||SPEED CONTROL OF AN INDUCTION MOTOR USING INDIRECT VECTOR CONTROL
Over the past two decades technological advances in power electronics and an increasing demand for high performance industrial machinery have contributed to rapid developments in digital motor control. This field of study has numerous applications in the areas of manufacturing, mining and transportation but sometime it is difficult to determine which technique is best suited to a particular application.
Induction motor is used as a variable speed drive in many industrial applications. In this case induction motor is fed by a current controlled inverter system. Pulse width modulation technique is used to derive switching sequence for the inverter system. Closed loop control of induction motor utilizes a proportional integral speed controller. An indirect vector control strategy is employed for current controlled inverter system. The performance analysis of a vector controlled cage induction motor drive is studied. To check the dynamic performance of the drive, the motor is operated in four-quadrant operation. The simulated response of an induction motor will be presented in different modes of operation.
Major improvements in modern industrial processes over the past 50 years can be largely attributed to advances in variable speed motor drives. Prior to the 1950â„¢s most factories use DC motors because three phase induction motors could only be operated at one frequency. Now thanks to advances in power electronic devices and the advent of DSP technology fast, reliable and cost effective control of induction motors is now commonplace.
In 1997 it was estimated that 67% of electrical energy in the UK was converted to mechanical energy for utilization. AT the same time the motor drive market in Europe was in excess of one billion pounds. The increase in the use of induction motors was largely attributed to major oil and mining companies converting existing diesel and gas powered machinery to run off electricity. Over the past five years however, the area of AC motor control has continued to expand because induction motors are excellent candidates for use in Electric or Hybrid Electric Vehicles.
In this application high performance control schemes are essential. Over the past two decades a great deal of work has been done into techniques such as Field Oriented Control, Direct Torque Control and Space Vector Pulse Width Modulation. Another emerging area of research involves the application of sensor less control. This differs from conventional methods because it doesnâ„¢t require mechanical speed or position sensors.
Removing these sensors provides a number of advantages such as lower production costs, reduced size and elimination of excess cabling. Sensors less drives are also more suitable for harsh inaccessible environments as they require less maintenance. This undergraduate thesis thoroughly investigated the aforementioned techniques and used them to develop a Field Oriented Control Scheme for use in an Electric Vehicle.
Induction motors are widely used in various industries as prime workhorses to produce rotational motions and forces. Generally, variable-speed drives for induction motors require both wide operating range of speed and fast torque response, regardless of load variations. The field oriented control is the most successful in meeting the above requirements. Due to advances in power electronics and microprocessors, variable-speed drives for induction motors using the field oriented control have been widely used in many applications, such as ac servo, electric vehicle drive systems, and so on. Using the field-oriented control, a highly coupled, nonlinear, multivariable induction motor can be simply controlled through linear independent decoupled control of torque and flux, similar to separately excited dc motors. High performance torque control requires fast enough current response for the current regulator to track the reference current. However, due to limitations of voltage and current ratings on the inverter de link, input voltage and current of an induction motor are limited accordingly. Hence, developed torque in the motor should be limited for safe operation under these input constraints.
The objective of a variable-speed control system for higher productivity is to track the reference speed as fast as possible. Therefore, under the constraints of input voltage and current, a control scheme which yields the maximum, torque over the entire speed range can be usefully applicable to minimum-time speed control of induction motors. However, most researchers who deal with the speed control of induction motors have not considered the maximum-torque generation scheme.
FIELD ORIENTED CONTROL OR VECTOR CONTROL
In a separately excited DC machine the axes of the armature and field currents are orthogonal to one another. This means that the magneto motive forces established by the currents in these windings are also orthogonal. This means that the flux is dependent solely on the field winding current. If the flux is fixed then the torque is varied directly by the armature current. It is for this reason that DC machines are said to have decoupled or independent control over torque and flux. If iron saturation is ignored the developed torque is equal to
Te = Ktiaif
Unfortunately the operation of induction machines is much more complicated. Induction motors are coupled, non-linear, multivariable systems whose stator and rotor fields are not held orthogonal to one another. In order to achieved decoupled control over the torque and flux producing components of the stator currents a technique known as Field Oriented Control is used. In the vector control, an AC machine is controlled like a separately excited DC machine. This analogy is explained in the following figure.
