electrical power and machines- - three-phase induction motors (2)
TRANSCRIPT
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7/26/2019 Electrical Power and Machines- - Three-Phase Induction Motors (2)
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Chapter 13: Three-Phase
Induction Motors (2)
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Squirrel-cage Motors
Typical performance over the power range: 1 kW to 20 MW
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Squirrel-cage Motors
Motor operating at no-load
the no-load current is similar
to the exciting current in a
transformer
composed of a magnetiingcurrent that creates the
revolving flux! "m
small active power component
that supplies the iron! windage!
and frictional losses
considera#le reactive poweris needed to create the
revolving flux
a larger air-gap re$uires greater
amounts of reactive power
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Squirrel-cage Motors
Motor operating under load
active power delivered from
the source increases
proportionally to the
mechanical loado efficiency at full-load is
particularly high
the stator and rotor currents
produce mmfs that are
similar to those found in theprimary and secondary
windings of a transformer
o leakage fluxes are created in
the stator and rotor circuits
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Squirrel-cage Motors
Motor with a locked rotor
the locked-rotor current is % to & times the full-load current
I2R losses are 25 to 36 times higher than normal
the rotor must never remained locked for more than a few seconds
a strong mechanical tor$ue is developed
even though no power is delivered
power factor is low #ecause the leakage fluxes are larger
the stator and rotor windings are not tightly coupled due to the air-gap
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Estiating Motor Currents
The full-load current approximate value #ased on
empirical models
where I = full-load current
Ph = output power in HP
E = rated line voltae
The starting current
% to & times the full-load current
The no-load current
0'( to 0'% times the full-load
current
E!aple estimatethe full-load current!
locked rotor current! and no-load
current of a %00 hp! 2(00 )! (-
phase induction motor
estimatethe apparent powerdrawn under locked-rotor
conditions
statethe nominal rating of the
motor in kW
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E!aple
estimatethe full-load current! locked rotor current!
and no-load current of a %00 hp! 2(00 )! (-phase
induction motor
estimatethe apparent power drawn under locked-
rotor conditions statethe nominal rating of the motor in kW
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"cti#e Po$er %lo$s
*otor power
the power flowing across the
airgap is the product of therotating flux speed and the
magnetic tor$ue
Mechanical power
the developed power is e$ual to
the power transmitted to therotor less the rotor copper losses
and the windage and friction
losses
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"cti#e Po$er %lo$s
*otor power the power flowing across the
airgap is the product of the
rotating flux speed and the
magnetic tor$ue
Mechanical power
the developed power is e$ual to
the power transmitted to the
rotor less the rotor copper lossesand the windage and friction
losses
*otor copper losses
for constant power and speed!
the magnetic tor$ue and
mechanical load tor$ue must #e
e$ual
Motor tor$ue the tor$ue developed at any
speed is
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"cti#e Po$er %lo$s
E!aple
+ (-phase induction motor draws ,0 kW synchronous speed is 1200 rpm
stator copper and iron losses are % kW
windage and friction losses are 2 kW actual speed is 11%2 rpm
calculate
the active rotor power
the rotor copper losses
the developed mechanical power the delivered mechanical power
the motors efficiency
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E!aple
+ (-phase induction motor draws ,0 kW
synchronous speed is 1200 rpm
stator copper and iron losses are % kW
windage and friction losses are 2 kW
actual speed is 11%2 rpm
!alculate
a' the active rotor power
#' the rotor copper losses
c' the developed mechanical power
d' the delivered mechanical power
e' the motors efficiency
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"cti#e Po$er %lo$s
E!aple
+ (-phase! &0 .! ,-pole! s$uirrel-cage! induction motor stator copper and iron losses are % kW and 1 kW! respectively
the motor a#sor#s /0 kW
calculate the tor$ue developed #y the motor
what is the relationship #etween the developed tor$ue and the shaft
speed
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"cti#e Po$er %lo$s
E!aple
+ (-phase! 100 hp! &00 )! induction motor synchronous speed of 1,00 rpm
stator iron loss is 2 kW! windage and friction losses are 1'2 kW
stator copper resistance #etween two terminals is 0'(/ ohms
two-watt-meter reading e$ual to 0 kW and line current of , + at a rotor
speed of 1&( rpm
calculate
a' power supplied to the motor
#' rotor copper losses
c' mechanical power delivered to the load
d' efficiency
e' developed tor$ue
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E!aple: "cti#ePo$er %lo$s
+ (-phase! 100 hp! &00 )! induction motor
o synchronous speed of 1,00 rpm
o stator iron loss is 2 kW! windage and friction
losses are 1'2 kW
o stator copper resistance #etween two
terminals is 0'(/ ohms
o two-watt-meter reading e$ual to 0 kW andline current of , + at a rotor speed of 1&(
rpm
alculate
a' power supplied to the motor
#' rotor copper losses
c' mechanical power delivered to the load
d' 3fficiency
e' developed tor$ue
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E!aple: "cti#ePo$er %lo$s
+ (-phase! 100 hp! &00 )! induction motor
o synchronous speed of 1,00 rpm
o stator iron loss is 2 kW! windage and friction
losses are 1'2 kW
o stator copper resistance #etween two
terminals is 0'(/ ohms
o two-watt-meter reading e$ual to 0 kW andline current of , + at a rotor speed of 1&(
rpm
alculate
a' power supplied to the motor
#' rotor copper losses
c' mechanical power delivered to the load
d' 3fficiency
e' developed tor$ue
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Torque-speed Characteristic
The developed tor$ue depends upon the speed
the relationship is more complex than a single e$uation
characteristics are provided #y graphs with tor$ue-speed curves
shown on a per-unit #ase with the full-load tor$ue " as the #ase tor$ue
important points starting
pull-up
#reakdown
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Torque-speed Characteristic
4tarting tor$ue tor$ue at ero speed
typically 1'% times the full-load
tor$ue
5ull-up tor$ue the minimum tor$ue developed #y
the motor while accelerating from
ero speed
greater than full-load tor$ue6 less
than starting tor$ue
7reakdown tor$ue the maximum tor$ue that the motor
can develop
typically 2'% times the full-load
tor$ue
8ormal operation at full-load! the motor runs at n
rpm
rotor speed decreases slightly from
synchronous speed with increasing
load tor$ue motor will stall when the load
tor$ue exceeds the #reakdown
tor$ue
for small motors 91% kW; the
#reakdown tor$ue is a#out ,0< of
the synchronous speed
for large motors 9=1000kW; the
#reakdown tor$ue is a#out >,< of
synchronous speed
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&otor &esistance
The tor$ue-speed characteristics are greatly affected #y the
rotor resistance
the rotor resistance of a s$uirrel-cage rotor is essentially constant
the rotor resistance of a wound rotor can #e modified #y adding an
external three-phase resistance load
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&otor &esistance
+ high rotor resistance produces a high starting tor$ue and a
relatively small starting current
desira#le for motor starting
pro#lems appears as a rapid fall-off in speed with increasing load! a
high slip at rated tor$ue! and large copper losses
?nder running conditions it is prefera#le to have a low rotor
resistance
speed decreases much less with increasing load
slip is small at rated tor$ue efficiency is high
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'oe$or
5ro#lems: 1(-1! 1(-20! 1(-2(! and 1(-2/