14356481937. Comparison of Direct and Indirect Vector Control of Induction Motor

[11][22] An induction motor is said to be in vector control mode , if the decoupled components of the stator current space vector and he reference decoupled components defined by the vec

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Comparison of Direct and Indirect Vector

Control of Induction Motor

B Srinu Naik

Abstract-Vector control is becoming the industrial standard for induction motor control The

vector control technique decouples the two components of stator current space vector: one providing the control of flux and the other providing the control of torque The two components are defined in the synchronously rotating reference frame With the help of this control technique the induction motor can replace a separately excited dc motor The DC motor needs time to time maintenance of commutator, brushes and brush holders The main effort is to replace DC motor

by an induction motor and merge the advantages of both the motors together into variable speed brushless motor drive and eliminate the associated problems The squirrel cage induction motor being simple, rugged, and cheap and requiring less maintenance, has been widely used motor for fixed speed application So with the implementation of vector control, induction motor replaces the separately excited dc motor The vector control technique is therefore a better solution so that the control on flux and torque become independent from each other and the induction motor is transformed from a non-linear to linear control plant With the advent of field oriented control; the induction motor has become an attractive option In this report we will come to know the concept of vector control and different types of vector control techniques available And finally we will be able to compare them.

Index Terms-Induction Motor, Vector Control, Speed Control, AC Motors.

1 INTRODUCTION

Modern method of static frequency conversion has liberated the induction motor from its historical role as

a fixed speed machine The inherent advantages of adjustable frequency operation cannot be fully realized unless a suitable control technique is employed The choice of technique is vital in determining the overall characteristics and performance of the drive system Also the power converter has little excess current capability; during normal operation the control strategy must ensure that motor operation is restricted to the regions of high torque per ampere, thereby matching the inverter ratings and minimizing the system loses Overload or fault conditions must be handled by sophisticated control rather than over design.

Now a days more than 60% of all the electrical energy generated in the world is used by cage induction machines have been mostly used at fixed speed for more than a century On the other hand, D.C machines have been used for variable speed applications In DC machines mmf axis is established at 90˚ electrical to the main field axis The electromagnetic torque is proportional to the product of field flux and armature current Field flux is proportional to the field current and is unaffected by the armature current because of orthogonal orientation between armature mmf and field mmf Therefore in a separately excited DC machine , with a constant value of field flux the torque s directly proportional to the armature current Hence direct control of armature current gives direct control of torque and fast response Hence they are

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B S Nayak / International Journal of New Technologies in Science and Engineering

Vol 1, Issue 1, Jan 2014, ISSN XXXX-XXXX

simple in control and offer better dynamic response inherently Numerous economical reasons, for instance high initial cost, high maintenance cost for commutators, brushes and brush holders of DC motors call for a substitute which is capable of eliminating the persisting problems in dc motors Freedom from regular maintenance and a brushless robust structure of the three phase squirrel cage induction motor are among the prime reasons, which brings it forward as a good substitute.

The ac induction motors are the most common motors used in industrial motion control systems, as well

as in main powered appliances Simple and rugged design, low cost and low maintenance are some of the main advantages of 3 phase ac induction motors Various types of induction motors are available in the market Different motors are suitable for different application The speed and torque control of 3 phase induction motors require great understanding of the design and characteristics of these motors.

2 BRIEF THEORY OF VECTOR CONTROL (FIELD ORIENTED CONTROL)

The control of separately-excited dc machines is straightforward due to the inherent decoupled nature between flux and torque As a consequence, torque linearization can be obtained easily by armature current control with constant field flux DC motors have been widely used in high performance domains such as robotics, rolling mills and tracking systems where fast dynamic torque control is required AC machines are always preferable to dc machine due to their simpler and more robust construction; there are no mechanical commutators However, the electrical structures of ac machines are highly nonlinear and involve multivariable inputs and outputs Therefore, additional effort is required to decouple and linearize the control

of these machines In practice, intricate control algorithms are involved if ac drives have to match the dynamic performance of dc drives Due to advancements in microelectronics and power electronics, high performance control of ac motors can now be implemented at a reasonable cost This technological breakthrough has stimulated in turn the application of sophisticated control algorithms and the widespread use of ac drives in high performance domains [2][22].

