10.11.2011 | Autor / Editor: N.K. Romani / Marcel Dröttboom
The ability to control the drive acceleration torque providing a smooth soft start while maintaining belt tensions within specified safe limits is critical for belt conveyors performance. Soft controlled start drives on a long conveyor with multiple drives protect the belt and other conveyor components and help reduce investments costs.
In the good old days, a single horse driven cart was extended to two, four or six horse stage coach to cope with the increasing demand of passenger load. To ensure comfort to passengers from a sudden starting ‘jerk’, the horses were trained to move the coach very gently and gracefully. And when the horses galloped to full speed, they were whipped and controlled to ensure proper load share, thus avoiding an overturn or failure of components. Similarly, when belt conveyors grow longer, from 1 to 2 kilometres to 8 to 15 kilometres and haul 3000 to 4000 tonnes per hour over significant inclines and declines, the complex profiles place unusually stringent demands on the starting/stopping procedures, drive configuration on primary and secondary pulleys at the head end, tripper stations in the mid-sections and the booster station at the tail end.
A conveyor belt is a non linear visco-elastic medium in which strain can propagate at some finite wave speed. Long conveyor belt is a gigantic rubber band full of elasticity and load inertia dynamics which can result in destructive consequences if not properly understood. Perhaps the most difficult conveyor characteristics to overcome are the velocity and tension waves present in the belt. In a conveyor with head drives only, these waves tend to propagate to the slack side of the belt where they are attenuated by friction and the take-up mechanism. When a fully loaded conveyor belt is started abruptly or with no acceleration control, high acceleration forces induce tension waves that can adversely affect the belt fabric, belt splices, drive pulleys, idler pulleys, shafts, bearings, speed reducers, motors and couplings. This can cause performance problems in vertical curves, excessive belt take-up movement, loss of drive pulley friction, spillage of materials, and festooning of the belt fabric. Therefore the drive system must produce a minimum torque powerful enough to start the conveyor, and controlled such that the acceleration forces are within safe limits.
The ‘break away’ is the most crucial zone of conveyor going into motion from zero speed. There should be no ‘jerk’. Generally, drive systems offer a linear velocity ramp (to minimize static acceleration force), or do not attempt to attain any particular shape of the velocity ramp. To understand better the velocity/acceleration ramp relationship, let us assume a conveyor starts with velocity increasing linearly at 0.1 meter per second. The acceleration will remain constant at 0.1 m/s2. After 40 seconds the belt attains its full speed of 4 m/s. At this point the acceleration will drop to zero. This is not a good method of starting a conveyor as the jerk at the very first instant will be a huge spike, then remains constant, which could be detrimental to conveyor performance. If the belt splice section happens to be at the drive pulley at the instance when a fully loaded belt is started abruptly, the starting jolt can rip the belt.
On the other hand, if velocity is controlled such that it makes an ‘S’ shape curve (Fig. 1), the acceleration curve will be sinusoidal. The jerk curve, in this case, is continuous except at the beginning and end of the acceleration ramp.
V(t) = V/2 · (1 – cos(π/t ·T))
V = Design running speed of the belt [m/s]
t = time [s]
T = time to accelerate the stationery belt to speed V [s]
Starting the conveyor this way will have almost no ‘jolt effect’, thereby no chances to propagate velocity and/or tension waves. It should be observed closely that when velocity is 50 per cent, acceleration is 100 per cent whereas the jerk is zero. This is the zone where motor will deliver its maximum power or torque, thereafter, as velocity continue to increase to reach 100 per cent speed, acceleration goes down sinusoidal and jerk goes negative. It has been shown  that having an ‘S’ shaped velocity curve (or a sinusoidal acceleration curve); the belt safety factor can be reduced by approximately 15 per cent. For long conveyors, this reduction will typically yield cost savings that exceed any price premium for the right drive system. The belt cost usually constitutes at least 25 to 35 per cent of the total investment on a major conveyor project. If you are putting a 10 km long conveyor, as a thumb rule of 1 million USD per kilometres, you are investing roughly USD 10 million. The belt itself may cost around USD 3.5 million. By using an ‘S’ shaped velocity curve, you can switch over to a belt of next lower ST rating, saving 10 to 15 per cent on the cost. Besides this, having to have the peak belt stress, peak motor starting torque and peak tensions reduced by 15 per cent, a significant reduction in belt weight leads to selection of more economical idlers, less stringent specifications for the pulleys, as well as smaller motors, drive systems, couplings, holdbacks etc., which finally yields tremendous amount of cost savings .
The need for longer velocity ramps has led to the “30 seconds per kilometre (of conveyor length)” thumb rule. For belts 1 to 3 kilometres, the ‘S’ velocity ramp time is generally kept anywhere between 40 to 100 seconds, but for conveyors exceeding 3 kilometres, the results of the dynamic analysis can be used for determining an acceptable velocity ramp time.
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