Leeson Micro Series Compact Inverters Manual do Utilizador descarregar pdf (Página 21)

 “Variabletorque”referstothefactthatthetorquerequiredvarieswiththesquareofthespeed.Also,the

horsepower required varies with the cube of the speed, resulting in a large reduction in horsepower for

even a small reduction in speed. It is easily seen that substantial energy savings can be achieved by reducing

thespeedofafanorpump.Forexample,reducingthespeedto50%resultsina50HPmotorhavingto

produceonly12.5%ofratedhorsepower,or6.25HP.Variabletorquedrivesusuallyhavealowoverload

capacity (110%-120% for 60 seconds), becausevariabletorque applications rarely experience overload

conditions.To optimizeefficiencyand energysavings, variabletorquedrives areusually programmedto

followavariableV/Hzratio.

 Theterm“constanttorque”isnotentirelyaccurateintermsoftheactualtorquerequiredforanapplication.

Manyconstanttorqueapplicationshavereciprocatingloads,suchasvibratingconveyorsandpunchpresses,

wheretherotationalmotionofthemotorisbeingconvertedtoalinearmotion.Insuchcases,thetorque

requiredcanvarygreatlyatdifferentpointsinthecycle.Forconstanttorqueloads,thisfluctuationintorque

isnota directfunctionof speed, asit is withavariable torqueload.As a result,constant torque drives

typically have a high overload rating (150% for 60 seconds) in order to handle the higher peak torque

demands.Toachievemaximumtorque,constanttorquedrivesfollowaconstantV/Hzratio.

 MICROSeriesproductlineshavefulloverloadcapacity(150%for60seconds,180%for30seconds),so

thateitheronecanbeusedforeithertypeofapplication.TheV/Hzratiocanalsobechangedtooptimize

performance for either type of application.

6.2 DRIVE FUNCTION DESCRIPTION

 TheMICROSeriesisa16bitmicroprocessorbased,keypadprogrammable,variablespeedACmotordrive.

Therearefourmajorsections;aninputdiodebridgeandafilter,apowerboard,acontrolboard,andan

output intelligent power module.

6.2.1 DRIVE OPERATION

Incoming AC line voltage is converted to a pulsating DC voltage by the input diode bridge. The DC

voltage is supplied to the bus filter capacitors through a charge circuit which limits inrush current to the

capacitors during power-up. The pulsating DC voltage is filtered by the bus capacitors which reduces

theripplelevel. ThefilteredDCvoltageenterstheinvertersectionofthe drive,composedofsixoutput

intelligentinsulatedgate bi-polartransistors(IGBT’s)whichmakeupthethreeoutputlegs ofthedrive.

Each leg has one intelligent IGBT connected to the positive bus voltage and one connected to the negative

bus voltage. Alternately switching on each leg, the intelligent IGBT produces an alternating voltage on each

ofthecorrespondingmotorwindings.ByswitchingeachoutputintelligentIGBTataveryhighfrequency

(knownasthecarrierfrequency)forvaryingtimeintervals,theinverterisabletoproduceasmooth,three

phase,sinusoidaloutputcurrentwavewhichoptimizesmotorperformance.

6.2.2 CIRCUIT DESCRIPTION

 Thecontrolsection consistsof acontrol boardwitha16bitmicroprocessor,keypadanddisplay. Drive

programming is accomplished via the keypad or the serial communications port. During operation the

drivecanbecontrolledviathekeypad,bycontroldeviceswiredtothecontrolterminalstrip,orbytheserial

communications port. The Power Board contains the control and protection circuits which govern the

sixoutputIGBT’s.ThePowerBoardalsocontainsachargingcircuitforthebusfiltercapacitors,amotor

currentfeedback circuit,avoltagefeedbackcircuit, anda faultsignal circuit.Thedrive hasseveral built

in protection circuits. These include phase-to-phase and phase-to ground short circuit protection, high

andlowlinevoltageprotection,protectionagainstexcessiveambienttemperature,andprotectionagainst

continuousexcessiveoutputcurrent.Activationofanyofthesecircuitswillcausethedrivetoshutdown

in a fault condition.

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