Comparison oftheRedundant DesignsOptions Theperformance characteristics obtained withthedifferent redundancy options areprovided inTable1and arediscussed further. Theyarebasedonthefollowing assumptions: Onlyrelative parameter valuesaregiven(inpercents) allowing aneasiercomparison ofthedifferent designs. Thetwo-sectors electrical redundancy optionisconsidered asthereference caseandthe parameters otherthanthe100%reference caseareinboldcharacters. _,Themagnetic circuitisidentical forallthedifferent units,exceptthatitiseitherinoneorintwo separate pieces. Winding changes areimplemented toreflecttheselected typeofredundancy. _,Itisnotpossible tocompare ingeneral theoptions' efficiencies sincetheyaretoodependent uponthe specific speed/torquedemand. Thecomparison islimitedtothecopperlossescorresponding tothe majorlossfactorforlow-speed applications. Sincethelamination stackshavethesamesizeforallthe different options, theironlossescanbeconsidered identical throughout. Thecopperlossesare provided foragivenspecific "nominal" working point.Nevertheless, theserelative copperlossvaluesare validforanytorquedemand ifthemotorhasalineartorque-to-current characteristic (i.e.,brushand brushless DCmotors). Formagnetically saturated motors(i.e.steppers) andfordifferent torqueloads, moredetailed information arepresented laterontheexacttorque-to-power behavior. >,Forthemotormassanddimensions, onlythemotorframeless electric/magnetic partsareconsidered (Figure4),excluding allotherrelatedmechanical parts(i.e.,housing &flanges, bearings, outputshaft, outputcablesandconnectors), whicharetoohighlyinfluenced bythespecific application constraints (e.g.,useoftitanium, aluminum orsteelforthehousing orsizeofthebearings). A"size23"reference motorisconsidered inthiscalculation [2],witha0.059-m external diameter (2.3 in)anda0.030-m totalaxiallength(1.2in).Adjustments mayberequired formotorswithsignificantly different sizes. >,Forinformation, a"non-redundant" column hasbeenadded,providing theperformances reached bya non-redundant motormadewithamagnetic lamination stackidentical totheoneusedfortheelectric redundancy concepts. Thismotoris,consequently, moreefficient sinceitutilizes 100%oftheunit's magnetic andelectrical capabilities. Table1.Relative Performances oftheRedundant Options PARAMETERS Resistance Inductance Electrical Time Constant Back-emf Constant CopperLosses Frameless Mass TotalAxialLengthMechanical Redundancy FullIPartial 150 150 100 100 150 150 100 150 120 150iO0 150Electrical Redundancy Two Sectors 100% 100% IO0% 100% 100%2Parallel Wires 100 5O 200 100 100ISharedSlots 100 5O 200 100 IO0NonRedundant 5O 5O 100 IO0 50 100 120 100% 100 100 I150 100% 100 100 100 Electrical Characteristics andCopperLosses Thewinding designs ofTable1options havebeenadjusted inordertohaveaback-emf constant identical for eachofthedifferent options. Itreflects, inanunsaturated condition, anidentical torque-to-current characteristic forallthedifferent windings. Onthisbasis,foragiventorquetheresistance isanindirect indication ofthemotor's copperlosses. Addingtherelative inductance values, itispossible tohaveanidea ofthemotor's electrical impedance, necessary forthedriverdesign. Table1clearlypointsoutthe impedance increase resulting fromtheimplementation ofaredundant winding intoanexisting lamination stack.Redundancy options leading toalowerelectrical timeconstant shallinsomecasesbepreferred 182