The realization of fast decoupling control requires that both the magnitude and phase of the machine currents

be controlled accurately Depending on the design philosophy and the type of ac machine, there can be many different approaches to synthesize the machine currents to provide fast decoupling control Among the different approaches of torque and flux decoupling control techniques, the emerging consensus is that the method of field-orientation yields the best overall performance The field-oriented control (F.O.C.) is by far the most widely accepted method of control in high performance ac drive domains While F.O.C represents a single, unified control concept, the application strategies, complexity of implementation and drive responses vary with different drive motors.

In a Dc machine, a number of coils are distributed around the armature surface and inter connected to form a closed winding Stationary poles with dc-excited field windings or permanent magnets establish magnetic field in which the armature rotates Current is supplied to the armature through the commutator brushes so that the armature mmf axis is established at 90 degrees electrical to the main field axis.

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The DC motor analogy

Where torque (T)  Ia.If

And where Ia represents the torque component and If the field.

The orthogonal or perpendicular relationship between flux and mmf axes is independent of the speed of rotation and so the electromagnetic torque of the dc motor is proportional to the product of the field flux and armature current Assuming negligible magnetic saturation, field flux is proportional to field current and is unaffected by armature current because of the orthogonal orientation of the stator and rotor field Thus in a separately excited dc motor with constant value of field flux, torque is directly proportional to armature current.

The principle behind the field oriented control or the vector control is that the machine flux and torque are controlled independently, in a similar fashion to a separately excited DC machine Instantaneous stator currents are transformed to a reference frame rotating at synchronous speed aligned with the rotor stator or air gap flux vectors, to produce a d-axis component current and a q-axis component current (SRRF).In this work, SRRF is aligned with rotor mmf space vector, the stator current space vector is split into two decoupled components, one controls the flux and the other controls the torque respectively [11][22]

An induction motor is said to be in vector control mode , if the decoupled components of the stator current space vector and he reference decoupled components defined by the vector controller in the SRRF match each other respectively Alternatively, instead of matching the two phase currents (reference and actual) in the SRRF, the close match can also be made in the three phase currents (reference and actual) in the stationary reference frame Hence in spite of induction machine’s non linear and highly interacting multivariable control structure, its control has becomes easy with the help of FOC Therefore FOC technique operates the induction motor like a separately excitedly DC motor.

The transformation from the stationary reference frame to the rotating reference frame is done and controlled by with reference to specific flux vector (stator flux linkage, rotor flux linkage) or magnetizing flux linkage) In general, there exits three possibilities for such selection and hence, three vector controls They are stator flux oriented control, rotor flux oriented control and magnetizing flux oriented control As the torque producing component in this type of control is controlled only after transformation is done and

is not the main input reference, such control is known as indirect torque control The most challenging and

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ultimately, the limiting feature of field orientation is the method whereby the flux angle is measured or estimated Depending on the method of measurement, the vector control is sub divided into two sub categories: direct vector and indirect vector control In direct vector control, the flux measurement is done

by using flux sensing coils or the hall devices [2][22]

FOC uses a d-q coordinates having the d-axis aligned with rotor flux vector that rotates at the stator frequency The particular solution allows the flux and torque to be separately controlled by the stator current d-q components The rotor flux is a flux of the d-axis component stator current ids.The developed torque is controlled by the q – axis component of the stator current iqs.The decoupling

between torque and flux is achieved only if the rotor flux position is accurately known This can be done using direct flux sensors or by using a flux estimator.

3 TYPES OF VECTOR CONTROL TECHNIQUES OF INDUCTION MOTOR

The synchronously rotating reference frame (SRRF) can be aligned with the stator flux or rotor flux or magnetizing flux (field flux) space vectors respectively Accordingly, vector control is also known as stator flux oriented control or rotor flux oriented control or magnetizing flux oriented control Generally in induction motors, the rotor flux oriented control is preferred This is due to the fact that by aligning the SRRF with the rotor flux, the vector control structure becomes simpler and dynamic response of the drive

is observed to be better than any other alignment of the SRRF.

