Machinedesign 2059 Full Voltage Drives 0 0

Off to a Good Start

June 1, 2004
Conveyor drive technology offers several choices for reliable, performance-driven systems. Depending on budget constraints, cost may have a greater influence than technology. Regardless, the best designs are those that beat or meet starting and running criteria

After a facility’s conveyor design is finalized, its drive system must be specified. The ability to control drive acceleration torque is paramount to smooth, soft starts and consistent belt tensions within specified safe limits. For load sharing on multiple drives, torque and speed control must also be considered.

Conveyor drive control advancements have made for more cost-effective and performance-driven conveyor systems. Mechanical, hydraulic, electrical, or combination systems can be installed to provide more dependable, soft starts for conveyor systems. In fact, any number of drive control systems might satisfy one facility’s conveyor design requirements. Even so, selection typically depends on a drive system’s flexibility and reliability to meet all performance expectations and its cost compared to available budget.

Full-voltage starters — direct drive

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

In short, this is the simplest, lowest cost, and most reliable starting system. However, due to a full-voltage starter’s inability to control starting and maximum stall torques, its application is limited to low power, simple profile belt drives.

Reduced-voltage starters — direct drive

As conveyor power requirements increase, controlling applied motor torque during acceleration becomes increasingly important. Since motor torque is a function of voltage, motor voltage can be controlled with reduced-voltage starters. Many of these starters control voltage with timers, so each voltage increment creates a step change in the applied motor torque. To eliminate the torque spikes that often occur with each step change, silicon controlled rectifiers (SCRs) are recommended, as they allow continuous control of motor voltage throughout starting.

An SCR-reduced voltage starter begins with a low voltage (to take up conveyor belt slack) followed by a timed linear ramp-up to full voltage and belt speed. Keep in mind, though, this starting method doesn’t produce constant acceleration of the conveyor belt. When acceleration is complete, SCRs lock into full-conduction position and provide full line voltage to the motor.

One final note: Motors with higher rotor and pull-up torques (such as NEMA C and D motors) provide more desirable starting torque when combined with SCR starters. They are available up to 750 kW.

Variable frequency control — direct drive

VFC drives provide variable frequency and voltage to the induction motor at all times, resulting in excellent starting torques and acceleration rates for belt conveyor drives. These VFC drives, available from fractional to several thousand kW, are electronic controllers that rectify ac line power to dc and then (through an inverter) convert dc back to ac with frequency and voltage control. Due to the electronics’ current limit, VFC drives will not run overloaded; for this reason, correct size selection is important.

VFC drives are mechanically simple, but electronically complex. On most installations, voltagecontrolled transformers and extensive surge protection are required. But the drives are very reliable if properly installed and electrically protected.

When equipped with the proper electronics, VFC drives provide excellent speed and torque control when starting conveyor belts, and can be designed to provide load sharing for multiple drives. In general, the cost of a VFC drive is higher than other systems, particularly where high-voltage motors are specified or power requirements are above 200 kW. VFC 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

A modified configuration of a standard ac induction motor, wound rotor induction motors are connected directly to the drive system reducer. Putting resistance in series with the motor’s rotor windings, the modified motor control system controls motor torque. So during conveyor starting, resistance is placed for low initial torque. Then as the conveyor accelerates, resistance is slowly reduced to maintain constant acceleration torque. On multiple drive systems, an external slip resistor is sometimes left in series with the rotor windings to aid in load sharing.

Wound rotor motor systems make motor selection relatively simple. But the control system can be highly complex, based on computer control of the resistance switching. Today, the majority of these control systems are custom designed to meet a conveyor system’s particular specifications. For conveyor applications, the control system generally consists of a full-voltage starter combined with a 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. Note: Wound rotor motors, which are custom designed, are most easily justified for systems requiring over 400 kW.

Dc motor — direct drive

Dc motors, available from fractional to thousands of hp or kW, are designed for constant torque below base speed and constant power 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.

The most common method for controlling dc drives is with SCR devices that allow for continual variable speed operation. Dc drive systems are mechanically simple but can include complex, custom-designed electronics to monitor and control the complete system. This option is expensive when compared to other soft-start systems, but is reliable and cost-effective where torque, load sharing, and variable speed are primary considerations. For this reason, dc motors are generally applied to higher-power, complex-profile conveyors with multiple drive systems, booster tripper systems needing belt tension control, and conveyors requiring a wide variable speed range.

