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EE538EE538
ASSIGNMENT No:2ASSIGNMENT No:2Vector Controlled Induction MotorsVector Controlled Induction Motors
E/05/185 Lokugamage A.U.
E/05/295 SiriwardhanaA.S.L
E/05/298 Sooriyadasa S.M.D.P.K
E/05/321 Thrikawela M.M.E/05/341 WickramarathneW.J.C.
E/05/354 Wijesurendra K.P.N.U
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IntroductionIntroduction
controlled electrical drives has undergone rapid expansiondue mainly to the advantages of semiconductors in bothpower and signal electronics
This leads to AC drive control with ever lower powerdissipation hardware and ever more accurate control
structures. In vector control, speed and torque can be controlled by
controlling both the magnitude and the phase of each phasecurrent and voltage vectors
Control of current/voltage associated with field, hence thisstrategy is called field oriented controlled as well.
Two control strategies are filed orientation withcurrent control and field orientation with voltagecontrol (FOC)
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comparisoncomparisony Controlled strategies we have discussed such as inverter fed
IM, provided good steady state response but not in
transients (high current flow in transients which can damage
drive system)
y Deviation of air gap flux linkage in magnitude as well as in
phase, from their set values is the reason for this.
y other controllers utilized stator phase voltage/current
magnitude and frequency but not their instantaneous
phase, which deviate linkage flux from its set value.
Air gap flux
oscillation
Oscillation
of torque
Oscillation
of speedDamage to the system
Current
oscillations
Large stator currents
High rated equipments
required
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Comparison contnd..Comparison contnd.. Now its clear that for good dynamic characteristics control of
phase in above quantities is necessary which is addressed by
vector controlled system
In FOC both phase and magnitude of stator quantities are
controlled. Hence good dynamic performance
In FOC the ease of reaching constant reference (torque
component- IE and flux component of the stator current- IF)
Independent control of torque and flux is possible as dc drives
handling system limitations and achieve higher power conversion
efficiency compared to other techniques Suitable for high performance application such as servos, process
drives, metal rolling mills etc.
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Field Orientation with Current ControlField Orientation with Current Control
y In IM current in rotor which generate torque should begenerated by induction.
Figure 1.a Figure 1.b Figure 2.a Figure 2.b
I1 generates ] then apply I2 , since linked flux of rotor bars is changed it
induces I3 (refer vector diagram) to neutralize this (Fleming law+ lense law)
How ever due to induced I3 requires a field change (current caring conductorin a field) vector diagram changes as figure 2.b after certain time.
If rotor is assumed to be locked, then stator is rotates such that field and I1again parallel and again orientation is restored as figure 4.a
In reality stator is stationary and rotating vector is formed by rotating current
IE & IF (figure 5)
I1
I2 I3 ]
I1
I2
I3
]
I1
I2 I3
I1
I2 I3
]
J
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FOC with current control contnd..FOC with current control contnd..I1
I2
I3
I3
]
I2
J
Figure 4.a Figure 4.b
I2
IE
IF
I1
I
IE
I2
J
IF
Figure 5.aFigure 5.b
IE=I1cosJ- I2sinJtorque component IF=I1sinJ+ I2cosJflux
component)
Above relationship is obtained using vector rotor
(VR) which rotates current vector by the angle of field
I1
I2
sinJ cosJ
+
+
+
-
In puts to VR are set point values I1*
& I2* and field angle which is taken
from Hall generators which are at
deferent angles in air gapFigure 6
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FOC with current control contnd..FOC with current control contnd..
y Vector Analyzer (VA) converts angle (J) into sinJ & cosJ
y What actually VR does is transformation of current
vector I from field oriented coordinate system to stator
oriented coordinate system. Refer figure 7
y Output of VR fed in to variable current static controllerU to obtain IE and IF (vector multiplier)
I1
I2
CosJ sinJ
SinJ + cosJ U
IE*
IF*
IE
IF
Figure 7
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Application of vector rotor for fieldApplication of vector rotor for field
orientation in an IMorientation in an IMy This gives separate access to the field current andseparate access to torque producing current
y Thus it is possible to operate an IM in same manner as a
separately excited dc motor with current control
J
U
VA
IE
IFIF*
VR
I1*
I2*
IE*
sinJ cosJ
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Field orientation with voltage controlField orientation with voltage control
Open loop Control
To achieve the field orientation,
It is necessary to determine voltage
positioning values UE and UF
correspond to the current reference
values I and I
This relationship can be obtained in two steps
cosJ
U
VA
IE
IFIF*
VR 1
I1*
I2*
IE*
sinJ
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Step 1
Voltage vector U] in the field coordinate system is formed from the
current vector U] I
and I
U] vector contains,
Vectors for the resistive & inductive voltage drops of the
current
Vector for the back emf of the motor
This relationship is established in a computation circuit E
To do this, E needs information from the motor and contains a
simulation of the structure of the motor
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Step 2
U]
is transformed to Us* (Stator coordinate system)
Use Us* = D] U]
This transformation is done by the VR1
Result (UE and UF
) is then fed to the static convertor as
manipulated variables
Motor resistance is varying with the operating temperature
Thus, current at the stator deviates from the reference values I and I
The operating temperature of the motor cannot generally be taken in to
account in the computational circuit
There fore, Closed loop system is required
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Closed loop control
At steady state, components ofI] remain constant
And I] can be obtained by measurement of the stator oriented
current vectorIs and subsequent transformation of the field coordinate
system,
I] =D-1]Is
I] =D]Is
VA2
cosJ
U
VA
IE
IFIF*
VR 1
I1*
I2*
IE*
sinJ
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This transformation is done by VR2
Since actual I] is known error can be calculated
Then error is fed to integrator type controller (PI)
Hence required current can be maintained even
though the operating temperature is varying
Closed loop control Ctd
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This model along with the
Clark and Park
transformations can be used
as a alternative method for
deriving a time independentcoordinate system.
Below procedure can be
used to derive the time
independent coordinatesystem.
Isd and Iaq are the two
current vectors which are
important.
A Mathematical Model for
Coordinate Transformation
IM
Motor
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A Mathematical Model, cont Space Vector definition and projection
The (a ,b ,c)->( ,) projection (Clarketransformation)
The ( ,)->(d ,q) projection (Park
transformation)
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referencesreferences
y EE538 course note -: Siemens review
y Electric Motor Drives by R. Krishnan
y Field Orientated Control of 3-Phase AC-
Motors, Literature Number:
BPRA073:Texas Instruments Europe