation to the lightly loaded motor. This maintains a balanced and symmetrical power distribution between the master and slave motors in real time, dynamically and automatically adjusting to achieve synchronous operation of both motors.
3. The block diagram of the on-site frequency converter system is shown in Figure (7).
Figure (7) Field inverter frame diagram
4. On-site power balancing solutions
When multiple inverters are operating, the power balance of the motors becomes the key control point during operation.
The HIVERT series general-purpose high-voltage frequency converters use fiber optic communication with a communication rate of up to 500kbps, which is sufficient to meet the communication needs of the field. CRC check is also added to ensure the accuracy of data transmission.
The HIVERT series general-purpose high-voltage frequency converter starts the frequency converter after the host receives a normal start signal. At the same time, it sends the running signal, running frequency and torque current to the slave. The slave starts, runs and controls the output torque according to the data sent by the host. The slave itself has torque current regulation and compares its own torque current with the torque current sent by the host. The slave will automatically adjust its own running frequency to achieve power balance between the two frequency converters. The host's operating interface is shown in Figure (8); the slave's operating interface is shown in Figure (9).
Figure (8) Host operating interface; Figure (9) Slave operating interface
V. Summary
After the frequency conversion technology upgrade, the belt conveyor is operating smoothly, completely realizing soft start and soft stop operation modes for the high-power belt conveyor, as well as dual closed-loop regulation of speed and power balance, greatly improving the system's power factor and efficiency. The upgraded system can automatically adjust the output frequency and torque according to load changes, changing the previous constant speed operation mode of the motor at the industrial frequency, thus saving a significant amount of electrical energy. The operation after the upgrade has proven that the HIVERT series general-purpose high-voltage frequency converter has unparalleled superior product performance and unsurpassable technological advantages. Its application in energy-saving upgrades in the coal industry can create huge economic and social benefits, playing an important role in creating an energy-saving and environmentally friendly society.