The vector control can be classified into (i) Direct vector control and (ii) indirect vector control.

Scope of work

In vector control the dynamic performance of the induction motor improves to a great extent The squirrel cage induction motor behaves similar to a separately excited dc motor with control of field and torque being independent of each other Therefore the drive exhibits quick starting response, fat reversal response and quick change over from one operating point to another With proper choice of speed controller, the drive can be further improved in terms of performance indices such as starting time, reversal time, and dip

in speed on load application, overshoot in speed on load removal, steady state speed error on load etc The VCIMD can be operated in two modes of operation (a) operation below base speed and (b) operation above base speed When the drive operates below base speed, the flux component of the stator current (ids*) is maintained constant and torque is dependent on the torque component of the stator current (iqs*) and when the drive operates above base speed, the flux component of stator current (ids*) is reduced for control, with the torque component (iqs*) at the maximum possible level.

The excitation current for rotor flux (imr*) depends on the speed of the motor ( r) in inverse proportion

for the operation of the motor above base speed.

The voltage source inverter I operated in current controlled (CC) mode The CC mode of VSI gives a quick and fast response as the winding currents are regulated in accordance to vector control mode of an induction motor.

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Normally uncontrolled ac-dc converters are used to feed vector controlled induction motor These uncontrolled ac-dc converters draw non-sinusoidal current from the ac mains and behave as non-linear loads This leads to power quality problems But a number of techniques have been proposed for improving power quality at ac mains.

Direct vector control method

In direct vector control method we have seen that it determines the magnitude and position of the rotor flux vector by direct flux measurement or by a computation based on terminal conditions It also called flux feedback control is method in which required information regarding the rotor flux is obtained by means of direct flux measurement or estimation The flux is measured by the sensors like Hall Effect sensor, search coil and this is a part of the disadvantages Because fixing of number of sensors is a tedious job and this increases the cost factor [2]

The quantities generated from flux sensors are used in the outer loop of the drive control structure Alternatively, in place of flux sensors, the flux models can also be used for which the stator currents and voltages become the feedback signals and he rotor flux angle is given as its estimated output.

Figure.2 shows a simplified block diagram of a field control scheme the two axis reference currents, iqs

and ids are the demanded torque and flux components of stator current, respectively and are governed by

the outer control loops Currents, iqs and ids, undergo a coordinate transformation to two phase stator

based quantities, followed by two phase to three phase transformation which generates the stator reference currents ias*, ibs*, ics*.These reference current are reproduced in the stator phases by the current controlled PWM inverter [2]

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Figure-2: Basic field oriented control system for an induction motor with a current controlled PWM inverter.

Thus the external reference currents iqs and 

ds

i are reproduced within the induction motor Control is

executed in terms of these direct and quadrature axis current components to give decoupled control of flux and torque as in a dc machine

Disadvantages

1 Fixing of number of sensors is a tedious job.

2 The sensors increase the cost of the machine.

3 Drift problem exist because of temperature.

4 Poor flux sensing at lower temperatures.

These disadvantages lead to another technique called in-direct vector control technique.

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In-direct vector control Method

Figure.3 shows the basic block diagram of induction motor operating in indirect vector control mode The motor speed is used as feedback signal in the controller The controller calculates reference values of the two decoupled components of stator current space vector in the SRRF which are iqs*and ids*for the control

of torque and flux respectively [2] The two components of the currents are transformed into three phase currents which are ias*,ibs*,ics*in the stationary reference frame of reference Now as a balanced load, two

of the phase currents are sensed and the third one is calculated from the two sensed currents The current controller controls the reference currents close to sensed three phase currents in the stationary reference frame and operates the voltage source inverter to feed three phase induction motor This ensures a high level of performance of the vector controlled induction motor (VCIMD).Because of the smooth, efficient and maintenance free operation of VCIMDs, such drives are finding increasing applications in many drive application s such as air conditioning, refrigeration, fans blowers, pumps, waste water treatment plants ,elevators, lifts traction motors, electric vehicles, etc[2][7]

figure-3:basic block diagram of indirect vector control mode

The field-weakening controller receives the speed signal ( r ) as an input signal and provides reference

value of the excitation current ( imr*) as an output signal Therefore the two signals are the reference signals for the vector controller In the vector controller the d-axis component ( ids) and the q- axis

component ( iqs) of the stator current signals are computed which are responsible for the flux and torque

control respectively The slip frequency signal ( 2* ) is also computed in vector controller to evaluate

the flux angle The slip angle is computed using slip frequency ( 2*), rotor speed ( r ) and sampling

