Influence of Technology on Conveyors

Today's Conveyor Drive Technology: Choices for Reliable, Performance-Driven Systems

Your facility's new conveyor design has just been finalized. Now, the next step is specifying the drive system. Critical to belt conveyors' performance is the ability to control drive acceleration torque in order to provide a smooth, soft start while maintaining belt tensions within the specified safe limits. For load sharing on multiple drives, torque and speed control is another important consideration in the drive system's design.

Thanks to advancements in conveyor drive controls technology, a multitude of alternatives are available, resulting in more reliable, cost-effective and performance-driven conveyor drive systems. Mechanical, hydraulic and electrical devices, or a combination thereof, can now be installed to provide more dependable, smooth, soft starts for conveyor systems.

In many cases, more than one drive control system can satisfy a facility's conveyor design requirements. Selection typically depends on a drive system's flexibility and reliability to meet all performance expectations and its cost compared to available budget. For a better understanding of the advancements and choices in conveyor drive controls, current equipment options will be reviewed.

Examples are:

Full-Voltage Starters - Direct Drive

With a full-voltage starter design, the conveyor head shaft is direct-coupled to the motor through the gear drive (Fig. 1). On relatively low power, simple profile conveyors, direct full-voltage starting of a standard NEMA B-A.C. motor is adequate. With direct full-voltage starters, no control is provided for various conveyor loads and, depending on the ratio between full- and no-load power requirements, empty starting times can be three or four times faster than full load.

This is the simplest, lowest cost, most reliable, maintenance-free starting system. However, due to a full-voltage starter's inability to control starting torque and maximum stall torque, its application is limited to low power, simple profile belt drives.

Reduced-Voltage Starters - Direct Drive

As conveyor power requirements increase, it becomes increasingly important to control the applied motor torque during the acceleration period. Since motor torque is a function of voltage, motor voltage must be controlled. This is achieved through reduced-voltage starters.Many reduced-voltage starters control voltage with timers, whereby each voltage increment creates a step change in the applied motor torque. To eliminate the torque spikes that often occur with each step change, a silicon controlled rectifier (SCR) is recommended, allowing continuous control of motor voltage throughout starting.A common starting method with SCR reduced-voltage starters is initially applying low voltage, in order to take up conveyor belt slack, and then applying a timed linear ramp up to full voltage and belt speed. This starting method, though, will not produce constant acceleration of the conveyor belt. When acceleration is complete, SCRs are locked in full conduction, providing full line voltage to the motor.Motors with higher rotor torque and pull up torque, such as NEMA design C and D motors, provide better starting torque when combined with SCR starters, which are available up to 1,000 HP (750 kW).

Variable Frequency Control(V.F.C.) - Direct Drive

Variable frequency control (V.F.C.) devices provide variable frequency and voltage to the induction motor at all times, resulting in excellent starting torque and acceleration rate for belt conveyor drives. V.F.C. drives, available from fractional to several thousand HP (kW), are electronic controllers that rectify A.C. line power to D.C. and, through an inverter, convert DC back to AC with frequency and voltage control (Fig.2). V.F.C. Drives will not run overloaded, due to the electronics' current limit, so correct size selection is important.

V.F.C. Drives are mechanically simple, but electronically complex. On most installations, a voltage-controlled transformer and extensive surge protection are required. The drives are very reliable, if properly installed and electrically protected. In general, the cost of a V.F.C. drive is higher than other systems, particularly where high voltage motors are specified or power requirements are above 250 HP (200 kW).When equipped with the proper electronics, V.F.C. Drives provide excellent speed and torque control when starting conveyor belts, and can be designed to provide load sharing for multiple drives. V.F.C. controllers are frequently installed on lower-powered conveyor drives, retrofits where standard induction motors are used, and higher-powered belt systems where sophisticated variable speed operation is required.


Wound Rotor Induction Motors - Direct Drive

Wound rotor induction motors are direct connected to the drive system reducer and are a modified configuration of a standard AC induction motor. By inserting resistance in series with the motor's rotor windings, the modified motor control system controls motor torque.For conveyor starting, resistance is placed in series with the rotor for low initial torque. As the conveyor accelerates, the resistance is slowly reduced to maintain a constant acceleration torque. On multiple drive systems, an external slip resistor may be left in series with the rotor windings to aid in load sharing.Wound rotor motor systems are relatively simple, as far as motor selection. However, the control system can be highly complex, based on computer control of the resistance switching. Today, the majority of control systems are custom designed to meet a conveyor system's particular specifications.For this type of application, the control system generally consists of a full voltage starter combined with multiple secondary contact that controls the external resistance and provides step increases of motor torque. This contact can be actuated automatically by timing, frequency or current relays.Wound rotor motors, which are custom designed, are appropriate for systems requiring over 500 HP (400 kW).

DC Motor - Direct Drive

DC motors, available from a fraction to thousands of HP (kW), are designed for constant torque below base speed and constant HP above base speed to the maximum allowable RPM. With the majority of conveyor drives, a DC shunt wound motor is used, where the motor's rotating armature is externally connected (Fig. 3).

The most used method for controlling DC drives is an SCR device (discussed under "Reduced Voltage Starters") that allows for continual variable speed operation.The DC drive system is mechanically simple, but can include complex, custom designed electronics to monitor and control the complete system. This system option is high cost when compared to other soft start systems, but is a reliable, cost-effective drive where torque, load sharing and variable speed are primary considerations.

DC motors are generally applied to higher power conveyors, including complex profile conveyors with multiple drive systems, booster tripper systems needing belt tension control, and conveyors requiring a wide variable speed range.

Available at: http://pt.rexnord.com/customer_support/technical_information/conveyor.asp

Izz