ALTERNATING.CURRENTMOTORS An a.c.motor receivescurrentfrom an a.c.generator,sowe havea reversedaction because,insteadof mechanicalpowergeneratingelectromotiveforce,we have electromotiveforcegeneratingmechanicalpower. Themostimportantclassificationof a.c.motorsis betweensingle-phase motorsand polyphasemotors. A single-phase motor is onein which the winding consistsof a singlesetof coils so connected thatthey generatea singlewaveof alternatingmagnetomotive force. A polyphasemotorhastwo or morephasewindingswhich areso connected thattrryo rnore waves or of alternatingmagnetomotive force,differentin phase,areproduced at the sametime. Themostusedtypesof single-phase motorsarethe repulsionstart,induction-run motorsandthe synchronous motors. The synchronous a.c.motorhasa wide usein industrialfield whereprecisespeed regulationis necessary, or whenenergyconservation is required. Polyphasemotorscanbe two-phaseor tlree-phase.Thesemotorshavetheir windings connectedandplacedso asto producea rotatingmagneticflux whenpolyphase currentis supplied.The windings areplacedin the stator,while the rotor, which in realrtyis the "field" of the machine,is not connected to anysourceof alectrical energy.Therotor is basicallya pieceof metal,which is exftemelydurableandlong lasting,hencethe greatreliability of inductionmotors. Currentis inducedin the rotor asthe flux lines revolving aboutthe primary cut the conductorsof the secondary. Polyphasemotorsproducebetweenfwo-to-fourtimestheir frrll loadtorquewhdl starting,sothe motorsarecapableof accelerating up to fulIspeedwithouttheneedof any auxiliarywindingsor startingdwices,asin tha caseof single-phase motors. Three-phase motoris far morecommonthantwo-phasetype,but the secondtypeis usedin servomotors. Servomotors areinductionmotorsusedin servomechanisms andautomaticcontrolsvstems.