Three Phase Induction Motors Engineering Essay

Three Phase instauration Motors design EssayThree build installing moves atomic number 18 one of the well-nigh astray employ industrial machines. This is mainly due to their simplicity, in high spirits reliability and robustness. Although three shape facility gos thrust the same physical stator coil as a synchronous machine, they do have different rotor coil coil constructions. In addition, three var. consequence forces argon sh ard into ii groups squirrel-cage and wound rotor. The magnitude of the desegregate in the quadrature axis vertebra in the stator diminishes, so that the fuse pass on be zero, but the zipper is increase above the synchronous despatch. The rotor authentic comes from rotor emf, so the final result backward rotating for the mix in parcel permits the magnetomotive force (mmf) so the feed is domineering in the quadrature axis. So, the zip up at which the magnetic field rotates cease be determined. If the repulse is conne cted to rated frequency, the rush of the magnetic field is called the synchronous speed (Ns).The foldepressioning table shows the relationship amongst image of poles and synchronous speed when the frequency equals 50HZ.Pole p personal credit lines12346Number of poles246812 coexisting speed300015001000750500Table (3.1) relationship in the midst of number of poles and synchronousThe relationship between number of poles and synchronous speed is in return proportional.3.2 Equivalent dress circle of an elicitation motor.The per phase very(a) circuit is real important for inductive reasoning motors and can be utilize to provide a great deal of understanding and prediction of per physiqueance of the induction motor in a stable state. The induction motor take ons to provide for operation on the induction potential and up-to-the-minute in the rotor circuit from the stator circuit (transformer action) this is beca utilise the potency and present-day(prenominal) in the rotor ci rcuit of an induction motor is radicalally a transformer operation. So the equivalent circuit of the induction motor is similar to the equivalent circuit of the transformer. (In the transformer, uncreated equivalent the stator in the induction motor, secondary in the transformer want the rotor in the induction motor.In an equivalent circuit per phase of an induction motor, as shown in figure (3.1), Rs and Rr are the stator and rotor winding resistance per phase. Ls and Lr are stator and rotor leakage generalization per phase. However, the aim of test and analysis is to be able to refer the rotor resistance and reactance to the stator circuit and in that respectby to necessitate an equivalent circuit referred to the stator. frame of reference (3.1) induction motor per-phase equivalent circuits.(3.1)(3.2)(3.3)(3.4)(3.5)(3.6)(3.7)(3.8)(3.9)(3.10)(3.11)(3.12)(3.13)In this circuit between a and b apply TheveninFigure (3.3) Simplification of equivalent circuits of an induction moto rOpen Circuit voltage at ab(3.14) short(p) Circuit generate(3.15)(3.16)(3.17)(3.18)Maximum occurs(3.19)(3.20).3.3 Overview on give of consequence motors.Induction motors with squirrel-cage motors are used in industry because the advantage of these fibers of motors is their relatively wretched cost and simple construction. Induction motors always work at a nearly constant speed. However, king electronic converts it can work to go away the speed of an induction motor. The induction motor finds can be divided into groups based on their applications (a) Adjustable-speed drive. integrity important application of this drive is in process chequer by arrestling the speed of fans, compressors, pumps. (b) Servo drive by means of sophisticated enclose, induction motors can be used as servo drives in computer peripherals, machine tools, and robotics acknowledgement.I would equivalent to give a brief explanation of two regularitys that are used in the avow of induction motors an d I entrust go into more(prenominal) than detail about these method actings later. sender keep back is a method of rule of induction motors so the stator current is throwled in the field rotating part apply PWM inverter Reference. The rotor intermingle and stator desegregate linkages are equal by ar (t) and as(t) depends on the angle of the rotor m because the joint induction between the stator windings and rotor windings position is connected. However, the main creator for the q and d axis analysis in machines like the induction machines is to manoeuvre them properly, for example transmitter maneuver reference, the method of sender get wind of induction motor drives produces better dynamic achievement than scalar throw reference. The side by side(p) block diagram shows the direct torque cut back system of rules of an induction motor.Figure (3.4) shows direct torque program line of induction motors.This alternative type of visit of an induction motor i s very simple and basic in terms of construction. It consists of a switch table, hysteresis ascendances, flux estimator and torque. It is much easier to agree in a block diagram compared to the block diagram representing the sender conquer system due to the absence of coordinate transformation between the synchronous raise and stationary frame and alike it does non need a pulse width. Direct torque see drives are experienceled by the method of a close loop-the-loop system without using a current regulation loop and are also cogitate to use of a stationary d-q reference frame as well as having the d-axis aligned with the stator q axis. Moreover, the flux and torque are controlled by the stator voltage lacuna vector defined in this reference framereference. scalar control is another method of control of induction motors and is also the graduation method of control before the vector control method. The advantage of this method is simple control and ease of use. The motor drive is described by three performers (a) frequency (b) voltage (c) parameters of the motor and its power supply reference. The scalar variable is strictly one represented by magnitude alone. This method uses either close loop or blossom forth loop control and any feedback loop such as that for speed. This use of scalar quantities gives the