This chapter focuses on the basic theory of vector control and scalar control. The scalar control is methods of control of induction motor and also the scalar control methods is approach in other word is the same as variable voltage and variable frequency. The scalar control always relationships to the based steady state, usually only frequency and magnitude are controlled, so it is not relates to the space vector orientation. However, the voltage magnitude is proportional to the frequency, then the results constant stator flux, desirable to increase the capacity of the motor. Variable frequency is the principles of scalar control methods, therefore, the voltage improve at low frequency, usually, reduce large effect of stator resistance at low speed. The scalar control method does not use a "proportional-integral-derivative" (PID) algorithm and does not utilize feedback inside. For this reason, its torque control capabilities are not enough for demanding applications which have been historically kept for dc motors and drives.
The theory of vector control is the basis of a special control method for induction motor drives. With this control method of the study showed that induction motors can successfully replace the expensive DC motors. Also shown this method has become widespread in the induction motor drives of high precision. The most important advantages of induction motors are their simplicity and price, and greater reliability especially in harsh industrial environments. Induction motors requires complex control algorithms, because there is no linear relationship between the stator current and either the torque or flux. This means that it is difficult to control the speed or torque, because transients until the motor reach its new steady state.
The vector control method is the most popular of AC induction motors. In special reference frame, and the induction motors, " the control is usually performed in the reference frame (d-q) attached to the rotor flux space factor". Therefore, the implementation of vector control requires information on the module and the angle of the rotor flux space vector. The stator currents of induction motor can be separated into flux and torque is producing of component by using transformation to the d-q coordinate system, which "directs axis (d) is aligned with the rotor flux space vector". However, always the q - axis component of the rotor flux space vector is zero. And .
5.1.2 Scalar control
Scalar control methods is kind of control of induction motor, therefore, the motor is fed with variable frequency signals generated by the PWM control from an inverter. The v/f ratio kept constant over the whole operating range and other point of scalar control is controlled of induction motor drives only variable of magnitude, voltage and frequency are controlled; generally, in this method control, it is not have any feedback (open loop) but this method is low cost and simple to implement solution and also its simplicity it is easy and fast to program and requires only few calculation capabilities. Voltage is controlling the air gap flux and frequency or slip may be used to control of the torque. However, the torque and the flux are functions of voltage and frequency, respectively, but this coupling is not taken into account in the scalar control. Figure ( 5.1) shows the block diagram of scalar control of induction motor; in this block diagram can be used simple control principle and also this block diagram is shown torque is controlled by the slip Wr * which can get from the torque loop.
Figure (5.1) block diagram of scalar control of induction motor.
5.1.3 The aim of scalar control
The aim of the scalar control is controlling induction machine at quasi-steady state, by changing the amplitude of the basic supply voltage and its frequency. The amplitude of the essentially voltage is calculated from the stator pulsation by taking an experiential function that compensates the stator resistive fall and keeps the magnitude of the flux around at constant.
5.1.4 the most effect factors of scalar control
This simple and direct approach, however, does not work fit in actuality due to a number of factors, the most important are:
Effect of source voltage variations.
Power of stator resistance.
Speed and torque non-ideal characteristic.
Non- linearities introduced by the PWM inverter.
Operation at low frequency is particularly difficult to achieve given that these effects are greater at low voltages. In addition, the nonlinearities in the converter, they are not compensated, giving a voltage .Release very distorted, which, in turn, produces exciting , torque leading to vibration and acoustic noise increase.
5.1.5 Scalar control-overall conclusions
The scalar control has main weakness of control strategies from their philosophical base. Also the widely assumptions are made of drive and load characteristics or simple trying are made to measure control variable. In this case, cab be using control strategy the steady state torque can be controlled precisely, but the transient torque is not good enough, therefore, the uncontrolled, the machine is starting directly online, the response is uncontrolled and extremely, so the motor is full load the torque can easily developed, at the same time the current will be six times rated. An" inverter drives enable these pulsating transients to be same affect arises in equivalent two phase stator coil system". IS` vector results , consonantly, IS` current vector , results from currents pass in two coils, so exchange between three phase and two phase coil system , is usually vector controller system is in generalised machine theory.