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period (  T ).These signals of flux ( ids and torque ( 

qs

i ) are in the synchronously rotating reference

frame and these are transformed into stationary reference three phase currents ( ias*, ibs*, ics*)

For current controlled VSI fed vector controlled induction motor, the reference currents ias*, ibs*, ics* and sensed currents ( ias, ibs, ics) are fed into the pulse width modulated (PWM) current controller A triangular

carrier wave is generated at the required switching frequency (fs) The point of intersection of the triangular carrier wave and modulating signals acts as the point of state change over for the resulting PWM signals, which are fed to the driver circuit of VSI feeding an induction motor[2][7]

The indirect vector controlled induction motor is reshown in figure.3 below with blocks consists of the speed sensor, speed controller ,limiter, the field weakening controller , the two phase rotating frame to three phase stationary frame converter, PWM current controller, CC-VSI and three phase squirrel cage induction motor The functions are described as follows

qs

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Speed sensor

It measures the motor speed Since in the indirect vector, the accurate measurement of position of rotor flux vector is given by the sensors which have high resolution and precision Normally shaft encoders are used for the closed loop vector control of the cage induction motor drive.

Speed controller

The measured speed ( r ) is compared with the set reference speed ( *

r

 ) in the error detector and the

resulting output is known as speed error ( e ) is processed in the speed controller The output of the

controller is the control signal for the torque command ( T ) The command input may be positive or negative depending upon the set reference speed and the motor shaft speed The speed error ( e ) is

processesed in the speed controller which may be of different types depending upon the required dynamic performance of the drive And accordingly the controller is used.

When the drive operates in the transient conditions such as starting, reversing or load application or load removal the speed controller output (T) may be very high value to achieve the steady state condition of the drive as fast as possible, as, a result the controller output signal (T) may become quite high and in some cases it may become higher than the breakdown torque of the motor Such a situation may be rather dangerous for the motor and may take the drive into instability In order to avoid certain circumstances, it becomes very much necessary to apply certain limit on the output of the speed controller The output of the speed controller after the limit is considered as the reference torque(T*) to the vector controller and used to obtain the value of stator current torque component of the stator current space vector As a result the limit of the torque also ensures over current protection to the drive.

4 FIELD WEAKENING CONTROLLER

The field weakening operation of a VCIMD is similar to the field controller of a separately excited dc motor This operation is implemented when the drive speed is controlled above the base speed The input

to the field weakening controller is the feedback speed of the motor The output of the controller is the excitation current Below the abase speed the excitation current remains constant Above the based speed the excitation current varies in inverse proportion to the speed [13-14]

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component ( ids)

And also to calculate the slip frequency ( 2* ).The torque ( iqs) and the flux components ( ids ) are the

respective decoupled components of the stator current ( is) in the synchronously rotating reference frame.

synchronously rotating reference frame (SRRF) aligned with rotor inclined at flux angle ( ) with respect

to stationary reference frame [19]

Mathematically these equations for calculating these two components of the current are given as follows:

n T

  n i

n i

mr r

qs



*

……… Eq (3)

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Where,  r is the rotor time constant defined as

P is the number of poles, ids  n

L is the magnetizing inductance

Two phase rotating frame to three phase stationary frame converter

Two phase rotating frame to three phase stationary frame converter transforms the two decoupled components of stator current namely, ( idsand 

qs

i ) in synchronously rotating reference frame into three

phase currents namely ias*, ibs* and ics* in three phase stationary reference frame The conversion

process requires the flux angle ( ), which is calculated by the integration of the synchronous speed.

Synchronous speed is obtained by addition of slip speed ( 2* ) and motor speed ( r )

Transformation equations can be written as follows:

3 sin

2

1 sin

3

*

qs ds

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