Since the current If or the corresponding field flux is decoupled from the armature current Ia., the torque sensitivity remains maximum in both transient and steady state operations. This mode of control can be extended to an induction motor also if the machine is considered in a synchronously rotating reference frame where the sinusoidal variable appear as DC quantities. In the above figure the induction motor with inverter and control is shown with two control inputs ids and iqs. The currents ids and iqs are the direct axis component and quadrature axis component, respectively of the stator current, where ids is analogous to the field current if and iqs is analogous to the armature current Ia of a DC machine. Therefore the torque can be expressed as
Te = Ktiaif = Ktiqsids
THE VECTOR CONTROL CONCEPT
Initially, the emphasis of this paper will be on vector control as applied to the AC induction machine it then goes on to describe the necessary adaptations to this method for controlling a PMSM. In a typical AC induction motor, 3 alternating currents electrically displaced by 120 degrees are applied to 3 stationary stator coils of the motor. The resulting flux from the stator induces alternating currents in the Ëœsquirrel cageâ„¢ conductors of the rotor to create its own field these fields interact to create torque.
Unlike a DC machine the rotor currents in an AC induction motor cannot be controlled directly from an external source, but are derived from the interaction between the stator field and the resultant currents induced in the rotor conductors. Optimal torque production conditions are therefore not inherent in an AC Induction motor due to the physical isolation between the stator and rotor. Vector control of an AC induction motor is analogous to the control of a separately excited DC motor. In a DC motor the field flux f produced by the filed current Ia is perpendicular to the armature flux a produced by the armature current Ia. These fields are decoupled and stationary with respect to each other. Therefore when the armature current is controlled to control torque the field flux remains unaffected enabling a fast transient response.
Vector control seeks to recreate these orthogonal components in the AC machine in order to control the torque producing current separately from the magnetic flux producing current so as to achieve the responsiveness of a DC machine.
DC motor performance is extended to an induction motor in a synchronously rotating reference frame (d-q), where the sinusoidal variables appear as dc quantities in steady state. To understand the vector control, we have to be aware of dynamic d-q model of an induction machine (Bose. BK, 2003).
IMPLEMENTATION OF VECTOR CONTROL:
It has been established that iq and id of the rotating reference frame must be controlled to provide good dynamic control of the induction motor. Using closed loop control ordered quantities of iq and id are compared with the actual valued measured from the motor.
In order to obtain the motor values we have to perform transformations on the measured 3 phase stator currents into the direct and quadrature components of the rotating reference frame. The resulting error terms are then transformed back to 3 phase quantities and applied to the motor. Figure shows a representation of this process.
The purpose of the flux position calculator shown in figure is to produce the correct field orientation by ensuring alignment of ids with the rotor flux. The angular position of the rotor flux in an AIM can either be measured directly using sensors embedded in the motor or (more commonly) indirectly. The indirect method involves calculating the angle of slip between the stator and rotor fields using known characteristics of the rotor and summing this with the physical position of the rotor. The physical position is measured (usually) using an incremental encoder fitted to the motor shaft The difference (errors) between the ordered and actual id and iq components are input to Proportional Integral (PI) controllers.
Note that the PI controllers do not form part of vector control but are usually included in this type of system to provide optimum closed loop control of the motor. The output terms from the PI controllers that are referenced to the rotating reference frame are transformed back to the static frame using the inverse transform of equation 3 and then transformed from the static frame back into the 3 phase components using the inverse transformation of equation 2.