5.1.6 Voltage /Hertz Control
The function or the purpose of constant volts/ Hertz is controlling of the magnitude of the stator voltages and also the stator voltage is proportional with frequency though ignoring the coupling effects of the motor. Moreover, the major advantages of applying a scalar control methods to control induction motor quite simply to implement, and the scalar control widely used in applications that do not require accurate speed control such as fans for heating or pumps, so this method can be based on torque speed curve for an induction motor. The basic strategy of the scalar control method is controlling in reduce speed range applications ( wmin / wmax ≈ 1:10).When they don't need high dynamic behaviour such as pumps, ventilators. The functions range during the operation of machine, so could be done by regarding of the magnetic field in the core is keep or maintain at a particular point of speed the rated voltage at rated frequency. Figure (5.2) shows the voltage curve versus frequency and constant voltage to frequency. If the magnetic field is higher than the desired level, then the magnetic circuit is saturated and lead to increase in the loss and harmonics. If the magnetic flux is a fewer than the wanted rank then the torque will be developed by the machine can be decreased for a given quantity of current.
Figure (5.2) Real frequency versus voltage curve
5.1.7 Types of control
There are two types of control open and close loop. Open loop control has no feedback from the process. Closed loop control is obtained using the feedback of the process variable.
5.1.7.1 Open-loop scalar speed control
While the command and feedback signals are DC quantities, which are proportional to the respective variables. This is in contrast to the vector control, where both the magnitude and the phase of a vector variable are controlled.
Figure (5.3) Open-loop scalar speed control
The Figure shown above is used as an example of a simple open-loop scalar (Vs/ï·s) speed control method for an induction motor for illustration reason. The frequency ï·s is the command variable. It is close to the motor speed when the small slip frequency is neglected. The scheme is defined as Vs/ï·s control because the voltage command Vs is generated directly from the frequency signal through a Vs/ï·s gain constant K. In the steady-state operation the machine air gap flux m is approximately related to the ratio Vs/ï·s. As the frequency approaches zero near the zero speed the voltage drop at the neglected stator resistance will be relatively higher and higher. An auxiliary compensation signal is added to compensate the mentioned voltage drop.
5.1.7.2 Closed-loop speed control
A closed-loop scheme using PID control can be applied externally to refine the output of a drive with scalar control. The control algorithm had been external to the drive, but drives now generally include on-board PID control capability, requiring only a process sensor input to do on-board, closed-loop process control. Scheme is shown in Figure (5.4) .Scalar control is typically achieved by controlling the voltage to frequency ratio in an open or closed loop. Optimized motor efficiency can be achieved by implementing slip regulation.
Figure (5.4) Closed-loop speed control
5.2 Vector Control of an Induction Motor
5.2.1 Vector control
Vector-controlled drives are best understood by comparing them to dc drives because their speed, torque, and hp curves are nearly identical. Furthermore, dc motor fundamentals are easy to understand and translate into ac vector control properties. In a dc motor, current in a conductor reacts with a magnetic field to produce torque. The field stems from either permanent magnets embedded in the stator or current in stator windings. Windings in the armature (rotor) carry the reaction current which switches from coil to coil via commutator segments and brushes. In a constant-speed motor, the switching mechanism maintains sufficient armature current to generate maximum torque. However, in a variable-speed application, the magnitude and direction of armature current, a vector quantity, must be regulated electrically to maintain the torque. Ac induction motors, by contrast, contain conductors cast in rotors that resemble "squirrel cages" and magnetic field windings wrapped around stators. Currents in stator windings produce a magnetic field that has magnitude and direction, a vector. Regulating currents in so-called u, v, and w windings of a three-phase ac motor controls the field vector.
Sine wave currents, displaced by 120° and applied to the stator windings, generate a rotating field. This rotating field induces current in the rotor and produces torque and rotation. However, the resulting rotor speed is several rpm less than the rotating stator field. The difference, called "slip," is necessary to keep the stator field ahead of the rotor to produce torque. Because motor torque is proportional to the magnitude and direction of the stator field, a vector quantity, designers speak of controlling the stator field "vector" to control motor output torque. Figure (5.1) shows a scheme variable frequency control methods of induction motor. However, it is divided two main groups: Vector based control and Scalar based control. In this project was investigated into control methods of induction motor.
Figure (5.1) Control methods for induction motor
5.2.2 Aim of the vector control
The major goal of the vector control of induction motors is, as in DC machines, to independently control the flux and torque. Vector control is the best of modern high performance drives. It is also known as decoupling, orthogonal, or transvector control. Vector control techniques are divided into groups: direct control and indirect control, and there is also classification of control based on orientation rotor flux (Ψr ) or stator flux (Ψs). For the purposes of vector control cage motor when I investigate this project is to obtain a good dynamic response of torque. Ideally, in practice, can be change of the step changes in the torque, so fast changes are achieved, similar with the best similar from an armature - controlled dc machine of similar size.
For the purposes of vector control cage motor when I investigate this project is to obtain a good dynamic response of torque. Ideally, in practice, we can change the step changes in torque, rapid changes are achieved, even with the best similar from an armature-controlled
dc machine of similar size. The vector control has a mathematical model deals with, voltage, current, fluxes torque and the motor parameters. Also it is can control the instantaneous stator currents, control the magnitude and position of is`.
