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Synchronous Motors

Constant-speed machine

Propulsion for SS Queen Elizabeth II

!! M"

#$ %&

'$ (z )$ pole

#!! r*min

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Synchronous Motors +continued,

Construction

Stator identical to that of a three-phaseinduction motor no called the armature

Ener.ize from a three-phase supply and

de/elop the rotatin. ma.netic field

0otor has a 1C /olta.e applied +e2citation,

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Synchronous Motors +continued,

3peration

Ma.netic field of the rotor loc%s ith therotatin. ma.netic field rotor turns at

synchronous speed

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Cylindrical +0ound, 0otor

Constructed from solid steel for.in. to ithstand lar.e

centrifu.al stresses inherent in hi.h-speed operation

4sed for hi.h speed5 lo inertia loads +lo startin. tor6ue,

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Salient-Pole 0otorE2citation "indin.s

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Salient-Pole 0otor

ith shaft-mounted 1C e2citer

7eed carbon brushes to ma%e

contact ith the commutator

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Salient-Pole 0otor ith brushless e2citation

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Synchronous Motor Startin.

8et motor to

ma2imum speed

+usually ith no load,

Ener.ize the rotorith a 1C /olta.e

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9he &:0IS930 or resistor in shunt ith the field indin. pre/ents hi.h

/olta.e from bein. induced durin. loc%ed-rotor and acceleration;

9he induced current helps to accelerate the rotor by pro/idin. additional

tor6ue;

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(o it or%s

=re6uency-sensiti/e Control circuit >oo%s at emf induced in the field

fr ? sfs:t loc%ed-rotor5 s?#5 fr? fs

Close SC0# bloc% current from field

3pen SC0@ connect dischar.e resistor across thefield

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(o it or%s +continued,

:s the speed approaches ns5 fr.ets small5

fr? sfsapproaches $ 3pen SC0@ disconnects the dischar.e resistor

Close SC0# allos field current to flo

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Salient-Pole Motor operatin. at

both no-load and loaded conditions

:n.le A is thepower angle5 load angle5 or torque angle

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0otatin. =ield =lu2 and Counter-emf

0otatin. field flu2 fdue to 1C current in the rotor;

: speed /olta.e5 counter-emf5 or e2citation/olta.e Efis .enerated and acts in opposition tothe applied /olta.e;

Ef? nsf%f

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:rmature-0eaction &olta.e

0otatin. armature flu25 aris caused by the

three-phase stator currents; 9he induced speed/olta.e caused by the flu2 arcuttin. the statorconductors;

Ear? nsar%a

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:rmature-0eaction &olta.e +continued,

Ear? nsar%a

arproportional to armature current Ia Ear? +Ia,+Bar,

here ar? armature reactance +D*phase,

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E6ui/alent Circuit of a Synchronous Motor

:rmature +3ne Phase,

( )

T a a a l a ar f

s l ar

T f a a s

T f a s

V I R I jX I X E

X X X

V E I R jX

V E I Z

= + + +

= +

= + +

= +

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Phasor 1ia.ram for one phase of a

Synchronous Motor :rmature

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Synchronous-Motor Poer E6uation

In most cases5 the armature resistance is

much smaller than the synchronous

reactance5 so the synchronous impedance

sis appro2imately e6ual to Bs;

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9he E6ui/alent-Circuit and Phasor 1ia.ram

IascosFi? -EfsinA

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9he Synchronous-Motor Poer E6uation

&9IacosFi? -+&9Ef*s,sinA

&9IacosFi? poer input per phase ? Pin5#G

-+&9Ef*s,sinA ? ma.net poer per phase

de/eloped by a cylindrical-rotor motor+a function of Efand A,

Pin5#G? -+&9Ef*s,sinA is the synchronous-machine poer e6uation

=or three phases5 Pin? H+&9IacosFi,proportional to Iacos Fi

Pin? H+-&9Ef*s,sinAproportional to EfsinA