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machine

h only amortisseur windings is called an induction machine. Such machines

called induction machines because the rotor voltage (which produces

rotor current and the rotor magnetic fi e ld) is induced in the rotor windings

her than being physically connected by wires. The distinguishing feature of anuction motor is that no de field current i s required to run the machine.

ere are two diferent

pes o induction motor rotors which can be placed inside the stator. One is called

age rotor, while the other is called a wound rotor.

age induction

tor rotor consists oa series o conducting bars laid into slots carved in the ace

the rotor and shorted at either end by large shoT1ing rings. This design is reerred

as a cage rotor because the conductors, i examined by themselves,

uld look like one o the exercise wheels that suirrels or hamsters run on.

e other type of rotor is a wound rotor. A wound rotor has a complete set

thr ee- phase windings that are mirror images of the windings on the stator. The

ee phases of the rotor windings are us ually V-connected and the ends of the

ee rotor wir es are tied to slip rings on the rotor!s shaft. TIle rotor windings are

orted through brushes riding on the slip rings. "ound-rotor induction motor s

ref ore have their rotor currents accessible at the stator brushes where they can

e#amined and where e#tra r esistance can be inserted into the rotor cir cuit. It isssible to ta$ e advantage of this feature to modify the tor%ue- s peed characteristic

the motor.

ou nd-rotor induction motors are more e#pensive than cage induction motor s

d they re%uire much more maintenance because of the wear associated

h their brushes and slip rings. As a result wound-rotor induction motors are

ely used.

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hr ee-phase set of voltages has

n applied to the stator and a three-phase set of stator currents is flowing. These

r e nts prod uce a magnetic field &s which is rotating in a countercloc$wise

ection

e speed o the magnetic ! eld"s rotation is given by

e voltage induced in a given rotor bar is given by the euation

s the relative motion o the rotor compared to the stat or magnetic !eld

t prod uces induced voltage in a rotor bar. The velocity o the upper rotor bars

ative to the magnetic !eld is to the right, so the induced voltage in the upper

rs is out o the page, while the induced voltage in the lower bars is into the page.

s results in a current $ow out o the upper bars and into the lower bars. %owever,

ce the rotor assembly is inductive, the peak rotor current lags behind the

ak rotor voltage &see 'igure ())*b+. The rotor current $ow produces a rotor magnetic

d Br.

resulting tor%ue is countercloc$wise. Since the rotor induced tor%ue is counterc

$wise the rotor accelerates in that direction.