A.O.Smith Motor Catalog - Universal Electric Motors

Speed Engineered® MotorsMotors specially designed, tested andwarranted to be Corona-Free forcompatible inverter duty are markedin this catalog with a J.E-Plus ®Speed Engineered ®Inverter Duty MotorWhy Specify SpeedEngineered ® InverterDuty Motors?Variable frequency drives (VFDs),while offering advantages of greatercontrol and energy savings to commercialand industrial motor users, can alsocause premature winding failurein motors not designed specificallyfor inverter duty. Now A. O. Smithengineers have developed a solidsolution...Speed Engineered ® InverterDuty Motor.Speed Engineered Inverter DutyMotors are specially designed andconstructed to eliminate the destructiveforces that can occur when motorsare applied with drives. The SpeedEngineered “Corona-Free” solutioneliminates the causes of prematurewinding failure.All Speed Engineered motors meetor exceed NEMA MG1-31 performancestandards, in addition to carrying A. O.Smith’s Speed Engineered warranty forinverter duty applications.The Causes of PrematureMotor FailureResearch we conducted identifiedwhy motors can fail when used withvariable frequency drives under certainoperating conditions. The resultswere published in a white paper, TheSimple Truth About Motor/DriveCompatibility, which is available fromA. O. Smith. Our findings revealedthat “corona” as well as other potentialhazards, can materialize and eventuallydamage motors applied with adrive.What is Corona?VFDs create high voltage pulses atthe motor, especially when the motorand drive are separated by long powerleads. Those high voltage pulses (orvoltage spikes) develop voltage potentialbetween adjacent conductors inthe motor winding.When the voltage generated in theair between the conductors is highenough, the air breaks down.This breakdown is known as“corona.” The discharge that is createdforms ozone, which causes themotor’s magnet wire insulation todisintegrate, causing premature failure.This phenomenon has been aroundfor a long time and affects a limitednumber of earlier vintage motor/driveapplications. But with drives becomingmore sophisticated, inverter switchingrates increasing and the percentage ofmotors operating with drives growingrapidly, incidents of downtime are alsogrowing, and corona is now getting alot of attention in the motor/driveindustry.There are several techniquesemployed in the market to increasemotor tolerance to corona. Althoughsimpler and less costly, these practicesare not always effective sincecorona is not cured...only bandaged.The only way to be sure the destructiveefforts of corona will not compromiseyour motor/drive application is toeliminate corona altogether. This iseasily accomplished by specifyingA. O. Smith Speed Engineered motorson your next project.What Makes Corona-FreeSpeed Engineered ® MotorsBest For Motor-DriveCompatibility?There are several solutions to theproblem of motor insulation stresscaused by inverters. Rather than justsquelching the voltage overshoot whichleads to corona, as mentioned earlier,the preferred method and the approachused by A. O. Smith is to design themotor to be corona free at expectedpeak voltage. We begin with a designpremise of understanding the magnetwire corona inception voltage (CIV)and distribution of voltage in the motor.From that, our design approachbecomes simply to:Choose a winding layoutthat minimizes the proximityvoltage differences and reliablypositions insulation materialsto improve dielectrics above thethreshold of corona…You may recognize this as thedesign approach for any motor,regardless if it is line operated ordriven by an inverter. The difference isthat with an inverter you must anticipatea much higher peak voltage and therapid rise times of these potentiallyharmful pulses.www.aosmithmotors.com