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7/30/2019 Chapter 6 Induction Motor Construction
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5/9/2013 360 Chapter 7 Induction Motor 1
7.2 Construction
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Induction Motors
For industrialapplications, thethree-phase
induction motoris used to drivemachines
Figure 7.2 Large
three-phaseinduction motor.(CourtesySiemens).
Housing
Motor
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Induction Motors
Figure 7.3
Induction motor
components.
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Induction Motors
The motor housing consists of three parts:
The cylindrical middle piece that holds the stator ironcore,
The two bell-shaped end covers holding the ball bearings.
This motor housing is made of cast aluminum or cast iron.Long screws hold the three parts together.
The legs at the middle section permit the attachment of
the motor to a base.
A cooling fan is attached to the shaft at the left-hand side.This fan blows air over the ribbed stator frame.
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Induction Motors
Figure 7.4 Stator
of a large
inductionmotor.
(Courtesy
Siemens).
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Induction Motors
The iron core has cylindricalshape and is laminated with slots
The iron core on the figure haspaper liner insulation placed insome of the slots.
In a three-phase motor, the threephase windings are placed in theslots
A single-phase motor has two
windings: the main and thestarting windings.
Typically, thin enamel insulatedwires are used
Figure 7.5 Stator iron core without windings
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Induction Motors
A single-phase motor has
two windings: the main and
the starting windings
The elements of the
laminated iron core are
punched from a silicon iron
sheet.
The sheet has 36 slots and 4holes for the assembly of
the iron core.Figure 7.6 Single-phase stator with main windings.
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Induction Motors
Figure 7.7 Stator iron core sheet.
The elements of the
laminated iron core
are punched from a
silicon iron sheet.
The sheet has 36 slots
and 4 holes for theassembly of the iron
core
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Induction Motors
Figure 7.8 Stator and rotor
magnetic circuit
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Induction Motors
Squirrel cage rotor. This rotor has a laminated iron core with slots,
and is mounted on a shaft.
Aluminum bars are molded in the slots and thebars are short circuited with two end rings.
The bars are slanted on a small rotor to reduceaudible noise.
Fins are placed on the ring that shorts thebars. These fins work as a fan and improvecooling.
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Induction Motors
Rotor bars (slightly skewed)
End ring
Figure 7.11 Squirrel cage rotor concept.
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Induction Motors
Figure 7.10 Squirrel cage rotor.
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Induction Motors
Wound rotor.
Most motors use the squirrel-cage rotor because of therobust and maintenance-free construction.
However, large, older motors use a wound rotor withthree phase windings placed in the rotor slots.
The windings are connected in a three-wire wye.
The ends of the windings are connected to three slip rings.
Resistors or power supplies are connected to the slip ringsthrough brushes for reduction of starting current and speedcontrol
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Induction Motors
Figure 7.9 Rotor of a large induction motor. (Courtesy
Siemens).
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7.3 Three-Phase Induction Motor
7.3.1 Operating principle
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Induction Motors
This two-pole motor has threestator phase windings, connectedin three-wire wye.
Each phase has 2 3 = 6 slots.
The phases are shifted by 120
The squirrel cage rotor has short-circuited bars.
The motor is supplied bybalanced three-phase voltage atthe terminals.
The stator three-phase windingscan also be connected in a deltaconfiguration.
A+
A-
B+
B-
C+
C-
A BC
Figure 7.12 Connection diagram of a
two-pole induction motor with
squirrel cage rotor.
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Induction Motors
Operation Principle
The three-phase stator is supplied by balanced three-
phase voltage that drives an ac magnetizing currentthrough each phase winding.
The magnetizing current in each phase generates a
pulsating ac flux.
The flux amplitude varies sinusoidally and thedirection of the flux is perpendicular to the phase
winding.
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Induction Motors
Operation Principle
The three fluxes generated by the phase
windings are separated by 120 in space andin time for a two-pole motor
The total flux in the machine is the sum of thethree fluxes.
The summation of the three ac fluxes resultsin a rotating flux, which turns with constantspeed and has constant amplitude.
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Induction Motors
Operation Principle
The rotating flux induces a voltage in the short-circuited bars of the rotor. This voltage drives
current through the bars. The induced voltage is proportional with the
difference of motor and synchronous speed.Consequently the motor speed is less than thesynchronous speed
The interaction of the rotating flux and the rotorcurrent generates a force that drives the motor.
The force is proportional with the flux density andthe rotor bar current
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Induction Motors
The figure shows the threecomponents of the magnetic field at a
phase angle of60.
Each phase generates a magnetic fieldvector.
The vector sum of the componentvectorsFa, Fb, Fc gives the resultingrotating field vectorrot,
The amplitude is 1.5 times theindividual phase vector amplitudes,and rot rotates with constant speed.
A+
A-
B+
B-
C+
C-
Fb
Fa
Frot
Fc
Fb
Fc
Figure 7.13 Three-phase winding-
generated rotating magnetic field.
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Induced Voltage Generation
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Induction Motors
Faradays law
Voltage is induced in a
conductor that movesperpendicular to a
magnetic field,
The induced voltage is:
Magnetic field B into page
Conductor
moving
upward with
speed v
Induced voltage V
Conductor length L
v v
Figure 7.14 Voltage induced in
a conductor moving through a
magnetic field.
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Induction Motors
The three-phase winding onthe stator generates a rotatingfield.
