Vector Control
Application to Induction Motor Control
DSP in Motion Control - Seminar
Vector Control - Principle
The Aim of Vector Control is to Orient the Flux Producing
Component of the Stator Current to some Suitable
Flux Vector under all Operating Conditions
Different Schemes Possible if Use:
Rotor Flux Vector => RFO
Stator Flux Vector => SFO
Airgap Flux Vector => MFO
DSP in Motion Control - Seminar
Direct & Indirect Vector Control
Different Implementations are Possible Depending on whether or not
there is Direct Feedback of the Flux Magnitude and Orientation
DIRECT VECTOR CONTROL: Uses Sensors or Model to
Provide Feedback of the Flux
Magnitude and Orientation.
INDIRECT VECTOR CONTROL: Uses Assumed Slip Frequency
Relationship to Achieve
Field Orientation.
DSP in Motion Control - Seminar
Rotor Flux Orientation - RFO
Orientation to the Rotor Flux Vector is the only Scheme that
Provides Complete Decoupling of the Torque and Flux
Producing Components in the Induction Motor.
=> Most Popular Approach.
Both Direct and Indirect Schemes are Possible.
Indirect Schemes are Favored due to Problems of Good Flux
Estimation at Low Speeds
DSP in Motion Control - Seminar
Principle of RFO Control
jsβ
jsq
is
sd
ψr
isd
i sq
ρ
sα
DSP in Motion Control - Seminar
Direct RFO Orientation of IM
• Direct Sensing of Airgap Flux using Hall Sensors
– Original Method of Blaschke for Field Orientation
– Problems due to Signal Distortions due to Slot Ripple
– Thermal and Mechanical Fragility of Sensors
• Alternatively Monitor Rate of Change of Airgap Flux
using Search Coils
– Non-Standard Machine Construction
– Difficulty of Integrating Measured Signals at Low Speeds
• Most Modern Direct Field Oriented Controllers use a
Motor Model to Estimate the Rotor Flux Components
DSP in Motion Control - Seminar
Voltage Model for Rotor Flux
Estimation
ψs =
∫ (v
s
− R si s )dt
Lr
ψr =
(ψ s − σLsi s )
Lm
L2m
σ = 1−
Ls Lr
Total Leakage
Factor
ψ r = ψ 2rd + ψ 2rq
 ψ rq 

ρ = tan 
 ψ rd 
−1
DSP in Motion Control - Seminar
Current Model for Rotor Flux
Estimation
1
ψr =
Tr
∫ (L
)
i − (1 − jω r Tr )ψ r dt
m s
Lr
Tr =
Rr
Rotor Time
Constant
ψ r = ψ 2rd + ψ 2rq
 ψ rq 

ρ = tan 
 ψ rd 
−1
DSP in Motion Control - Seminar
Rotor Flux Estimation
• Voltage Model
– Difficult at Low Speeds as Rs Dominates Stator Voltage Equation
– Good Estimates at Medium and High Speeds
– Lr/Lm and σ are only Moderately Affected by Saturation
• Current Model
– Allows Low and Zero Speed Estimation
– Difficult at High Speeds Due to Cross Coupling
– Dependency on Rotor Time Constant
• Best Solution is to Combine both Models in a Closed Loop
Flux Observer
DSP in Motion Control - Seminar
Closed Loop Flux Observer
vs
Rs
is
e
Voltage
Model
+
− jθ r
Lm
1 + pTr
Current Model
-
σL s
ψrC
ej
θr
+
+
K1
-
+
+
1
p
+
L r ψr
Lm
K2
p
DSP in Motion Control - Seminar
Direct Vector Control of IM - RFO
T e*
*
iqs
+
+
-
*
vqs
-
e
ωr
ψ*
+
-
+
PWM
VSI
M
~
*
vds
*
ids
r
jρ
-
ids
-jρ
iqs
ψ
r
Te
e
ρ
2
3
Flux and
Torque Model
Measured
Motor
Signals
DSP in Motion Control - Seminar
Indirect Vector Control of IM
• Gained Popularity as Operation to Zero Speed is Possible
by Adding Rotor Position or Speed Transducer
• Compute Current Components in Synchronous Frame and
Slip Frequency to Maintain Field Orientation
2  L r  Te*

 *
i =
3p  L m  ψ r
*
qs
1  *
dψ *r 
 ψ r + Tr

i =
Lm 
dt 
*
ds
1
ω2 = ωe − ωr =
Tr
Apply Using
Closed-Loop
Current Control
 L mi *qs 
 * 
 ψr 
DSP in Motion Control - Seminar
Indirect Vector Control of IM - RFO
T e*
Lr
2 ___
__
3p L m
( )
N
..
__
*
i qs
Current
Regulated
PWM
VSI
D
ψ*
r
1
__
Lm
Encoder
θe
1
__
Tr
..
__
D
~
*
i ds
d
1 + T r __
dt
N
M
ω
2
∫
θ2
θr
+
+
DSP in Motion Control - Seminar
Indirect Vector Control of IM
Controller is a Feedforward Structure
=> Parameter Errors Produce:
Tr Errors Most Significant
due to Thermal and
Saturation Changes
• Incorrect Field Orientation
• Over- or Under-Fluxing of Machine
• Coupled Torque and Flux Responses
• Excessive Losses and Heating
• Inefficient Drive Operation
• Transient Torque Oscillations
DSP in Motion Control - Seminar
Slip Gain Tuning of Indirect RFO
Use a Model Reference Adaptive Controller (MRAC) to
track Variations in Rr or Tr.
Compute some function in two different ways, one
of which depends on the parameter to be tracked and
one of which does not.
Use difference to force parameter to converge to correct value.
Suitable Functions: Modified Reactive Power
Electromagnetic Torque
Various Voltage Terms
DSP in Motion Control - Seminar
MRAC for Slip Gain Tuning
Calculation #1
+
-
Adaptation
Mechanism
T$ r
Calculation #2
DSP in Motion Control - Seminar
Direct Stator Flux Oriented Control
Recently Direct Stator Flux Orientation (SFO) has emerged as
a possible alternative for IM Control.
Above a few Hz, can Reliably Estimate the Stator Flux using
the Voltage Model.
Compute Magnitude and Orientation for Control from:
ψ s = ψ 2sd + ψ 2sq
 ψ sq 

ξ = tan 
 ψ sd 
−1
DSP in Motion Control - Seminar
Direct SFO for IM
• Main Problem is that Torque and Flux Axis are not
Decoupled
• Need to Include a Parameter Dependent Decoupling
Network
• Better Torque/Ampere Performance in Field Weakening
than RFO
• Torque and Flux Control does not require Speed Feedback
• Operation to Low Speeds Difficult due to Inaccurate Flux
Estimation
• Good Alternative for Medium Performance Drives
DSP in Motion Control - Seminar