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.