4 Type Belt Conveyor Soft Start In Coal Mine

Belt conveyor is important continuous transportation equipment for raw coal in coal mines. With the continuous increase of mine transportation, the installed power of the belt conveyor has also increased from the original single-drive 90kW to the current four-wheel drive 1850kW. During the driving operation of the belt conveyor, the drive system is required to provide an adjustable and smooth starting torque without impact force to reduce the dynamic tension the operation of the belt conveyor. The belt conveyor has the following characteristics in the actual use process:

①Large amount of coal.

②Due to manual inspection of iron debris, large gangue, etc., there is a need for heavy-load start-stop equipment.

③ Check the tape joint needs to run slowly.The belt conveyor is required to stop smoothly under heavy loads and start smoothly under heavy loads.

Status of coal mine belt conveyor drive technology

At present, the soft start methods of belt conveyors used in A coal mine (One mine in China) have been maturely used: speed regulating hydraulic coupling, CST soft start, asynchronous motor variable frequency soft start, and synchronous motor variable frequency soft start. Except for not using the latest magnetic coupler, the main soft start is used. Among them, the No. 1 belt conveyor in lane 232204 was put into use for the first time with high-power permanent magnet synchronous variable frequency drive, and the equipment has performed well since its operation.

Variable speed hydraulic coupling

The speed-regulating hydraulic coupling consists of a pump wheel, a turbine, a rotating casing, a guide tube, and the like. The pump wheel converts the mechanical energy of the motor into the kinetic energy and potential energy of the working oil, while the turbine converts the kinetic energy and potential energy of the working oil into the mechanical energy of the output shaft, thereby realizing the flexible transmission of energy. 

The advantages of the speed-regulating hydraulic coupler:

①It can realize the no-load start of the motor and reduce the impact on the power grid.

②With overload protection function, it can protect the transmission equipment.

③ The structure is simple.

Disadvantages of the speed-regulating hydraulic coupler:

① The control precision is low, and the start-up process from start to full speed is not stable.

② Low efficiency. Due to the existence of slip, the efficiency loss is 8%~10%, which is lost in the form of heat energy, which is not conducive to energy saving.

③ The power unbalance is about 8%.

④The amount of maintenance in the later period is large.

CST soft start mode

Controllable start transmission device (hereinafter referred to as “CST”) is a driving system developed and produced by Dodge Lance Electric Company, which is specially used for large and heavy-duty belt conveyors. The connection method between the equipment and the driving roller of the belt conveyor is: asynchronous motor + CST + driving roller. CST has the dual functions of deceleration and soft start, and can control the wet clutch through the electro-hydraulic control system, so as to drag the belt conveyor to the rated belt speed within the set time, or make the belt speed at the set speed when stopped. time to zero. At the same time, the multi-drive power balance can be achieved through the PLC program, which is widely used in belt conveyors of 1.4m and above in A coal mines.

There are the following deficiencies in the actual installation and use process:

(1) The planetary gear reduction system and the mucus transmission are combined into one, which affects each other, and the installation and debugging are complicated. The equipment itself has many monitoring sensors, filters, and monitoring points. The corresponding equipment has many fault points during operation, and equipment installation and maintenance. A lot of work.

(2) It has high requirements for oil quality. It is imported Mobil 424 hydraulic transmission oil, and there is no domestic substitute. The service life of oil products is about 6 months. Once the oil quality is deteriorated if the replacement is not timely, the equipment failure rate and the frequency of filter replacement will be sharply increased. Overall maintenance costs are high.

(3) Equipment maintenance is highly dependent on the manufacturer’s after-sales service, and equipment debugging and matching require on-site debugging by the manufacturer’s technicians. Due to the decrease in the number of CSTs purchased by Ningxia Coal Industry Co., Ltd. of the National Energy Group, some CSTs have a service life of more than 8a, and all CSTs have different degrees of fatigue and wear, with high failure rates and high maintenance costs. At the same time, due to various reasons, the enthusiasm of the after-sales personnel of the manufacturer is low, and they cannot provide timely and effective after-sales service. Once the CST box fails, it needs to be dismantled and transported to the original factory to open the cover for maintenance. The maintenance cycle is long and the passiveness is large.

(4) The whole set of equipment is large in size and requires a large installation space. In order to ensure the heat dissipation effect and the safe maintenance distance when the three-wheel drive is driven, the construction work of the driving chamber is relatively large.

Variable frequency drive mode of asynchronous motor

The principle of variable frequency drive of asynchronous motor: the rotation speed of the motor is n=60f/p (f is the frequency of the power supply, and p is the number of pole pairs of the motor). The frequency of the power supply of the equipment is adjusted by the inverter and then supplied to the motor to achieve the purpose of adjusting the rotation speed of the motor.

The inverter is composed of rectifier unit, reactance unit, filter unit and inverter unit.

The connection method between the equipment and the driving roller of the belt conveyor is: asynchronous motor + reducer + driving roller. The variable frequency speed regulation of the asynchronous motor can realize the smooth transition of the speed of the asynchronous motor from 0 to the rated speed, reduce the impact on the mechanical part, and reduce the starting tension of the tape; it can realize that multiple frequency converters maintain the same speed setting, so that each drive motor Synchronous operation to achieve multi-drive power balance, the adjustment accuracy is 0.1Hz.

