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.

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.

Fig. 1. With a full-voltage starter conveyor drive design, the conveyor head shaft is direct-coupled to the motor through the gear drive, as shown in this application.

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.

Fig. 2. Variable frequency control drives provide excellent speed and torque control when starting conveyor belts.

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).

Fig. 3. A DC drive system provides a reliable, cost-effective option where torque, load sharing, and variable sped are primary considerations.

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.

Hydroviscous Clutch

The basic design consists of a series of driven and reaction clutch plates submerged in circulating hydraulic fluid for transfer of power and cooling. Hydraulic pistons apply force to moveable reaction clutch plates. As these plates move closer, the transmitted torque increases, due to higher hydroviscous shear force of the fluid between the plates, with the output torque directly proportional to the applied hydraulic pressure. At a certain pressure, the fluid between the clutch plates is forced out and the clutch locks up, providing a direct connection between the AC motor and driven equipment.

A control system to accurately control clutch pressure is necessary. With the simplest design, an input analog ramp signal is programmed at a set time to measure the conveyor's speed. Measured signals are compared and any deviation is used to adjust the clutch pressure.

Hydroviscous clutches, available up to several thousand HP (kW), are used on medium to high power conveyors. As the conveyor complexity increases, so does the control system. The installed cost ranges from moderate to high, depending upon the control requirements.

Hydrokinetic Coupling

Hydrokinetic couplings, commonly referred to as fluid couplings, are composed of three basic elements: the driven impeller which acts as a centrifugal pump, the driving hydraulic turbine known as the runner, and a casing that encloses the two power components. Hydraulic fluid is pumped from the driven impeller to the driving runner, producing torque at the driven shaft. Since circulating hydraulic fluid produces the torque and speed, there is no mechanical connection between the driving and driven shafts.

The power produced is based on the amount and density of the fluid circulated and the torque in proportion to input speed. Since the pumping action within the fluid coupling is dependent on centrifugal forces, the output speed is less than the input speed. This is referred to as slip and is normally between one and three percent.

Available in configurations from fractional to several thousand HP (kW), basic hydrokinetic couplings are devices of well-proven technology. Common designs are fixed fill fluid couplings, variable fill drain couplings, scoop control drives and scoop trim drives. Conveyor power and control requirements will determine the best coupling to specify.

Fixed Fill Fluid Coupling

Mounted between the AC motor and gear drive with a mechanical flexible shaft coupling, fluid couplings prevent potentially belt-damaging AC motor breakdown torque from being transmitted through the system (Fig.4). For low HP conveyor drives up to 75 HP (55 kW), simple standard fluid couplings are suggested.

Fig. 4. Mounted between the motor and gear drive, fluid couplings prevent belt-damaging motor breakdown torque from being transmitted through the system.

With fluid couplings, the AC motor starts virtually unloaded and, as the motor accelerates, torque smoothly increases from zero to conveyor breakaway torque. No compensation is provided for different conveyor loads, therefore unloaded starting can be considerably faster than loaded conditions.

Changing the coupling fill controls the accelerating torque. Decreasing the amount of fluid reduces starting torque, but increases slip. The coupling fill, though, must always be sufficient to carry the maximum load without exceeding the coupling's self-heat dissipation.

With a simple fluid coupling, load balancing on multiple drive systems can easily be accomplished. By observing the AC motor currents under full load conditions, the difference can be corrected by removing fluid from the higher loaded drive or adding fluid to the lower loaded drive.

Fixed Fill Fluid Coupling with Delay Chamber

For higher power conveyors with longer belts and more complex profiles using multiple drives, simple fluid couplings, as described above, are not sufficient. This need led to the development of fixed fill fluid couplings with delay chambers (Fig.5).

Fig. 5. Fixed fill fluid couplings with delay chambers are best for applications with higher power conveyors with longer belts and/or multiple drives.

Design changes include the addition of a separate chamber, referred to as the delay chamber, to the basic fluid coupling. At rest, a portion of the fluid drains back into the delay chamber so the amount of fluid in the coupling's working circuit is reduced on start-up, therefore lowering the initial torque produced and providing a soft start. Fixed fill fluid couplings with delay chambers also reduce the average acceleration torque, providing a longer, softer acceleration rate.

Fixed fill fluid couplings are the most used soft start devices for conveyors with simpler belt profiles and limited convex/concave sections. They are relatively simple, low-cost, reliable, maintenance-free devices that provide excellent soft starting systems to the majority of belt conveyors in use today.

