Load Sharing Applications For AC Drives

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AC DrivesLoad Sharing Applicationsfor AC Drives

Load Sharing Applications for AC DrivesOverviewLoad sharing is a term used by many to describe a systemwhere multiple drive and motor sets are coupled and used torun one mechanical load. In the strictest sense, load-sharingmeans that the amount of torque applied to the load from eachmotor is prescribed and carried out by each drive and motorset.Identifying Load Sharing ApplicationsThe first step in addressing the needs of an application withmultiple motors is to determine if the motors are required towork together to share the load. Load sharing involves multiplemotors and drives powering the same process. Each drive andmotor set must contribute its proportional share of power to thedriven load.Multiple motors that are run from a single drive is not loadsharing because torque control of individual motors is notpossible. Motors that are controlled by separate drives withoutany interconnection are also not sharing the load. The lack ofinterconnection defeats any possible comparison and errorsignal generation that is required to compensate for differencesin the load that is applied to any single drive and motor set.Control TopologiesThree categories areDroop, Torque Follower,and Speed Trim Follower.Control TopologiesThree categories of load sharing techniques will be presented,each having unique characteristics. The subtle differences willbe addressed to better identify how to implement each toensure a successful application. The categories are Droop,Torque Follower, and Speed Trim Follower.DroopThis is the simplest form of load sharing to set up and,therefore, the least precise and less flexible. The precision ofthis control is dependent on three factors, the drives controlalgorithm, the motor characteristics, and the type of load beingcontrolled.Since a volts/hertz drive does not have the ability to run in“torque mode”, a more loose interpretation of the term “loadsharing” is sometimes used. Load sharing on a volts/hertz driveis much less controllable and to a large extent dependent onthe motor and type of load coupling. Since a loaded inductionmotor has “slip” as an inherent characteristic, load sharing is anatural response to loading.

Load Sharing Applications for AC DrivesWhen two motors are coupled to the same load each will slipaccording to its portion of load, forcing load to be picked up by theother motor. In this regard, a high slip motor is better for load sharingapplications. However in a general sense, the slip of a motor isrepresentative of its efficiency rating. High slip motors tend to havehigh starting torque but have low efficiency. Though high efficiencymotors will slip when loaded, they may not have enough slip at full loadto achieve a reasonable degree of load sharing. For this reason manydrives offer a feature called “Droop”.The Droop feature enhances the natural slip of the motor byreducing the output frequency in proportion to load current. Ideally it isonly the torque-producing component of the load current thatdetermines the droop. The amount of droop can usually beprogrammed in the drive in hertz or % speed.Torque FollowerThis type of load sharing requires a drive having the capability ofoperating in “torque mode”, such as the 1336 IMPACT drive. If speedregulation is required, one of the drives may be in “speed mode”(called the “Master”). Keep in mind that a drive in speed mode is reallystill in torque mode. While in speed mode the speed regulator providesa torque command output. This torque command output can bedistributed to the other drives on the system (called the “Slaves” or“Torque Followers”). This torque signal may be scaled at the masteroutput or the slave input to divide the load sharing in any ratio desired.In this case, it is possible to have drive and motor sets of varyinghorsepower sizes each pulling load to the extent of their capability. Forexample, a 100 horsepower and a 75 horsepower drive and motor seteach running at 86% of its power rating could power a system requiring150 horsepower. The ratio of load sharing can be adjusted such thateach motor and drive set pulls a proportional amount of load under allconditions.Speed Trim FollowerThis configuration requires drives that can produce an internal torquereference or estimate torque current. Master and follower drives areoperated in speed regulation mode and receive the same speedreference. Interconnection of the drives is necessary so that torquereferences may be compared. The torque reference of the master driveis sent to the follower drives. Each follower drive compares its owntorque reference with that of the master drive. The output of thecomparator is an error signal that trims the speed of the follower.

Load Sharing Applications for AC DrivesConfigurationsAdvantages Simple No extra wiring forinterconnection High performance drivenot required No runaway conditionwith load lossDroopThe master and follower drive(s) receive the same speedreference and no interconnection of the drives is required. Thespeed of the motors will be dependent on the load and amountof droop that is programmed at full load current.Droop ConfigurationDisadvantages Poor speed regulation Limited speed range Sharing of load notpreciseDroop ApplicationsAir handling units with a common discharge.

Load Sharing Applications for AC DrivesContinuous belt conveyor with multiple driven rolls.Load Sharing Applications for AC DrivesIn both examples, the speed regulation is not critical.Torque FollowerThe master drive is operated in speed regulation. The followerdrive(s) are operated in torque regulation mode.Interconnection of the drives is required. The torque referenceof the master drive is sent to the follower drives and used asthe command.Torque Follower ConfigurationAdvantages Precise load sharing(act as one) Operation over theentire speed range Minimum torque modehelps prevent runawayDisadvantages Requires a torqueregulating drive Interconnectionrequired Load loss runaway iftorque regulation only

Load Sharing Applications for AC DrivesTorque Follower ApplicationsLarge diameter bull gear with multiple drives.Printing presses with inline drive shaft.

Load Sharing Applications for AC DrivesCoal car dumper, Cement kiln, Separator drum.In all examples, the coupling between the motors is rigid and speed regulation iscritical.Speed Trim FollowerThe master drive is operated in speed regulation. The follower drive(s) areoperated in speed regulation mode with a speed trim. The trim is a function ofthe comparing of torque commands of the master and follower drives.Two types of configurations can be used; one uses a single source (masterdrive) for a torque reference comparison. The follower drives compare themasters drive torque reference to their own internal value to create the errorsignal for speed trimming.Speed Trim Follower (Single Source)

Load Sharing Applications for AC DrivesAdvantages Continuous automaticcompensation Operation over theentire speed range Trim feature built intodrive Speed regulationThe second configuration cascades the torque referencecomparison. The first follower compares the master to itsinternal value. The second follower compares follower 1 and itsinternal value.Speed Trim Follower (Cascade)Disadvantages Requires highperformance drive forprecision Requiresinterconnection wiringSpeed Trim Follower Applications

Load Sharing Applications for AC DrivesIn these examples, the coupling between motors has a veryhigh potential for oscillation; therefore, the automaticcompensation inherent with the speed trim is best suited tothese types of applications.

Load Sharing Applications for AC DrivesNOTES

Load Sharing Applications for AC DrivesNOTES

Publication DRIVES-WP001A-EN-P — June 2000 2000 Rockwell International Corporation. All rights reserved. Printed in USA.

Coal car dumper, Cement kiln, Separator drum. In all examples, the coupling between the motors is rigid and speed regulation is critical. Speed Trim Follower The master drive is operated in speed regulation. The follower drive(s) are operated in speed reg

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