International Journal of Engineering Trends and Technology (IJETT) – Volume-43 Number-6 -January 2017

Comparison Between Direct and Indirect Field

Oriented Control of Induction Motor
Venu Gopal B T

Research Scholar, Department of Electrical Engineering UVCE, Bangalore University, Bengaluru

ABSTRACT - Vector control, also called field- solutions and performance. Thirteen years later, a new
oriented control (FOC), is a Variable frequency technique for the torque control of
Drive (VFD) control method in which induction motors was developed and presented by
the stator currents of a 3 phase induction motor are I.Takahashi
identified as two orthogonal components that can be as direct torque control (DTC) and by M. Depenbrock
visualized with a vector. The vector control of induction as direct self control (DSC). Since the beginning, the
motors is one of the most suitable and popular speed new technique was characterized by simplicity, good
control technique presently used. The vector control performance and robustness . Using DTC or DSC it is
technique decouples the two components of stator possible to obtain a good dynamic control of the torque
current space vector: one providing the control of flux without any mechanical transducers on the machine
and the other providing the control of torque. The two shaft. Thus, DTC and DSC can be considered as
components are defined in the synchronously rotating “sensor less type” control techniques. The basic scheme
reference frame. With the help of this control technique of DSC is preferable in the high power range
the induction motor can replace a separately excited dc applications.
motor. The scalar control technique is simple to
implement but have the coupling effect ultimately Now a days more than 60% of all the electrical energy
responsible for the sluggish response, which leads to generated in the world is used by cage induction
instability due to higher order system effect. The DC machines Variable speed or adjustable torque control of
motor needs time to time maintenance of commutator, electrical motor drives are crucial components in almost
brushes and brush holders. The main effort is to all-modern industrial manufacturing processes.
replace DC motor by an induction motor and merge the Traditionally variable speed electric machines were
advantages of both the motors together into variable based on dc motors, but for the last 45 years, Ac
speed brushless motor drive and eliminate the drives using induction machines are now finding
associated problems. The squirrel cage induction increasing acceptance in various industrial applications
motor being simple, rugged, and cheap and requiring because of the performance they can provide. The
less maintenance, has been widely used motor for fixed development of high performance control strategies for
speed application. The induction motor is transformed ac drives, driven by industry requirements has followed
from a non-linear to linear control plant. It is expected a rapid evolution during the last two decades. On the
that with increasing computational power of the DSP other hand, D.C machines have been used for variable
controllers, it will eventually nearly universally speed applications. In DC machines mmf axis is
displace scalar volts -per-Hertz (V/f) control. In this established at 90˚ electrical to the main field axis. The
paper we will come to know the concept of vector electromagnetic torque is proportional to the product of
control and different types of vector control techniques field flux and armature current. Field flux is
available. And finally we will be able to compare them. proportional to the field current and is unaffected by the
armature current because of orthogonal orientation
Key words-Induction Motor, Vector Control, Scalar between armature mmf and field mmf .Therefore in a
Control, Sluggish, Higher Order System Effect, DSP. separately excited DC machine , with a constant value
of field flux the torque s directly proportional to the
I. INTRODUCTION armature current.
ALMOST 45 years ago, in 1971 F. Blaschke presented
Electric drive has evolved over the year & so have the
the first paper on field-oriented control (FOC) for
techniques to control their speed & torque. There are a
induction motors. Since that time, the technique was
large number of researches activities taking place in
completely developed and today field oriented
order to achieve stable control techniques with growing
controlled drives are industrial reality and are available
capabilities in similar fields, the control techniques are
on the market by several producers and with different

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 International Journal of Engineering Trends and Technology (IJETT) – Volume-43 Number-6 -January 2017

