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英文文献
A Comparison of Soft Start Mechanisms for Mining Belt Conveyors
Michael L. Nave, P.E.
CONSOL Inc.
1800 Washington Road Pittsburgh, PA 15241
Belt Conveyors are an important method for transportation of bulk materials in the mining industry. The control of the application of the starting torque from the belt drive system to the belt fabric affects the performance, life cost, and reliability of the conveyor. This paper examines applications of each starting method within the coal mining industry.
INTRODUCTION
The drive pulley via friction between the drive pulley and the belt fabric must transmit the force required t
o move a belt conveyor. In order to transmit power there must be a difference in the belt tension as it approaches and leaves the drive pulley. These conditions are true for steady state running, starting, and stopping. Traditionally, belt designs are based on static calculations of running forces. Since starting and stopping are not examined in detail, safety factors are applied to static loadings (Harrison, 1987). This paper will primarily address the starting or acceleration duty of the conveyor. The belt designer must control starting acceleration to prevent excessive tension in the belt fabric and forces in the belt drive system (Surtees, 1986). High acceleration forces can adversely affect the belt fabric, belt splices, drive pulleys, idler pulleys, shafts, bearings, speed reducers, and couplings. Uncontrolled acceleration forces can cause belt conveyor system performance problems with vertical curves, excessive belt take-up movement, loss of drive pulley friction, spillage of materials, and festooning of the belt fabric. The belt designer Is confronted with two problems, The belt drive system must produce a minimum torque powerful enough to start the conveyor, and controlled such that the acceleration forces are within safe limits. Smooth starting of the conveyor can be accomplished by the use of drive torque control equipment, either
mechanical or electrical, or a combination of the two (CEM, 1979).
SOFT START MECHANISM EV ALUATION CRITERION
pulleysWhat is the best belt conveyor drive system? The answer depends on many variables. The best system is one that provides acceptable control for starting, running, and stopping at a reasonable cost and with high reliability (Llewellyn and Sudarshan, 1978).
Belt Drive System
For the purposes of this paper we will assume that belt conveyors are almost always driven by electrical prime movers (Goodyear Tire and Rubber, 1982). The belt "drive system" shall consist of multiple components including the electrical prime mover, the electrical motor starter with control system, the motor coupling, the speed reducer, the low speed coupling, the belt drive pulley, and the pulley brake or hold back (Zur, 1986). It is important that the belt designer examine the applicability of each system component to the particular application. For the purpose of this paper, we will assume that all drive system components are located in the fresh air, non-permissible, areas of the mine, or in non-hazardous, National Electrical Code, Article 500 explosion-proof, areas of the surface of the mine.
Belt Drive Component Attributes
Size.
Certain drive components are available and practical in different size ranges. For this discussion, we will assume that belt drive systems range from fractional horsepower to multiples of thousands of horsepower. Small drive systems are often below 50 horsepower. Medium systems range from 50 to 1000 horsepower. Large systems can be considered above 1000 horsepower. Division of sizes into these groups is entirely arbitrary. Care must be taken to resist the temptation to over motor or under motor a belt flight to enhance standardization. An over motored drive results in poor efficiency and the potential for high torques, while an under motored drive could result in destructive over speeding on regeneration, or overheating with shortened motor life (Lodi, etal., 1978).
Torque Control.
Belt designers try to limit the starting torque to no more than 150% of the running torque (Cema, 1979; Goodyear, 1982). The limit on the applied starting torque is often the limit of rating of the belt carcass, belt splice, pulley lagging, or shaft deflections. On larger belts and belts with
optimized sized components, torque limits of 110% through 125% are common (Elberton, 1986). In addition to a torque limit, the belt starter may be required to limit torque increments that would stretch belting and cause traveling waves. An ideal starting control system would apply a pretension torque to
the belt at rest up to the point of breakaway, or movement of the entire belt, then a torque equal to the movement requirements of the belt with load plus a constant torque to accelerate the inertia of the system components from rest to final running speed. This would minimize system transient forces and belt stretch (Shultz, 1992). Different drive systems exhibit varying ability to control the application of torques to the belt at rest and at different speeds. Also, the conveyor itself exhibits two extremes of loading. An empty belt normally presents the smallest required torque for breakaway and acceleration, while a fully loaded belt presents the highest required torque. A mining drive system must be capable of scaling the applied torque from a 2/1 ratio for a horizontal simple belt arrangement, to a 10/1 range for an inclined or complex belt profile.
Thermal Rating.