basic characteristics of satisfactory steady state behaviour, but miserablely controlled transient response.3.4 rendering of the Induction motor driveSince a motor drive plays a big part in the control system, it is necessary to have round background information about it. In a typical induction motor drive, power electronic devices are used to operate AC motors at frequencies other than the supply one. It consists of two main sections, a controller to set the operating frequency and a three-phase inverter to generate the postulate sinusoidal three-phase system from a DC bus voltage.Therefore, an induction motor requires a variable-frequen cy three-phase source for variable speed operation by using a power converter system consisting of a rectifier connected to an inverter through a DC link. The next figure shows a block diagram of the power circuit of a typical variable-frequency induction motor drive.Figure (3.5) shows Variable-Frequency Induction Motor DriveThe rectifier converts the power grid AC voltage into a opinionated DC voltage. An LC filter to provide a smooth DC voltage, which is then applied to the inverter input, filters out the harmonics.3.5 Induction Motor dominance Methods3.5.1 Vector controlThe vector control of induction motors has been wide used for high performance drives. There have been many studies developed and presented which allow an overview of vector control (reference). Many proposals for the theory of electric machines discuss using space vector control to represent sinusoidal distribution in the air violate and they also discuss types of control of ac drives including induction motor drives, permanent-magnet ac drives and switched reluctance drives (reference).Induction motor drives have performance control as the same high performance four-quadrant DC drive. In 1960 field-oriented control (FOC) was used in the area of induction motors, but in the bygone decades, induction motors have been controlled by using scalar control methods like the voltage/hertz. However, this manner is an old way used before vector control in the area of induction machines, but it was an easy method of controlling an induction motor (Reference).3.5.1.2 Concept of vector controlThe purpose of vector control of AC drives is related to a space phaser which provides a means of representing three phase variables in a machine, voltage, current and flux (Reference). Both flux and torque (DTC) are basically controlled by methods of unkindly loop so are methods of control of an induction motor, using disagreeable loop without current loop, similar to the formulaic vector control drives (R eference). So the stator current will be utilizing transformation to the d q synchronise system and direct axis with the rotor flux space factor therefore, the stator d q axis current is controlled dependently and the d q-axis for rotor flux will be zero.(3.21)The relationship between the stator current, rotor flux and electromagnetic are shown by this equation(3.22)WhereIs rotor flux linkage, Rr , Lr , Lr are rotor resistance, and Lm magnetizing inductance. (Reference).However, the space vector in the three phase inverter will produce eight output states 1 0 0 switch states, top(prenominal) in switch phase is represented by and b- is closed and c are open. The eight space vector represented by V0 0 0 0 and V7 1 1 1 are null and chronic six are of equal magnitude and arranged 600 part in space diagram as shown in figure. (Reference).Figure (3.6) shows witching voltage space vectors.3.5.1.3 Control CharacteristicsVector control has allowed the dynamic performance of AC drives so that they better DC drives the flux and the torque can be controlled reveally by using vector control producing components of the supply current. The depot voltage cannot be directly monitored, but can be using the dc link voltage and switching function of the inverter supplying the motor. Currently, the drive dynamic is largely used with the inverter control of the stator current of the induction machines this in turn is determined by the supply voltage and inductance of the machine. The main features of the direct torque control (DTC) are direct control of the torque and flux, and confirmatory main control of voltages and currents. This type of control has a number of advantages sinusoidal stator current and reduced torque oscillations handsome torque dynamics and the main advantage of (DTC) direct torque control, absence of coordinate transformations which related to vector control implementations. However, in this type of control there are some disadvantages possible prob lems during starting and low speed operation and also during change in torque command it also requires flux and torque estimators (Reference).The vector control theory provides independent control between torque and flux torque is controlled by the q-axis component of current if the flux is constant and oriented along the d-axis of the referred frame. The referred frame can be rotor flux-oriented control, stator flux-oriented control or air gap flux-oriented control. Thus, the phase angle and the modulus of the current or current vector have to be controlled. Figure 2.1(a), Reference, shows the rotor angle r with respect to the stator. Since the vector control is to be implemented in the rotor flux oriented reference frame, the induction machine is fixed in that reference frame by rotating the variable as appropriate. Figure (3.7) shows that the rotor flux reference frame rotates at speed (angle e) with respect to the stator reference and the d-q axes are fixed to the rotor flux spa ce phasor. This results in decoupling of the flux and torque which are separately controlled by stator direct-axis current ids and quadrature-axis current iqs. Reference(B) Vector rotation Figure (3.7) (A) Rotor angle3.5.2 Direct and Indirect Rotor Flux-Oriented ControlThe vector controls can be divided into two groups, substantiative and direct, for the confirmatory (slip frequency controlled) and the direct (field oriented). The characteristics of these two controllers have been considered to be the same, but there are some differences between these methods. However, the direct control type is a modern control theory and