The rotor bar cuts the
magnetic field lines as thefield rotates.
The rotating field induces avoltage in the short-circuitedrotor bars
The induced voltage isproportional to the speeddifference between therotating field and the spinningrotor
A+
A-
B+
B-
C+
C-
Fb
Fa
Frot
Fc
Fb
Fc
V = B L (vsynv m)
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Induction Motors
The speed of flux cutting isthe difference between themagnetic field speed and therotor speed.
The two speeds can becalculated by using the radiusat the rotor bar location and
the rotational speed.
A+
A-
B+
B-
C+
C-
Fb
Fa
Frot
Fc
Fb
Fc
mrotmot
synrotsyn
nrv
nrv
2
2
msynrotrotbarnnBrV 2
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Induction Motors
The voltage and current generation in the rotor bar require aspeed difference between the rotating field and the rotor.
Consequently, the rotor speed is always less than the magneticfield speed.
The relative speed difference is the slip, which is calculatedusing
sy
msy
sy
msy
n
nns
2p
fnsy The synchronous speed is
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Motor Force Generation
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Induction Motors
The interactionbetween the magneticfield B and the
current generates aforce
F = B L I+
B B B B
F
B
Figure 7.15 Force direction on a current-carrying conductor placed in a magneticfield (B) (current into the page).
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Induction Motors
Force generation in a motor
The three-phase windinggenerates a rotating field;
The rotating field induces acurrent in the rotor bars;
The current generation requiresa speed difference between therotor and the magnetic field;
The interaction between thefield and the current producesthe driving force.
Brotating
Force
Ir
Rotor Bar
Ring
Figure 7.16 Rotating
magnetic field generated
driving force.
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7.3.2 Equivalent circuit
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Induction Motors
An induction motor has two magnetically coupled circuits:the stator and the rotor. The latter is short-circuited.
This is similar to a transformer, whose secondary is
rotating and short-circuited.
The motor has balanced three-phase circuits; consequently,the single-phase representation is sufficient.
Both the stator and rotor have windings, which haveresistance and leakage inductance.
The stator and rotor winding are represented by aresistance and leakage reactance connected in series
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Induction Motors
A transformer represents the magnetic couplingbetween the two circuits.
The stator produces a rotating magnetic field that
induces voltage in both windings.
A magnetizing reactance (Xm) and a resistance connected inparallel represent the magnetic field generation.
The resistance (Rc) represents the eddy current and hysteresis
losses in the iron core
The induced voltage is depend on the slip and the turnratio
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Induction Motors
Stator Rotor
Xrot_m
= rot
Lrot
Rrot
Irot
Vrot= s V
rot_s
Rsta
Irot_t
Vsta
Vsup
Ista Xm
Rc
Xsta
= sy
Lsta
Figure 7.17 Single-phase equivalent circuit of a
three-phase induction motor.
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Induction Motors
In this circuit, the magnetizing reactance generates a flux thatlinks with both the stator and the rotor and induces a voltage inboth circuits.
The magnetic flux rotates with constant amplitude andsynchronous speed.
This flux cuts the stationary conductors of the stator with thesynchronous speed and induces a 60 Hz voltage in the statorwindings.
The rms value of the voltage induced in the stator is:
2
max systa
sta
NV
F
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Induction Motors
The flux rotates with the synchronous speed and the rotorwith the motor speed.
Consequently, the flux cuts the rotor conductors with the
speed difference between the rotating flux and the rotor.
The speed difference is calculated using the slip equation:
The induced voltage is:
ssymsy )(
22
)( m axm ax sNNV
syrotmsyrot
rot
F
F
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Induction Motors
The division of the rotor and stator induced voltageresults in:
This speed difference determines the frequency of the rotorcurrent
sVsVN
N
V srotstasta
rot
rot _
sy
symsyrotrot fs
sf
222
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Induction Motors
The division of the rotor and stator induced voltageresults in:
This speed difference determines the frequency of the rotorcurrent
The rotor circuit leakage reactance is:
sVsVN
NV srotsta
sta
rotrot _
sy
symsyrotrot fs
sf
222
sXsLLX rotsyrotrotrotmrot _
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Induction Motors
The relation between rotor current and the rotor-induced voltage is calculated by the loop voltageequation:
The division of this equation with the slip yields
The implementation of this equation simplifies theequivalent circuit
)( sXjRs rotrot rotrot_srot IVV
rot
rot Xjs
R
rotrot_s
IV
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Induction Motors
Stator Rotor
Xsta
Xrot
Rrot
/s
IrotV
rot_s
Rsta
Irot_t
Vsta
Vsup
Ista
Xm
Rc
Figure 7.18 Modified equivalent circuit of a three-phase
induction motor.
The rotor impedance is transferred to the stator side. This
eliminates the transformer
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Induction Motors
Figure 7.19 Simplified equivalent circuit of a three-phase
induction motor.
Stator Rotor
Xsta
Xrot_t
Rrot_t
/s
Irot_t
Vsta
Rsta
VsupIsta
XmRc
Air gap
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Induction Motors
Stator Rotor
Xsta
Xrot_t
Rrot_t
Irot_t
Vsta
Rsta
Vsup
Ista
XmRc
Air gap
Rrot_t(1-s)/s
Figure 7.20 Final single-phase equivalent circuit of a three-phase
induction motor.