During starting and running, each drive system may dissipate waste heat. The waste heat may be liberated in the electrical motor, the electrical controls,, the couplings, the speed reducer, or the belt braking system. The thermal load of each start Is dependent on the amount of belt load and the duration of the start. The designer must fulfill the application requirements for repeated starts after running the conveyor at full load. Typical mining belt starting duties vary from 3 to 10 starts per hour equally spaced, or 2 to 4 starts in succession. Repeated starting may require the debating or over sizin
g of system components. There is a direct relationship between thermal rating for repeated starts and costs.
Variable Speed.
Some belt drive systems are suitable for controlling the starting torque and speed, but only run at constant speed. Some belt applications would require a drive system capable of running for extended periods at less than full speed. This is useful when the drive load must be shared with other drives, the belt is used as a process feeder for rate control of the conveyed material, the belt speed is optimized for the haulage rate, the belt is used at slower speeds to transport men or materials, or the belt is run a slow inspection or inching speed for maintenance purposes (Hager, 1991). The variable speed belt drive will require a control system based on some algorithm to
regulate operating speed.
Regeneration or Overhauling Load.
Some belt profiles present the potential for overhauling loads where the belt system supplies energy to the drive system. Not all drive systems have the ability to accept regenerated energy from the load. So
me drives can accept energy from the load and return it to the power line for use by other loads. Other drives accept energy from the load and dissipate it into designated dynamic or mechanical braking elements. Some belt profiles switch from motoring to regeneration during operation. Can the drive system accept regenerated energy of a certain magnitude for the application? Does the drive system have to control or modulate the amount of retarding force during overhauling? Does the overhauling occur when running and starting?
Maintenance and Supporting Systems.
Each drive system will require periodic preventative maintenance. Replaceable items would include motor brushes, bearings, brake pads, dissipation resistors, oils, and cooling water. If the drive system is conservatively engineered and operated, the lower stress on consumables will result in lower maintenance costs. Some drives require supporting systems such as circulating oil for lubrication, cooling air or water, environmental dust filtering, or computer instrumentation. The maintenance of the supporting systems can affect the reliability of the drive system.
Cost.
The drive designer will examine the cost of each drive system. The total cost is the sum of the first capi
tal cost to acquire the drive, the cost to install and commission the drive, the cost to operate the drive, and the cost to maintain the drive. The cost for power to operate the drive may vary widely with different locations. The designer strives to meet all system performance requirements at lowest total cost. Often more than one drive system may satisfy all system performance criterions at competitive costs.
Complexity.
The preferred drive arrangement is the simplest, such as a single motor driving through a single head pulley. However, mechanical, economic, and functional requirements often necessitate the use of complex drives. The belt designer must balance the need for sophistication against the problems that accompany complex systems. Complex systems require additional design engineering for successful deployment. An often-overlooked cost in a complex system is the cost
of training onsite personnel, or the cost of downtime as a result of insufficient training.
Multiple Drives.
A simple belt is often driven by one drive pulley. Multiple, independent pulleys drive some more comple
x belts. These pulleys may be near each other, or at different belt locations. Multiple drives allow the belt designer to increase the driven horsepower, while maintaining or reducing belt fabric tensions. Multiple drives require a drive starting and running system that allows for coordination between drives. Multiple drives of different sizes or different belt wrap angles may require a load proportion scheme (load sharing). Load sharing requires one of the drives to operate at a lower torque rating during starting, or also during running conditions. Will the multiple drive belt system operate with one or more prime movers out of service? Multiple drives at different locations will require a distributed control system (Gallina, 1991; Sur, 1987).
Conveyor Jam.
It is possible for a running belt conveyor to encounter a mechanical jam of the belt fabric. The drive system will continue to impart torque to the fabric up to the slip of the fabric on the drive pulley. Different drives vary in the application of breakdown torque to the stalled machine. It is important for the drive designer to examine the rotating inertia in the drive system. The prime mover motor rotor and all other rotating parts may contribute significant kinetic energy to the stalled belt. The drive response to the stall and the application of the torque limit may vary.
Control System.
Each drive system will require a base case permissive control system for starting and running supervision. Most belt drive systems today use some form of computer control. The computer control systems all depend on field sensors for measurement and reporting of drive parameters. The belt drive designer must determine the minimum required number and location of the field devices for adequate control. The drive control system will require power switchgear and control switchgear with provisions for "lockout" for conveyor maintenance and service. Lockout applies to sources of electrical, hydraulic, pneumatic, and gravity energy.
SOFT START METHODS OVERVIEW
The system approach will group the electrical prime mover with control, the high and low speed couplings, the speed reducer, and the drive pulley for examination. The belt conveyor will require a force to initiate movement, termed breakaway torque. In movement, the conveyor will

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