field oriented control ANNAUNIVERSITY,SYLLABUS

1. SYLLABUS
Field oriented control of induction machines theory
DC drive analogy-direct and indirect methods
Flux vector estimation
Direct torque control of induction machines
Torque expression with stator and rotor fluxes
DTC control strategy
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2. Introduction
 Scalar control of ac drives produces good
steady state performance but poor dynamic
response.
 This manifests itself in the deviation of air gap
flux linkages from their set values.
 This variation occurs in both magnitude and
phase.
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3. Introduction (cont’d)
 Vector control (or field oriented control) offers more
precise control of ac motors compared to scalar
control.
 They are therefore used in high performance drives
where oscillations in air gap flux linkages are
intolerable, e.g. robotic actuators, centrifuges, servos,
etc.
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4. Introduction (cont’d)
Why does vector control provide superior dynamic
performance of ac motors compared to scalar
control ?
In scalar control there is an inherent coupling
effect because both torque and flux are functions
of voltage or current and frequency.
This results in sluggish response and is prone to
instability because of 5th order harmonics.
Vector control decouples these effects.
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5. Introduction (cont’d)
 Vector control is also known as decoupling,orthogonal
or transvector control.
 Vector control is applicabe to both induction and
synchronous motor drives
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6. DC drive analogy (cont’d)
 There is a close parallel between torque control of
a dc motor and vector control of an ac motor.
 It is therefore useful to review torque control of a
dc motor before studying vector control of an ac
motor.
 A vector controlled induction motor drive
operates like a separately excited dc motor drive
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7. DC drive analogy (cont’d)
A dc motor has a stationary field structure
(windings or permanent magnets) and a rotating
armature winding supplied by a commutator
and brushes.
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8. DC drive analogy (cont’d)
 The basic structure and field flux and armature MMF are
shown below:
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9. DC drive analogy
 The field flux f (f) produced by field current If
 The armature flux a (a) produced by the
armature current Ia.
 The developed torque Te can be written as:
 They are decoupled, i.e. the field current only
controls the field flux and the armature current
only controls the armature flux.
'
e t a fT K I I
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10. DC drive analogy
 DC motor-like performance can be achieved with
an induction motor
 The motor control is considered in the
synchronously rotating reference frame (de-qe)
where the sinusoidal variables appear as dc
quantities in steady state.
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11. DC drive analogy
(Vector controlled induction motor)
 Two control inputs ids * and iqs * can be used for a vector
controlled inverter as shown in the fig
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12. DC drive analogy
or
- is the peak value of the sinusoidal space vector
Compared to the dc machine space vectors,
induction motor space vectors rotate
synchronously at frequency
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13. DC drive analogy
This dc motor-like performance is only possible if iqs
*
only controls iqs and does not affect the flux Ψ, i.e. iqs
and ids are orthogonal under all operating conditions
of the vector-controlled drive.
Thus, vector control should ensure the correct
orientation and equality of the command and actual
currents.
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14. Equivalent Circuit and phasor diagram
The complex de-qe equivalent circuit of an induction
motor is shown in the below figure (neglecting rotor
leakage inductance).
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15. Equivalent Circuit and phasor diagram
The stator current vector Is is the sum of the ids
and iqs vectors. Thus, the stator current
magnitude, Is is related to ids and iqs by:
2 2
s ds qsI i i $
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16. Equivalent Circuit and phasor diagram
The steady state phasor (or vector) diagrams for an
induction motor in the de-qe (synchronously
rotating) reference frame are shown below:
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17. Equivalent Circuit and phasor diagram
---Terminal Stator voltage
--
--- magnetizing component of stator current
Air gap voltage
--Stator current
--- Rotor flux -- Air gap flux
----Torque component of stator current flowing in
the rotor circuit
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18. Equivalent Circuit and phasor diagram
The rotor flux vector is aligned with the de
axis and the air gap voltage is aligned with the
qe axis.
The terminal voltage Vs slightly leads the air gap
voltage because of the voltage drop across the stator
impedance.
iqs contributes real power across the air gap but
ids only contributes reactive power across the air
gap.
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19. Equivalent Circuit and phasor diagram
The first figure shows an increase in the torque component