There are the following deficiencies in actual installation and use:

(1) There are many electronic control units, the equipment installation space and safety distance are required to be large, and the installation and retraction workload is large. Coal mine 1118104 machine lane No. 1 belt conveyor drive system uses asynchronous frequency conversion speed control system. The 3 sets of drive department have a total of 12 frequency conversion supporting equipment and 9 frequency converter and reducer supporting equipment low-voltage switches.

(2) When the equipment is installed, there are many high-voltage cable joints between the electronic control units, the electrician wiring workload is heavy, and the installation period is long.

(3) The asynchronous motor still needs to be equipped with a reducer, the mechanical efficiency is low, and the daily maintenance and maintenance workload of the reducer is large.

(4) The output harmonics of the inverter have an impact on the underground power grid and affect the stable operation of the power grid.

Permanent magnet variable frequency drive technology

The main difference between permanent magnet variable frequency drive and asynchronous variable frequency drive is the configuration of the motor. The permanent magnet variable frequency drive is equipped with a rare earth permanent magnet motor, while the asynchronous variable frequency drive is equipped with a three-phase asynchronous motor. Compared with induction motors, permanent magnet synchronous motors do not need reactive excitation current, can improve power factor (up to 1, or even capacitive), reduce stator current and stator resistance losses, and have no rotor copper loss during stable operation, In turn, the fan (small-capacity motor can even remove the fan) and the corresponding wind friction loss can be reduced, and the efficiency can be increased by 2 to 8 percentage points compared with the induction motor of the same specification. Moreover, the permanent magnet synchronous motor can maintain high efficiency and power factor within the range of 25% to 120% of the rated load, which makes the energy saving effect more significant during light load operation. Motors generally have a start winding on the rotor, which has the ability to start directly at a certain frequency and voltage. In addition, the permanent magnet variable frequency motor will automatically adjust the frequency to change the motor power according to the changes of the user-end equipment, and match the output power of the motor well. Whether it is in the case of light load or overload, the work efficiency still maintains a high level. In the early stage of development, the permanent magnet motor was limited by the power of the process and could not meet the conditions of high-power working conditions. With the improvement of the process, the maximum installed power of a single machine can reach 1000kW.

The advantages of permanent magnet variable frequency drive technology:

High efficiency

The permanent magnet motor has no excitation loss, no rotor winding, no rotor copper loss, and no rotor iron loss. The efficiency is 3%~12% higher than that of the same capacity asynchronous motor, and higher efficiency can be obtained at 20%~120% load.

Small size

Compared with the asynchronous variable frequency drive, the permanent magnet variable frequency drive reduces the reducer, and at the same time, the electronic control part reduces 15 high and low voltage equipment, which reduces the construction volume of the installation chamber and the labor intensity of equipment installation, and shortens the construction period.

The starting torque is large

Conventional asynchronous motors have limited starting torque multiples and maximum torque multiples. In order to meet the starting requirements, an asynchronous motor with a larger capacity needs to be selected, and the efficiency and power factor are low. Due to the high number of stages, the permanent magnet synchronous motor can output torque up to several times that of the asynchronous motor, which can meet the starting torque requirement under heavy load.

Application comparison of 4 different soft start methods

Through the analysis of the procurement cost and post-maintenance cost of the 4 soft-start methods (compared with the use cycle 5a), the comprehensive use cost from low to high is the debugging hydraulic coupler, asynchronous variable frequency drive, synchronous variable frequency drive, CST soft start.

Comparing the installation difficulty of the four soft-start devices and the amount of construction work in the installation chamber, it can be seen that the order from easy to difficult is the speed-regulating hydraulic coupling drive, synchronous variable frequency drive, CST soft start, and asynchronous variable frequency drive. Through analysis, it can be concluded that

The speed regulating hydraulic coupler is mainly used for the transportation volume of coal less than 1000t/h, single drive or double drive drive with lower power balance requirements. The characteristics are: simple installation and low cost of use.

CST soft start, asynchronous variable frequency drive and synchronous variable frequency drive are mainly used for large coal transportation. Among them, due to the harsh conditions such as wet underground environment and large coal dust, CST soft start equipment has high use cost and insufficient maintenance cost with the increase of years.

Compared with the synchronous motor, the asynchronous motor has the advantages of no need to replace the motor and less one-time investment. However, due to the long installation period of electrical equipment, large workload, and insufficient start-stop characteristics under heavy load, through the analysis of the starting torque of the asynchronous motor and synchronous motor, it can be seen that the synchronous motor has a good heavy-load starting curve, which can reach 3~ 4 times rated torque start.

Summary of belt conveyor soft start

Through the analysis of the four soft-start methods, different application scenarios of the four soft-start methods can be summarized. At the same time, with the development of the synchronous motor technology, the cost is reduced, and the synchronous variable frequency drive enhances the safety and efficiency of the coal flow transportation system in the fully mechanized mining face. Reliability, eliminates the situation that can not start with load due to fault shutdown, the power saving efficiency is obvious, and has high practical application value and application prospect.