SCHEME OF VECTOR CONTROL
Representing machine model in asynchronously rotating reference frame by considering that inverter is having unity current gain. And it generates ia, ib and ic currents depending upon the ia, ib and ic command currents of the controller. The machine terminal phase currents ia, ib and ic are converted to ids and iqs by three phase to two phase transformation. These are converted to synchronously rotating reference frame by three-unit vector components Cose and Sine. The control currents ids and isq correspond to the machine control ids and iqs. And also the unit vector assures the correct alignment of ids current with the flux vector r and iqs perpendicular to it. (Bose. BK, 2003).
Vector control scheme depends on the field angle, classified as
1. Direct or feed back control.
2. Indirect or feed back forward control.
Vector control implementation principle with machine de-qe model:
In this, field angle is calculated by using terminal voltages and currents or Hal-sensors or flux sensing windings. (Krishnan. R. 2003) the above figure Represents the block diagram of a direct vector control method for PWM voltage fed inverter drive.
The control parameters ids and iqs which are dc values in synchronously rotating reference frame converted to stationery frame by using unit vectors generated from flux vector signals dr and qr. These flux signals are generated from the machine terminal voltages and currents with the help of the voltage model estimator. For precision control of flux control loop should be added. The torque component of current I qs is generagted form the speed control loop through iqs aligning of current ids in the direction of flux r and the current iqs perpendicular to it is explained in fig. De â€œ qe will rotate at synchronous speed we with respect to stationery frame ds qs. The angular position of the de axis with respect to the ds axis is e.
dr s = r Cos e
qr s = r Sin e
r = v( dr s2 + qr s2)
r vector is represented by magnitude r. These unit vector signals, when used for vector rotation, give a current ids on de axis and iqs on qe axis. When qr = 0 and dr = r, then the torque expression will be same as dc machine expression. In direct vector control the generation of a unit vector signal is from feed back flux vector.
INDIRECT OR FEED FORWARD VECTOR CONTROL
In this type of control, the rotor angle is obtained by using rotor position angle and partial estimation with only machine parameters but not any other variables. (Krishnan. R. 2003) the unit vector signals are generated in feed forward manner. The fundamental principle of this control will be explained with the help of phasor diagram. The ds â€œ qs axes are fixed on the stator, but the dr â€œ qr axes, which are fixed on the rotor, are moving at speed wr as shown in fig., and synchronously rotating axes de â€œ qe are rotating ahead of th dr â€œ qr axes by positive slip angle sl corresponding to slip frequency wsl.
e = r + sl
e = e dt = (r + sl) dt = r + sl
Rotor pole position is slipping to the rotor at frequency sl. For attaining decoupling control the stator flux component of the current ids should be aligned on the de axis and torque component of current iqs on the qe axis. Implementation of vector control to an inverter drive will be explained in the next chapter.
ADVANTAGES OF VECTOR CONTROL
Stable operation with large motors.
Better performance at current limit with improved slip control.
Decrease in the losses of the machine.
Excellent speed control with inherent slip compensation.
High torque at low speeds.
Increase in the overall performance of the motor.
A proven technique that has been used for some time.
DISADVANTAGES OF VECTOR CONTROL
Relatively low dynamic performance due to the presence of PI current regulator.
Parameter detuning causes high torque and flux magnitude errors.
The equipment required for vector control of induction motor is very costly.
Comprehensive study on dynamic d-q model and vector control has been made. The performance of closed loop speed controllers for variable speed operation of vector controlled induction motor drive is observed. To study the dynamic performance of an induction motor MATLAB SIMULINK toolbox is used. The drive system has been carried out. During transients, the controller brings faster response in the drive system. The current controlled voltage source inverter is found capable of providing four-quadrant operation of the drive.
1. Comparing the dynamic performance of the PI speed controller with Fuzzy PI speed controller.
2. Implementing space vector modulation for generating switching sequence to the inverter.
3. Realizing the vector controlled induction motor through hardware.
1. Electrical machinery
By Dr.P.S. Bimbhra
2. Electrical engineering