Conventional belt conveyors are powered by electric or electro-mechanical drives mounted at the conveyor’s head, tail, intermediate return span, or a combination of these. The following are main elements influencing the selection of the drive system: 1) dynamic characteristics, 2) economic factors such as capital cost, plant spares compatibility, reliability, availability, maintainability, and 3) environmental constraints.
The principal dynamic characteristics of concern are the reactions of belt forces generated by the drive system during starting of the conveyor.
Another attribute for consideration is the application of multiple driven pulleys used to 1) lower the conveyor’s belt tension requirements, 2) increase the expected tonnage throughput, allowing for possible drive component failures, and 3) increase the plant spares flexibility related to the drive assembly.
Soft Start Control
It is a known fact that with larger conveyor sizes it becomes economically advantageous to control the motor torque imparted to the belt during the starting process. The analysis of this starting process becomes more complex with increasing conveyor length, mass and power requirements.
Conventional AC squirrel cage induction motors, directly coupled to the conveyor, impart torques prescribed by their electrical speed-torque curve, which usually exceeds the nameplate torque rating by more than 200 percent. The magnitude of an uncontrolled (across-the-line) motor start associated with considerable belt length, mass, and power, generates a number of possible adverse effects during the start such as:
- Material load instability on sloped belts: The material acceleration force exceeds the net incline resistance of the material on the belt (e.g. sensitivity of prills and slimes).
- Mechanical equipment shock: The motor locked-rotor torque, as shown in Fig. 2, is rendered to the mechanical components at a very high stress rate which requires an additional service factor to be placed on the equipment.
Example: To upgrade the service factor from 1.25 to 1.50 means the mechanical components of bearings gears, etc. must be built for a 1.8 times equivalent life expectancy of the 1.25 service factor unit. - Belt overstressing: The belt manufacturers limit the amount of momentary overstressing to approximately 140 percent.
The control then accelerates the conveyor at a linear rate per a preset time. If the normal full load running resistance and mass have been calculated correctly, the total full acceleration torque can be preset by the acceleration time. There are two distinct advantages to this scheme. First, the acceleration torque can be programmed not to exceed the maximum breakaway torque, and second, since the acceleration time is fixed, the acceleration torque requirement reduces with the decrease in material mass on the belt. This allows the vertical curves to be designed to minimum values. Tripper stresses and belt lifting are also minimized. The torque limit control, set to a maximum design value, disengages power when the applied torque reaches the set limit.
Dual Drives
Conveyors driven by two pulleys, usually in tandem, offer a number of design advantages. They also compromise the design with some not too clear trade-off conditions on reliability, performance, and cost. The questions concerning 1) when the dual drive should be considered, and 2) what configuration of size and quantity offers the greatest utility are the objectives of the following discussion.
The dual drive’s most distinctive feature is in the lowering of the belt operating tension. The larger angle of belt wrap around two pulleys (Fig. 5) increases the potential tractive tension capacity of the drive at a given slack side holding tension. Conversely, with two pulleys, at a given tractive condition, the slack side holding tension is reduced.
Given 1000 Hp and a wrap angle of 200 °, 1000 Hp intersects 200° wrap (PT. 1) at a slackside tension of 13,000 lbs. When the wrap is increased to 400° (PT. 2) the slackside tension is 3000 lbs. By doubling the wrap angle, the slackside tension was lowered 77%. This example assumes an ideal load division can be obtained by intersecting the horizontal plane at (PT. 2) with the 200° wrap line (PT. 3). Reading the power vertically down from (PT. 3) the required secondary pulley power is equal to 230 Hp. The primary power is equal to the difference or 770 Hp. The ideal load ratio primary to secondary is approximately 3.4:1.
Reducing belt tension does save belt cost in most but not all cases. A specific application may require a belt strength rating for non-tension reasons.
The single motor drive is simple and reliable. As such, its frequency of failure is minimized. The addition of more drive assemblies will increase the expectation of a system failure. The planned use of multiple drives should therefore be formulated with the concept of just compensation for the lack of reliability. The multiple drive system partially compensates for its higher failure rate by allowing operation at reduced productivity. A measure of productivity and reliability are required to quantify the net gain or loss of the multiple drive in comparison with the single drive assembly.
Productivity and reliability must be combined into a common measure for an effective comparison between the single and dual drives. When the frequency of failure is multiplied by the capacity lost per failure, the net loss per drive system is obtained which can be used to rank the merits of each drive. A single drive frequency of failure is assumed to be equal to one. Carrying this concept to the next step, a service factor [SF] can be applied to increase the productivity and the expected reliability. The SF would then be an upgrading measure which makes all drives, single and multiple, relatively equal (unity), without compensation for maintenance cost.
A more comprehensive model should be studied which accounts for differences in bearing characteristics. The model presented does not adequately measure the factors of a multi-conveyor system connected in series. The use of it would only give a crude correction to the service rating of conveyors linked in series by their process flow.
The belt cost may be reduced with the dual drive, but at the same time the reducer, motor, couplings, base plates, foundations, etc. increase at the same designated power. As the size of the equipment increases, the unit cost per size decreases (i.e., two 250 Hp motors cost more than one 500 Hp motor, etc.). To gain a better understanding on the cost advantages of the dual drive, a study was made in which both the belting and drive assembly costs were evaluated.
With increasing belt slope, the dual drive benefit gradually disappears. A major factor, limiting the benefits of the single drive, is the assembly size. Handling, availability, and spares compatibility are some of the governing factors. The data presented limits the drive size to approximately 1200 Hp.
Multiplexing Conveyor Soft-Start Controls
The purpose of the soft-start control device is to initiate and regulate the acceleration motion of the conveyor until the conveyor reaches full speed. Upon reaching full speed, the acceleration control system is of no further use, assuming the drive is designed for constant speed operation. In fact, the control is a liability being used only a fraction of the conveyors operating time. Since it is always in the running circuit, any malfunction of the control’s components can lead to the conveyor’s shutdown. Present design practice requires that one control be applied to one motor. Thus, in a multi-conveyor plant, the reliability of the drive controls become critical to the plant operation.