also become better with time. In 1980s, DC motor mathematical transformations, thereby avoiding
drives were generally used in variable speed drives problems due to the commutator. The mentioned
because of the simplicity of control due to decoupling control techniques have undergone considerable
between armature current & the field current. research over the last 20 years, but several problems
The control & estimation of ac drives in general is remain. The implementation of those concepts was
considered more complex than those of dc drives, & possible due to technological developments such as the
this complexity increase substantially if performance DSP controllers and the new power semiconductors.
are demanded. To control the induction motor there are The electrical DC drive systems are still used in a wide
different types of control are there they are: range of industrial applications, although they are less
reliable than the AC drives. Their advantage consists in
 Variable supply voltage control simple and precise
 Variable rotor resistance control command and control structures.
 Constant Volts/Hz control (scalar control) There are two fundamental directions for the induction
 Direct torque control motor control:
 Vector Control • Analogue: direct measurement of the machine
parameters (mainly the rotor speed), which are
The name direct torque control is derived by the fact compared to the reference signals through closed
that, on control loops;
the basis of the errors between the reference and the • Digital: estimation of the machine parameters in the
estimated sensor less control schemes (without measuring the
values of torque and flux, it is possible to directly rotor speed), with the following implementation
control the methodologies:
inverter states in order to reduce the torque and flux
errors within the prefixed band limits. Unlike FOC, • Slip frequency calculation method;
DTC does not require any current regulator, coordinate • Speed estimation using state equation;
transformation and PWM signals generator (as a • Estimation based on slot space harmonic voltages;
consequence timers are not required). In spite of its • Flux estimation and flux vector control;
simplicity, DTC allows a good torque control in steady- • Direct control of torque and flux;
state and transient operating conditions to be obtained. • Observer-based speed sensor less control;
The problem is to quantify how good the torque control • Model reference adaptive systems;
is with respect to FOC. In addition, this controller is • Kalman filtering techniques;
very little sensible to the parameters detuning in • Sensor less control with parameter adaptation;
comparison with FOC. On the other hand, it is well • Neural network based sensor less control;
known that DTC presents some disadvantages that can • Fuzzy-logic based sensor less control.
be summarized in the following points:
The interest in sensor less drives of induction motor has
1) difficulty to control torque and flux at very low grown significantly over the past few years due to some
speed; of their advantages, such as mechanical robustness,
2) high current and torque ripple; simple construction and less maintenance. These
3) variable switching frequency behaviour; applications include pumps and fans paper and textile
4) high noise level at low speed; mills, subway and locomotive propulsions, electric and
5) lack of direct current control. hybrid vehicles, machine tools and robotics, home
appliances, heat pumps and air conditioners, rolling
It is now recognized that the two high-performance mills, wind generation systems etc.
control strategies for induction motor drives are Field
Oriented Control (FOC) and Direct Torque Traditionally, induction motors have been run at a
Control(DTC) . Both FOC and DTC are strategies single speed, which was determined by the frequency
allow torque and flux to be decoupled and controlled of the main voltage and the number of poles in the
independently. In the dc machine, this decoupling is motor. Controlling the speed of an induction motor is
obtained in an electromagnetic way by orienting the far more difficult than controlling the speed of a DC
current with respect to the stator flux using a motor since there is no linear relationship between the
commutator. motor current and the resulting torque as there is for a
In ac machines, this decoupling is obtained by DC motor. Vector control offers a number of benefits
implementing including speed control over a wide range, precise

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 International Journal of Engineering Trends and Technology (IJETT) – Volume-43 Number-6 -January 2017

speed regulation, fast dynamic response, and operation regulate the speed. “FOC has been solely developed for
above base speed. The choice of technique is vital in high-performance motor applications which can operate
determining the overall characteristics and performance smoothly over the wide speed range, can produce full
of the drive, during normal operation the control torque at zero speed, and is capable of quick
strategy must ensure that motor operation is restricted acceleration and deceleration”.
to the regions of high torque per ampere, there by
matching the inverter ratings and minimizing the The field oriented control consists of controlling the
system losses. Overload or fault conditions must be stator currents represented by a vector. This control is
handled by sophisticated control rather than over based on projections that transform a three phase time
design. and speed dependent system into a two coordinate (d
and q frame) time invariant system. These
2. FIELD ORIENTED CONTROL transformations and projections lead to a structure
similar to that of a DC machine control. FOC machines
Scalar control such as the “V/Hz” strategy has its need two constants as input references: the torque
limitations in terms of performance. The scalar control component (aligned with the q coordinate) and the flux
method for induction motors generates oscillations on component (aligned with d coordinate). The three-
the produced torque. Hence to achieve better dynamic phase voltages, currents and fluxes of AC-motors can
performance, a more superior control scheme is needed be analyzed in terms of complex space vectors. If we
for Induction Motor. With the mathematical processing take ia, ib, ic as instantaneous currents in the stator
capabilities offered by the micro-controllers, digital phases, then the stator current vector it defined as
signal processors and FGPA, advanced control follow:
strategies can be implemented to decouple the torque
generation and the magnetization functions in an AC
induction motor. This decoupled torque and
magnetization flux is commonly called rotor Flux
Oriented Control (FOC). Where, (a, b, c) are the axes of 3 phase system. This
current space vector represents the three phase
sinusoidal system. It needs to be transformed into a two
time invariant coordinate system. This transformation
can be divided into two steps: (a, b, c) → (α, β) (the
Clarke transformation), which outputs a two coordinate
time variant system. (a, β) → (d,q) (the Park
transformation), which outputs a two coordinate time
invariant system. The (a, b, c) → (α, β) Projection
(Clarke transformation), in this process, 3-phase
quantities either voltages or currents, varying in time
along the axes a, b, and c, can be mathematically
transformed into two-phase voltages or currents,
varying in time along the axes α and β by the following
transformation matrix:

Figure 1: Simplified Indirect FOC

Field Oriented Control describes the way in which the

control of torque and speed are directly based on the
electromagnetic state of the motor, similar to a DC
motor. FOC is the first technology to control the “real”
motor control variables of torque and flux. With
decoupling between the stator current components
(magnetizing flux and torque), the torque producing Assuming that the axis a and the axis α are along same
component of the stator flux can be controlled direction and β is orthogonal to them, we have the
independently. Decoupled control, at low speeds, the following vector diagram:
magnetization state of motor can be maintained at the
appropriate level, and the torque can be controlled to

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 International Journal of Engineering Trends and Technology (IJETT) – Volume-43 Number-6 -January 2017

Where, “θ” is the rotor flux position. The torque and

flux components of the current vector are determined
by the following equations:

These components depend on the current vector (α, β)

components and on the rotor flux position. If you know
the accurate rotor flux position then, by above
equations, the d,q component can be easily calculated.
At this instant, the torque can be controlled directly
because flux component (isd) and torque component
Figure 2: Space vectors in Stationary Reference Frame (isq) are independent now.

The above projection modifies the three phase system If the field angle is calculated by using terminal
into the (α, β) two dimension orthogonal system as voltages and currents or flux sensing windings and
stated below: rotor speed, then it is known as direct FOC. The field
angle can also be obtained by using rotor position
measurement and slip position by partial estimation
with only machine parameters but not any other
variables such as voltages or currents, this class of
control scheme is known as indirect FOC. The rotor
But these two phase (α, β) currents still depends upon field angle is obtained by submission of rotor speed and
time and speed. The (α, β) → (d.q) projection (Park slip frequency.
transformation) , this is the most important
transformation in the FOC. In fact, this projection
modifies the two phase fixed orthogonal system (α, β)
into d,q rotating reference system. The transformation
matrix is given below.

Where, „θ‟ is the angle between the rotating and fixed Figure 4: Simplified Direct FOC
coordinate system. If you consider the d axis aligned
2.A. Indirect Vector control
with the rotor flux,
There are essentially two general methods of vector
control. One, called the direct or feed- back method,
was invented by Blaschke and the other, known as the
indirect or feed forward method was invented by
Hasse. .The two methods differ in the way the rotor
angle is determined. In direct FOC the angle is obtained
by the terminal voltages and currents, while as in
indirect FOC, the angle is obtained by using rotor
position measurement and machine parameter‟s
estimation. Field orientation has emerged as a powerful
tool for controlling ac machines such as inverter-
Figure 3: Space Vectors in Rotating Reference Frame supplied induction motors/synchronous motors. The

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 International Journal of Engineering Trends and Technology (IJETT) – Volume-43 Number-6 -January 2017

complex functions required by field oriented control are Here, only the rotor-flux-oriented type of control, also
executed by intelligent controllers using termed ”Field-Oriented Control” (FOC), is considered.
microcontrollers or digital signal processors (DSP), FOC can be implemented as indirect (feed forward) or
thus greatly reducing the necessary control hardware. direct (feedback) depending on the method used for
An important requirement to obtain good control rotor flux identification. The direct FOC determines the
performance is to make the motor parameters in the orientation of the air-gap flux by use of a hall-effect
field-oriented controller coincide with the actual sensor, search coil or other measurement techniques.
parameters of the motor. The ability to inject currents However, using sensors is expensive because special
into the motor with a current source opened up new modifications of the motor are required for placing the
possibilities for parameter determination. It was flux sensors. Furthermore, it is not possible to directly
Takayoshi who described a new identification sense the rotor flux. Calculating the rotor flux from a
technique utilizing injected negative sequence directly sensed signal may result in inaccuracies at low
components. It is shown that the stator as well as rotor speed due to the dominance of stator resistance voltage
resistance and leakage inductance can be determined on drop in the stator voltage equation and inaccuracies
line while the motor is driving the load. The theory is due to variations on flux level and temperature. In
verified with a full-scale hybrid computer simulation of FOC, to perform the frame transformation, accurate
a field-oriented controlled PWM inverter based rotor flux position is needed to be acquired.
induction motor drive.
Because with inaccurate rotor flux position torque and
2.B. Direct Vector Control flux components are not be completely decoupled, as a
result of which dynamic response become poor . So,
In direct FOC the rotor angle or control vector is knowledge of rotor flux position is the core of the FOC.
obtained by the terminal voltages & currents directly by The measurement of the rotor flux position is different
using flux estimators. The direct vector control is also if we consider synchronous or induction motor. In
known as feedback vector control scheme. Similar to synchronous machine the rotor speed and rotor flux
Indirect Vector Control, various controllers have been speed are equal. Then rotor flux position is directly
implemented on direct vector controlled induction measured by position sensor or by integration of rotor
motor drives also to improve the performance of the speed. In the induction machine the rotor speed is not
drive. While the direct method is inherently the most equal to the rotor flux speed, then it needs a particular
desirable control scheme, it suffers from high cost and method to calculate field angle.
the unreliability of the flux measurement. Although the
indirect method can approach the performance of the 2.C. Comparison with direct vector control
direct measurement scheme, the major weakness of this
approach is centered upon the accuracy of the control The major disadvantage of direct vector method is the
gains which, in turn, depend heavily on the motor need of so many sensors. Fixing so many sensors in a
parameters assumed in the feed forward control machine is a tedious work as well as costlier. Due to
algorithm. this the scheme is prevented from being used. Several
other problems like drift because of temperature, poor
flux sensing at lower speeds also persists. Due to these
disadvantages and some more related ones, indirect
vector control is used. In indirect vector control
technique, the rotor position is calculated from the
speed feedback signal of the motor. This technique
eliminates most of the problems, which are associated
with the flux sensors as they are absent.

