Applying Adjustable Speed Drives to Conveyors
By Charles J. Cowie
This article is reprinted with permission
If you walk through almost any type of industrial plant, you will find conveyors moving material through the manufacturing process. Adjustable speed drives are used to control the operation of many of the conveyors used in a wide variety of industries. This article discusses the role of speed control in conveyor applications and presents the factors that should be considered in selecting and applying adjustable speed drives.
Conveyors can be used to move either bulk material such as coal or unit items such as automobile bodies. Unit handling types of conveyors include roller conveyors, belt conveyors and chains with hooks or carriers. Either conveyor belts or feed screws can be used to move bulk material. Although conveyors can have many mechanical configurations, their drive requirements are generally similar.
Above: Figure 1
Conveyors are inherently constant torque machines. That means a constant level of torque is required to drive the conveyor regardless of the operating speed. Figure 1 is a typical graph of load torque vs. speed for a conveyor. Note that the curve is essentially a horizontal line, indicating that the load torque remains relatively constant throughout the range of possible operating speeds.
In most cases, all of the power required to drive a conveyor is used to overcome the friction of the various mechanical elements of the load. With vertical or inclined conveyors, some of the driving power is used to lift a mass to a higher elevation. Occasionally, drives are required to provide braking as material is moved from a higher elevation to a lower elevation. In most conveyor applications, only a very small percentage of the torque supplied by the drive is used to accelerate inertia. Most conveyors have a relatively high friction load in comparison to the inertia reflected to the motor shaft. The drive may be subjected to shock loading when a mass that is moving relatively slowly is transferred or loaded onto a conveyor that is moving at a higher speed.
Sometimes conveyor drives must provide a high starting torque to overcome the static friction of the conveyor and the mechanical drive train components. In Figure 1, the starting torque requirement is shown as a nearly vertical segment of the torque vs. speed curve at zero speed. In some applications, high starting torque may be required because ice or solidified bulk material must be broken up as the conveyor starts to move.
Conveyors usually operate over a relatively narrow range of speeds near maximum speed, but extended periods of low speed operation may be required to accommodate set-up requirements. It may also be necessary to operate at low speed while making adjustments to the process or cleaning the equipment.
Performance Requirements and Control Strategies
Conveyors have a wide range of static and dynamic performance requirements. In most applications, an adjustable speed drive is used because the capability to adjust the operating speed provides a means for controlling the process. Understanding the relationship between the drive speed and the process is the key to defining the performance requirements of the drive.
Consider a bottle filling application in which a filling line is used for more than one size of bottle. Assume that the bottle filler can fill bottles at a rate of 120 gallons per minute regardless of bottle size. When the line is set up for one-gallon bottles, the conveyor speed must be adjusted to move bottles through the filler at 120 bottles per minute. If the line set up to fill half-gallon bottles, the conveyor must move the bottles at 240 bottles per minute.
Since each revolution of the screw meters out a fixed amount of material, speed controlled feed screw conveyors can be used to regulate the flow of bulk materials. For example, a screw conveyor can be used to control the rate at which coal is fed into a furnace. The feed rate might be changed to regulate the heat produced or to maintain a constant rate of combustion with variable fuel quality.
Most types of
adjustable speed drives will regulate speed to so that the operating speed
drops no more than 5% of the maximum speed when the load increases from
no-load to full-load. Many drives offer 3% regulation or even 0.5%
regulation as standard and have options available for 0.5% to 0.01%
regulation. Very accurate speed regulation is essential in some conveyor
applications, but 5% regulation is perfectly adequate for many applications.
Note that the load presented to the drive by a conveyor will not vary nearly
as much as the no-load to full-load variation that corresponds to the
specified maximum speed variation. It is also important to remember that
regulation is usually expressed as a percentage of maximum speed. If the
drive is operating at half of maximum speed, 5% of maximum speed represents
10% of set speed.
To determine how accurately speed must be regulated, it is necessary to determine how speed variation affects the process. In some applications, speed variations result only in variations in the amount of product produced per hour. A 5 or 10% hourly variation may be completely insignificant in comparison to other factors that affect daily output. Product quality frequently depends on accurate speed regulation. For example, if a drive feeding an ingredient to a process changes speed during the course of a day, there will be a variation in the percentage content of that ingredient in the end product. For example, if small variations in pigment content result in detectable color variations, very accurate speed regulation may be required for drives that feed pigment into a paint manufacturing process.
Most conveyor applications involve an elementary speed control strategy in which the drive simply regulates the operating speed at a setpoint that may be adjusted from time to time. The speed setpoint might be set manually by an operator or automatically by supervisory control equipment. When an adjustable speed drive is used with a conveyor to regulate a process variable, indirect control is usually utilized rather than direct control. Conveyor speed control is often used to control such processes as baking, drying or curing some material or product. Using indirect control, the conveyor is simply operated at the optimum speed that has been previously determined for a given process recipe.
Many applications require some degree of system control to provide coordination between the drive and the process. Many drives have built-in standard features or optional functions that allow the stand-alone drive to provide a basic level of system control. External control devices such as small PLC’s and specialized control modules can also be used to provide system control functions for stand-alone drives. An external master control system is usually employed in applications with complex process control requirements or coordinated control of multiple stand-alone drives.
In some applications, the speeds of several conveyors are matched so that they operate as if they were one continuous conveyor. The speeds of adjacent conveyors may need to be matched quite closely to prevent damaging products as they are transferred between conveyors. Adjacent conveyors are sometimes operated with a controlled difference between their speeds. A speed differential can be used to change the spacing between items as they transfer from one conveyor to another. If conveyors are used to transport a continuous sheet of material such as cloth or paper, the application is classified as a web process. Web process applications are beyond the scope of this article.
Position synchronization or relative position control is used to maintain a relative position relationship between points on two conveyors that are operating at the same speed. When two conveyors carry objects in buckets or slots, the objects can be smoothly transferred from one conveyor to the other if the buckets maintain their relative positions as they move beside each other at the transfer point.
Smooth acceleration and deceleration control is an important element of many conveyor applications. Linear acceleration and deceleration control limits the forces that are applied to accelerate or decelerate the load. Although linear acceleration limits the level of the force applied to the load, the full accelerating force is still applied all at once. This sudden application of force can apply a mechanical shock or jerk the load. With S-curve acceleration and deceleration control, the accelerating or decelerating force is gradually ramped up to full force. Applying the force gradually, limits the mechanical shock providing smoother starts and stops.
A conveyor is a mechanical system that is often spread out over a considerable distance. The power used to drive a conveyor is used to overcome the frictional load that is distributed over the entire length of the conveyor. If a single motor is used to drive the conveyor, power applied at that one location must be mechanically transferred throughout the conveyor. This means that all of the mechanical parts must withstand the stress involved in transferring power from one location to all parts of the conveyor. If several motors are located at various places along the conveyor, the power is transmitted electrically to locations closer to the points of use. There are various methods for designing a multiple motor conveyor drive system so that the load will be evenly shared among the motors. Load sharing systems will be the subject of a future article.
Charles J. Cowie is a Contributing Editor for drivesmag.com and a consultant with AFD Professional Services in Racine, Wisconsin. He is a Registered Professional Engineer in the State of Wisconsin and a member of The Institute of Electrical and Electronic Engineers.