5.2.3 Vector control strategy
A near the middle difference between IM's and DC motor that is IM air gap flux revolves at synchronous speed. However, the dc motor is stationary. The rotor magnetomotive force moves past the rotor at the slip speed. Thus, the angle of the rotor position magnetomotive force, is stationary in relative with the stator magnetomotive force, and is also the mutual produces of machine torque. The object of vector control is to control IM'S as the same of dc motors. However, it has good dynamic response. When the three stator phase currents are equal to two axis coordinate system can be used to control mathematical transformation. In three phase system form of the stator currents represented as: ( `Ias, ´Ibs, ´ Ic ) are fixed space, so the direction defined by the stator windings along a-b-c axes. Therefore, revolving two axis shape, then stator currents are solved into direct and quadrature (d-q) axis components, (id and iq) with the d-axis fixed to the machine flux. Thus, the d-q axes revolve in space synchronous speed.
Figure (5.2) shows the structure of vector control of induction motor.
Measure of the motor (phase voltages and currents).
Can be using a Clarke transformation when transform two phase system (β, α).
Calculate the position angle and rotor flux space-vector magnitude.
Using a park transformation to transform stator currents into the d, q reference frame.
The flux (isd) and stator current torque (isq) producing components are separately controlled.
By decoupling can be calculated the output stator voltage space vector.
The stator voltage space vector is transformed by an inverse transformation back to the Park d, q reference frame in the two-phase system with the fixed.
Using the modified vector space is created the output voltage of three phases.
Figure (5.2) Vector control transformations
5.2.4 Principle of voltage -oriented vector control
To make simpler the analysis and control of the induction motor, so can be following assumptions are conventional accepted:
Constant air-gap
Balanced armature.
Sinusoidal induction repartition.
Hysteresis and eddy current are negligible. So should be following equations park below the synchronous revolving reference.
(5.1)
(5.2)
(5.3)
(5.4)
) (5.5)
Where, WS and ∆w are represented by synchronous and slip angular speed respectively;
Vsd` Vsq` Isd`. Where the ; Isq` Ψsd` and Ψsq` are represented by q-axis and d-axis value of stator voltage , other values are represented current value and flux ; Ird , Irq , Ψrd , Ψrq , and are the q-axis and d-axis value of the rotor current and flux; Tem is the electromagnetic torque , and p is the number of pole pairs; Rr and Rs are represented of the resistance of rotor and stator respectively. Figure (5.2) shows the reference frame is constant to the direction of stator voltage vector and also can be described the voltage stator as: value of stator voltage.
Figure (5.3) voltage orientation
When the good control of motors, should be kept the flux as a constant. Which can be expressed as:
By equation (5.1) and (5.2) we can obtain equation (5.6)
. Furthermore, can be use of
so the same procedure to rotor equations (5.3) and (5.4) gives as: (5.6)
"By substituting all rotor variables with stator variables in (5.7) then we obtain" + = (5.7)
By the equation (5.7) can be obtained other equation as:
(5.8)
Where / Where the "is the rms value of the stator flux reference, Is; Is represented rms value of the stator phase current, so the equation is given by:
5.2.5 Switching table
On the foundation of the torque state of flux and stator flux hysteresis sector, which is represented by α, so that the direct torque control (DTC) algorithm chooses the inverter voltage vector applied to the induction machine from the table below. "The outputs of the switching table are the settings for the switching devices of the inverter". Figure ( ) shows the relative of the stator flux switching sectors and inverter voltage vector. (5.9)
d
dTe
ï¡(1)
sector 1
ï¡(2)
sector 2
ï¡(3)
sector 3
ï¡(4)
sector 4
ï¡(5)
sector 5
ï¡(6)
sector 6
1
1
0
-1
0
1
0
-1
Table 5.1 Switching table of inverter voltage vectors
Active switching vectors: (100); (110); (010); (011); (001); (101)
Zero switching vectors: (000); (111)
Figure (5.4) stator flux switching sector and inverter voltage vectors
Zidani, F., Nait-Said, M., Abdessemed, R. & Benoudjit, A. (1998) Comparative study by numerical simulation of induction machine performances in vector and scalar control: System Theory, 1998. Proceedings of the Thirtieth Southeastern Symposium on. 8-10 Mar 1998. ( scalar control)
Yongdong, L. & Juanjuan, S. (2009) Voltage-Oriented Vector Control of induction motor: Principle and dynamic performance improvement: Power Electronics and Applications, 2009. EPE '09. 13th European Conference on. 8-10 Sept. 2009.