2.D. Advantages of Indirect FOC

 The sensors are eliminated.

 The dynamic performance of the indirect
vector control is better than the direct vector
control
 The cost factor is decreased.
Figure 5: Basic FOC of Sensor less Induction Motor  There is no drift problem as in direct vector
Drive control.

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 International Journal of Engineering Trends and Technology (IJETT) – Volume-43 Number-6 -January 2017

2.E. Advantages of Field Oriented Control 5. REFERENCES

 Improved torque response.

 Torque control at low frequencies and low 1. Domenico Casade, Francesco Profum, Giovanni Serra,
Angelo Tani, “FOC and DTC: Two Viable Schemes for
speed. Induction Motors Torque Control”, IEEE
 Dynamic speed accuracy. TRANSACTIONS ON POWER ELECTRONICS, VOL. 17,
 Four quadrant operation. NO. 5, SEPTEMBER 2002
2. Mircea Popescu, “INDUCTION MOTOR MODELLING
 Short-term overload capability. FOR VECTOR CONTROL PURPOSE”, Helsinki
 Reduction in size of motor, cost University of Technology, Laboratory of Electromechanics
 Reduction in power consumption . Report, Espoo 2000, 144 p.
3. Rakesh Singh Lodhi1, Payal Thaku, “Performance &
Comparison Analysis of Indirect Vector Control of Three
3. SUMMARY Phase Induction Motor”, International Journal of
Emerging Technology and Advanced Engineering Volume
The stator currents are converted to a fictitious 3, Issue 10, October 2013
synchronously rotating reference frame aligned with 4. Ashish Chourasia, Vishal Srivastava,Abhishek Choudhary
and Sakshi Praliya, “Comparison study of Vector Control
the flux vector and are transformed back to the stator of Induction Motor Using Rotor Flux Estimation by Two
frame before feeding back to the machine. The indirect Different Method”, International Journal of Electronic
method is normally preferred over the direct method. It and Electrical Engineering, Volume 7, Number 3 (2014),
provides the independent control of flux and torque, pp. 201-206
5. Muhammad H. Rashid, “Power Electronics, Circuits,
control characteristics is linearised. Devices and Applications”, Third edition 2011
6. B.SrinuNaik,“ComparisonofDirectandIndirectVector
The vector control implementation with corresponding Control of Induction Motor”, International Journal of
feedback signal estimation is complex, and therefore, New Technologies in Science and Engineering Vol. 1,
Issue. 1, Jan. 2014,
digital control with high speed, powerful 7. Bimal K Bose, “ Modern Power Electronics and AC
microcontroller, or DSP is essential. It is expected that Drives”, 2002,
the vector control will eventually emerge as the 8. www.electrical4u.com
industry standard control method for induction motor
drives. Fuzzy logic and neural network based adaptive
controls constitute emerging technologies.

4. CONCLUSION

From the above discussion it can be concluded that the

control of induction motor is very necessary as it is the
common motor used in industrial motor control
systems. Hence a well established induction motor
drive which is simple, rugged, low cost and low
maintenance can serve the required purpose. This
paper reviews the various aspects in the field oriented
control of induction motor including the principles,
classification (direct and indirect FOC), and the flux
vector position determination. Many authors have
published several research papers on the vector control
techniques of induction motor. And studying vector
control techniques it is clear that the indirect vector
control technique supersedes the direct vector control
and is more used rather than the later one. Hence for
the further work the method adopted is the indirect
vector control technique.

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