of current iqs and the second figure shows an increase in
the flux component of current, ids.
The orthogonal orientation of these components,
The torque and flux can be controlled independently.
It is necessary to maintain these vector orientations under
all operating conditions.
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20. Principles of Vector Control
The basic conceptual implementation of vector control is
illustrated in the below block diagram
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21. Principles of Vector Control
The motor phase currents, ia, ib and ic are
converted to ids
s and iqs
s in the stationary
reference frame. These are then converted to the
synchronously rotating reference frame d-q
currents, ids and iqs.
In the controller two inverse transforms are
performed:
1) From the synchronous d-q to the
stationary d-q reference frame;
2) From d*-q* to a*, b*, c*.
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22. Principles of Vector Control
There are two approaches to vector control:
1) Direct field oriented current control
here the rotation angle of the iqs vector with respect
to the stator flux qr
s is being directly determined
(e.g. by measuring air gap flux)
2) Indirect field oriented current control
- here the rotor angle is being measured indirectly,
such as by measuring slip speed.
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23. Direct or feed back Vector Control
A block diagram of a direct vector control method using a PWM
voltage-fed inverter
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24. Direct or feed back Vector Control
In direct vector control the field angle is calculated
by using terminal voltages and current or Hall
sensors or flux sense windings.
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25. Direct Vector Control (cont’d)
The principal vector control parameters, ids
* and iqs
*,
which are dc values in the synchronously rotating
reference frame, are converted to the stationary
reference frame
(using the vector rotation (VR) block) by using the
unit vector cose and sine.
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26. Direct Vector Control (cont’d)
 These stationary reference frame control
parameters ids
s* and iqs
s* are then changed to the
phase current command signals, ia
*, ib
*, and ic
*
which are fed to the PWM inverter.
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27. Direct Vector Control (cont’d)
A flux control loop is used to precisely control the flux.
Torque control is achieved through the current iqs
*
which is generated from the speed control loop
The torque can be negative which will result in a
negative phase orientation for iqs in the phasor diagram.
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28. Direct Vector Control (cont’d)
From the vector diagram
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29. Direct Vector Control (cont’d)
Here the de-qe frame is rotating at synchronous
speed e with respect to the stationary reference
frame ds-qs,
Any point in time, the angular position of the
de axis with respect to the ds axis is e (=et).
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30. Direct Vector Control (cont’d)
The cose and sine signals in correct
phase position are shown below:
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31. Direct Vector Control (cont’d)
These unit vector signals, when used in the vector
rotation block, cause ids to maintain orientation
along the de-axis and the iqs orientation along the
qe-axis.
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32. Summary of Salient Features of Vector
Control(feed back vector control)
A few of the salient features of vector control are:
 The frequency e of the drive is not controlled (as in
scalar control).
 The motor is “self-controlled” by using the unit vector
to help control the frequency and phase.
 There is no concern about instability because limiting
within the safe limit
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33. Summary of Salient Features of VectorControl
(feed back vector control)
 Transient response will be fast because torque
control by iqs does not affect flux.
 Vector control allows for speed control in all four
quadrants (without additional control elements)
since negative torque is directly taken care of in
vector control.
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34. INDIRECT VECTOR CONTROL
Indirect vector control is similar to direct vector
control except the unit vector signals (cose and
sine) are generated in a feedforward manner.
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35. Indirect Vector Control (cont’d)
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36. Indirect Vector Control (cont’d)
Features of this implementation:
 Diode rectifier front-end with a PWM inverter with a
dynamic brake in the dc link.
 Hysteresis-band current control.
 Speed control loop generates the torque component of
current, iqs
*.
 Constant rotor flux is maintained by using the desired ids
*.
 The slip frequency sl
* is generated from the desired iqs
*.
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37. Indirect Vector Control (cont’d)
 Slip gain Ks is given by:
 e and e are given by:
and
 The incremental encoder is necessary for indirect
vector control because the slip signal locates the
rotor pole position with respect to the dr axis in a
feedforward manner.
µ
*
*
sl m r
s
qs r r
L R
K
i L