also has high performance and it is more well-known than indirect control (Reference).In this two methods above are considered of voltage and current of a stator. The first method, indirect, is related to stator current control and the second method is related to stator voltage control. In both cases the system inputs are torque and flux reference which is requ ired current values for isd and isq. Field oriented control to induction motor operation in a synchronously rotating d-q reference from that is aligned with one of the motor flux. So, control of the torque and flux is decoupled such as the d- axis component of the stator current and rotor flux magnitude and the q-axis component control , the output torque , where the ids stator current of d-axis component and is the rotor flux magnitude demand, so can be given in equation as(3.23)Where, Lm = magnetizing inductance.For the q-axis component of the stator current iqs, the torque demand as(T* em) so can be determined as the equation(3.24)The d-axis of the synchronously rotating reference frame to be aligned with the rotor flux, the slip relation.(3.25)Direct rotor flux-oriented control (RFOC) has a control loop for flux where the measuring stick is performed using flux sensing coils (or Hall-effect devices) or by the flux model. In indirect rotor flux-oriented control (RFOC), the roto r flux angle is not measured but is estimated from the equivalent circuit model. One of the techniques for estimating the rotor flux angle (e) is based on the slip relation, which requires measurement of the rotor speed () and slip frequency (). The slip frequency is hooklike on rotor time constant (r) and estimated rotor flux amplitude. This indicates that indirect methods are easier to implement since they do not require a flux model, but are less accurate. Nevertheless, if the model were perfect, the performance obtained would be identical to direct torque control. In 6, the relationships between direct and indirect approaches have been analyzed. It proves that they have the same control but have differences in coordinate of state variable, the rotor flux and stator currents. A new indirect vector control with an observer has been presented which has the same characteristic as the conventional direct torque control.Figure (3.8) direct vector controlFigure (3.9) indirect vector c ontrol3.5.3 scalar control methodScalar Control manner is related to AC machines and can use voltage fed-inverters. In addition, the scalar control is related to control of the magnitude of a variable only, also using of applications for constant voltage/hertz supply at the motor terminals are given constant air gap flux, so it could be that the stator voltage is dropped. (Reference) The scalar control methods are considered just for study state behaviour, but have poorly controlled transients. (Reference)This method applies to use in either close loop or open loop, and in any feedback loop such as that of speed. The scalar control method was basically developed for study state operation. However, it is a method also used in variable speed applications. (Reference)In this curve, the voltage and frequency are applied on the stator, therefore, when the supply frequency is constant, the speed will be constant, but the torque can be ever-changing as the square of the applied voltage. I will give more details about scalar control in another chapter.3.5.4 comparability of vector control (VC) and scalar control (SC).Induction machines are widely used in various industries as prime workhorses to produce rotational motions and forces. However, the squirrel-cage type is a simple and rugged electrical machine with low cost and minimal maintenance this is reason that the squirrel-cage types are most widely used in industrial electrical motors reference. With regard to the scalar control methods for an induction machine, only the motor model is considered for steady state and the scalar control methods are controlled based of the induction motor, but this method will not give good performance transients for an induction motor and it is also poor in terms of dynamics but the vector control methods considered above are valid for transient conditions and the vector control will give a dynamic performance far superior to that of scalar control reference. The scalar control met hod to control an induction motor is simple to execute and easy to programme, but the vector control is related to the varying magnitude and phase alignment of the vector quantities of the motor. Moreover, the scalar control is related to the voltage per hearts v/f control and is normally used for low cost drives where high dynamic performance is not a key requirement. The applications include fans, blowers and pumps where the applied load is known. In this method, the form is simple, the control does not require any sensors and the control algorithm can be implemented in a relatively low performance microprocessor. Vector control is related to a mathematical model which deals with voltage, current, flux torque and the motor parameters. We can control the instantaneous stator currents, control the magnitude and position is.The following diagram shows a feedback control system for measuring currents.When the controller has a fast response then is vector can be imposed on the stator r apidly.Advantages and disadvantages of vector control and scalar control referenceMethodAdvantagesDisadvantagesSCSimplest method of obtaining variable speed. kickoff cost and easy to implement solution.Is widely used.Is not as entangled as other control methods.Poor transient performance and poor dynamic.It cannot control torque directly.The transit response such as control is not fast.VCFor those d component and q component are two decouple components can be independently controlled by passing through separate PI controller.This control method has an fantabulous torque and speed curve.It has excellent dynamic performance.Sensitive parameter variation use PI current regulators that decrease transient performance.Is more complicated than any other control method.Cannot control torque before transformation is done.Table (3.2) advantages and disadvantages of scalar control and vector control