Hydroviscous clutches

This design consists of a series of driven and reaction clutch plates submerged in circulating hydraulic fluid for transferring power and cooling. Hydraulic pistons apply force to moveable reaction clutch plates. As these plates come closer together, higher hydroviscous shear force of the fluid between the plates increases the transmitted torque. In effect, output torque is directly proportional to the applied hydraulic pressure. Finally, at a certain pressure, the fluid between the clutch plates is forced out and the clutch locks up, connecting the ac motor and driven equipment.

A 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. Then measured signals are compared and any deviation is used to adjust the clutch pressure.

Hydroviscous clutches, available up to several thousand hp or 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 couplings

Hydrokinetic couplings, commonly referred to as fluid couplings, are composed of three basic elements: a driving hydraulic turbine or runner, a driven impeller that acts as a centrifugal pump, 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. Power produced is based on the amount and density of the fluid circulated, and torque produced, in proportion to input speed. Since the pumping action within the fluid coupling is dependent on centrifugal forces, output speed is less than input speed. Called slip, this difference is normally between 1 and 3%.

Available in configurations from fractional to several thousand kW, basic hydrokinetic couplings are a 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 determine the most appropriate version.

Fixed-fill fluid couplings

Mounted between an ac motor and a gear drive with a mechanical flexible shaft coupling, fluid couplings prevent potentially belt-damaging ac motor breakdown torque from transmitting through the system. For low-power conveyor drives up to 55 kW, simple standard fluid couplings are suggested.

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, so 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. But the coupling fill 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 is easily accomplished. By observing the ac motor currents under full-load conditions, the difference can be corrected by removing fluid from the more loaded drive or adding fluid to the less loaded drive.

Delay chambers

For higher power conveyors with longer belts and more complex profiles using multiple drives, simple fluid couplings are not sufficient. But fixed-fill fluid models with delay chambers provide needed control. What does the feature add? Design changes include the addition of a separate chamber - 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 startup; this lowers the initial torque produced and provides softer starts. Fixed-fill fluid couplings with delay chambers also reduce the average acceleration torque, providing a longer, softer acceleration rate.

Couplings with delay chambers are the most commonly used soft-start devices for conveyors with simpler belt profiles and limited convex/concave sections. The design is relatively simple, economical, and reliable.

Variable fill drain couplings

Drainable fluid couplings work on the same principle as fixed-fill couplings. A coupling’s impellers are mounted on the ac motor, and its runners on the driven reducer’s highspeed shaft. Housing is mounted to the drive base to enclose the working circuit. The coupling’s rotating casing contains bleed-off orifices that allow fluid to continually 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 refilling of the fluid coupling.

To control the starting torque of a single drive conveyor system, the ac motor current must be monitored 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 fluid escaping from the bleed-off orifices; this reduces the coupling torque. When the motor current falls to the lower limit, oil supply is restored until current reaches its maximum limit again. This cycle repeats until the conveyor reaches full speed. (On multiple drive systems, the control system becomes more complex and expensive. Information must be processed using sophisticated computer programs.)

Variable fill drain couplings are utilized in medium to high-power conveyor systems. The drives can be mechanically complex and, depending on the control parameters, systems can be electronically intricate. Cost is medium to high, depending upon size.

Hydrokinetic scoop control drives

Scoop control fluid couplings consist of the three standard fluid coupling components: driving runner, driven impeller, and casing (to enclose the working circuit.) 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 it rotates, 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% filled.

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 positions. 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 and reliable means for soft, controlled starts of conveyors having simple to complex profiles. Scoop control couplings are applied on conveyors requiring single or multiple drives from 150 to 750 kW, and their cost is moderate.

Scoop trim drives

These hybrid scoop controls include a stationary rather than rotating reservoir. A scoop chamber on the coupling’s impeller assembly allows a sliding scoop tube (housed in a stationary manifold) for the regulation 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 startup, a constant-displacement pump provides 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% filled for full output torque at maximum speed. Since the transfer of working fluid is determined by the scoop tube position and not by bleed-off orifices, the scoop trim drive provides a fast and accurate response to torque change requirements.

This responsiveness works well for 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 make the scooptrim drive an excellent choice for simple to complex conveyor drive systems. Cost depends on control complexity. This option is generally utilized with conveyor drives of 375 to several thousand kW.

Falk Corp. > (800) 852-3255

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