 
*
e sl r    edt  
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38. Indirect Vector Control (cont’d)
phasor diagram explaining the indirect vector control
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39. Indirect Vector Control (cont’d)
The speed control range can be extended into the field
weakening region by incorporating the dotted line part of the
implementation
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40. Indirect Vector Control (cont’d)
The ds-qs axes are fixed on the stator and the dr-qr
axes are fixed on the rotor. The de-qe axes are
rotating at synchronous speed and so there is a
slip difference between the rotor speed and the
synchronous speed given by:
we can write:
e r sl   
e edt  
e r sl   
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41. Indirect Vector Control (cont’d)
In order to ensure decoupling between the stator
flux and the torque,
the torque component of the current, iqs, should be
aligned with the qe axis and the stator flux
component of current, ids, should be aligned with
the de axis.
We can use the de-axis and qe-axis equivalent
circuits of the motor (shown on the next slide) to
derive control expressions.
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42. Indirect Vector Control (cont’d)
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43. Indirect Vector Control (cont’d)
The rotor circuit equations may be written as:
( ) 0dr
r dr e r qr
d
R i
dt

     
( ) 0
qr
r qr e r dr
d
R i
dt

     
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44. Indirect Vector Control (cont’d)
The rotor flux linkage equations may be written
as:
These equations may be rewritten as:
dr r dr m dsL i L i  
qr r qr m qsL i L i  
1 m
dr dr ds
r r
L
i i
L L
 
1 m
qr qr qs
r r
L
i i
L L
 
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45. Indirect Vector Control (cont’d)
Combining these with the earlier equations allows us to
eliminate the rotor currents which cannot be directly
obtained. The resulting equations are:
where .
0dr mr
dr r ds sl qr
r r
d LR
R i
dt L L

     
0
qr mr
qr r qs sl dr
r r
d LR
R i
dt L L

     
sl e r   
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46. Indirect Vector Control (cont’d)
For implementing the indirect vector control strategy, we need
to take these equations into consideration as well as the
equation:
Note:
A constant rotor flux results in the equation:
so that the rotor flux is directly proportional to ids in steady state.
µ
m dsr L i 
e r sl   
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47. Direct torque and flux control
 Advanced scalar control technique known as direct
torque and flux control (DTFC or DTC) or direct self
control was introduced for voltage fed PWM inverter
drives
 Nearly comparable performance with vector controlled
drives
 Direct control of torque and stator flux of a drive by
inverter voltage space vector selection through a
lookup table
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48. Torque expression with stator flux and
rotor fluxes
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49. Torque expression with stator flux and rotor fluxes
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Stator flux is
Stator current is
Torque equation is

50. Torque expression with stator flux and rotor fluxes
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Magnitude of torque is-----
-
Incremental torque
expression is

51. Control strategy of DTC
(Direct torque and flux control block diagram)
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52. Control strategy of DTC
 The speed control loop and the flux program as a
function of speed are as usual
 The errors are processed through hysteresis band
controllers
 The flux controllers has two levels of digital output
according to the following relations
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53. Control strategy of DTC
 The torque control loop has three levels of digital
output , which have the following relations
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54. Control strategy of DTC
( Trajectory of stator flux vector in DTC control)
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Total hysteresis
band width of
the flux
controller

55. Control strategy of DTC ( inverter voltage vectors and
corresponding stator flux variation in change in time)
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56. Control strategy of DTC
(Switching table of inverter voltage vectors)
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57. Control strategy of DTC (inverter voltage vectors )
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58. Control strategy of DTC
 The feedback flux and torque are calculated from the
machine terminal voltages and currents.
 The signal computation block also calculates the
sector number s(k) in which the flux vector lies
 There are six vectors
 The drive can easily operate in the four quadrants and
speed loop and field weakening control can be added
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59. Special features of DTC
 No feedback current control
 No traditional PWM algorithm is applied
 No vector transformation as in vector control
 Feedback signal processing is somewhat similar to
stator flux oriented vector control
 Hysteresis band control generates flux and torque
ripple and switching frequency is not constant
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60. FLUX VECTOR ESTIMATION
 Voltage model
 Current model
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61. FLUX VECTOR ESTIMATION(voltage model)
 In this method the machine terminal
voltage and currents are sensed and the
fluxes are computed from the equivalent
circuit .
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62. FLUX VECTOR ESTIMATION(voltage model)
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63. FLUX VECTOR ESTIMATION(voltage model)
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64. FLUX VECTOR ESTIMATION (voltage model)
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65. FLUX VECTOR ESTIMATION (voltage model)
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 Feed back signal estimation with help of a
microcomputer is shown
 Stator fluxes,airgap fluxes and torque also estimated
 Low pass filtering and 3 phase to 2 phase conversion
with help of operational amplifiers before conversion
by the A/D converter

66. FLUX VECTOR ESTIMATION(current model)
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