Variable Fill Drain Coupling

Drainable fluid couplings work on the same principle as fixed fill couplings. The coupling's impellers are mounted on the AC motor and the runners on the driven reducer high-speed shaft. Housing mounted to the drive base encloses the working circuit. The coupling's rotating casing contains bleed-off orifices that continually allow fluid to exit the working circuit into a separate hydraulic reservoir. Oil from the reservoir is pumped through a heat exchanger to a solenoid-operated hydraulic valve that controls the filling of the fluid coupling.

In order to control the starting torque of a single drive conveyor system, the AC motor current must be monitored, in order to provide feedback to the solenoid control valve. When the motor current reaches an upper limit, the coupling's oil supply is cut off and the coupling fill is lowered by the fluid escaping from the bleed-off orifices, reducing the coupling torque. When the motor current falls to the lower limit, the oil supply is restored until the current reaches the maximum limit. This cycle repeats until the conveyor reaches full speed.

On multiple drive systems, the control system becomes more complex and expensive. The information must be processed using sophisticated computer programs.

Variable fill drain couplings are utilized in medium to high HP (kW) conveyor systems and are available up to thousands of HP (kW). The drives can be mechanically complex and, depending on the control parameters, the system can be electronically intricate. The cost of the drive system is medium to high, depending upon size specified.

Hydrokinetic Scoop Control Drive

The scoop control fluid coupling consists of the three standard fluid coupling components: driven impeller, driving runner and casing which encloses the working circuit (Fig.6). The casing is fitted with fixed orifices that bleed a predetermined amount of fluid to a reservoir. A stationary manifold holds a sliding tube with an opening parallel to the outside diameter of the reservoir. As the reservoir rotates, the oil passes through the scoop to an external cooler and directs the oil to the working circuit of the fluid coupling. When the scoop tube is fully extended into the reservoir, the coupling is 100 percent filled.

Fig. 6. Scoop tube fluid couplings are a simple, reliable, and low-maintenance method for soft controlled starts of conveyors having simple to complex profiles.

The scoop tube, extending outside the fluid coupling, is positioned using an electric actuator to engage the tube from the fully retracted to fully engaged position. This control provides reasonably smooth acceleration rates. Although adding complexity to the control system, computer-based systems and feedback devices can be integrated to provide constant acceleration rates for all loading conditions.

Since the amount of fluid in the coupling determines the torque output, scoop tube fluid couplings are a simple, low maintenance and reliable method for soft controlled starts of conveyors having simple to complex profiles. Scoop control couplings are applied on conveyors requiring single or multiple drives from 200 HP to 1,000 HP (150 kW to 750 kW), and its cost is considered moderate.

Scoop Trim Drive

The scoop trim drive is a hybrid scoop control where a stationary, rather than rotating, reservoir is used. A scoop chamber on the coupling's impeller assembly allows a sliding scoop tube housed in a stationary manifold to regulate the amount of fluid in the working circuit. The mechanical connection to the scoop tube is external to the housing and is positioned using an electric actuator for automatic control.

At start-up, a constant displacement pump provides a fixed fluid flow from the reservoir to an external cooler into the fluid coupling working circuit. The scoop tube trims the level of working fluid in the coupling, thereby controlling output torque. As the scoop tube is withdrawn from its chamber, the amount of fluid in the working circuit is increased, along with the torque output. When the tube is fully withdrawn, the fluid coupling is 100 percent filled, providing full output torque at maximum speed. Since the transfer of working fluid is determined by the scoop tube position, and not dependent on bleed-off orifices, the scoop trim drive provides a fast and accurate response to torque change requirements.

The high response rate of the scoop trim drive allows the addition of various feedback devices on single and multiple drive systems. These include tachometers to provide constant acceleration under all loading conditions and AC motor current feedback for load sharing or torque limiting control.

The control flexibility and torque control response makes the scoop trim drive an excellent choice for simple to complex conveyor drive systems. Cost varies from medium to high, depending upon the control complexity. This option is generally utilized with conveyors drives of 500 HP (375 kW) to several thousand HP (kW).

Conclusion

So, what is the best starting system for conveyor drives? The answer is a system flexible enough to meet all performance, reliability and cost expectations. The designer must specify the requirements for each belt system and select the drive that best meets the design and performance criteria. Depending upon budget constraints, cost may have a greater influence than technology (Fig. 7).

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