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International Journal of Computer Information Systems, Vol. 2, No. 5, 2011 Simulink Model of Induction motor and Different Methods for Speed Control of Induction motor Prakash P.K Shivkumar E.G Asst.Professor, ECE dept Asst.Professor, EE dept APS College of engineering University Visvesvaraya College of Engineering Bangalore Bangalore prakashapk@rediffmail.com egshiva@yahoo.com Abstract— The AC Induction motor is the most widely used nature of the AC wave to turn the field coils on and off type of electric motor in the world. AC motors are primarily sequentially. The AC induction motor does not need brushes used as a source of constant-speed mechanical power but are because the rotor is essentially a passive device that is increasingly being used in variable speed-control applications. These motors are simple in construction, reliable and low cost. continuously being pulled in one direction. Due to these advantages over DC machine, they are more AC induction motors are classified in to different commonly used as electromagnetic drive for industrial, groups depending on the type of power they use: single- commercial and other domestic applications. This paper phase, two-phase and three-phase. Each type is based on the mainly focusses on simulink model of induction motor and same operating principles and each type has its own different existing methods of speed control of induction motors. advantages and disadvantages. Keywords- Induction motor; Simulink; DTC; FOC. Single-phase AC motor is most commonly used in domestic and commercial purposes, typically for applications requiring 1 or 2 hp. The motor has major I. INTRODUCTION problem i.e. it can not start itself. The most common way to Among all the existing motors on the market there are start a single-phase motor is to use a second set of windings, three ‘classical’ motors: the Direct Current with called the start windings, which are only energized during commutators (wound field) and two Alternating Current the start-up period. In general, it becomes temporarily two- motors; the synchronous and the asynchronous motors. phase motor, which is self-starting. This type of motor is These motors, when properly controlled, produce constant also known as split-phase motor. The AC in start winding instantaneous torque (very little torque ripple) and operate should ideally be 90 degree out of phase with the run from pure DC or AC sine wave supplies. The Figure.1 winding. shows the classification of motors. The two-phase is not available directly from the power company; it must be created, usually from single-phase. The three phase motor is simpler and smaller than its single-phase counterpart, but it can be used only where three phase is available. Usually used for industrial purpose where power required is more than 2hp.It has three sets of stator windings, with each set of windings powered by one of the phase voltages. The natural timing of sequence of the three individual phase voltages produces the rotating stator field that pulls the rotor around. The rotor is the squirrel cage Figure.1 type. The Figure.2 shows asynchronous motor. The theory of operation of the AC induction motor has some similarities to that of the stepper motor or Brushless DC motor. These motors work by having their field poles energized in sequence around the stator. The rotor is pulled around because it is attracted to the sequentially energized poles. With stepper motors and brushless DC motors, special switching circuits are required to turn the field windings on and off. The AC motor also works by rotating Figure.2 the stator field, but it makes use of the natural alternating May Issue Page 49 of 53 ISSN 2229 5208 International Journal of Computer Information Systems, Vol. 2, No. 5, 2011 II. THEORITICAL BACKGROUND equations: A. Dynamic Model s Let us first consider the stator circuit. The resistance Rs dψ s s s = vs − Rs i s ( stator ) of the stator winding is (for all practical purposes) equal in dt all three phases. From the law of induction it follows that (2.5) s the part of the stator voltage which is not dissipated in the dψ s s stator resistance will build up a flux in the stator winding. r = j ω rψ − Rr ir (rotor ) Hence, with vss as the stator voltage space vector, the dt r following relation must hold: s The Figure.3 shows induction machine s s dψ v −R i − s s s dt s =0 ---------- (2.1) Where iss and ψss are the space vectors for stator current and stator flux linkage respectively. The rotor circuit, with winding resistance Rr, can be treated in a similar way. Suppose that the rotor is observed from a coordinate system (rotor coordinates) which rotates with the same speed as the rotor ωr. Let us denote rotor coordinates with superscript "r". as the coordinate. System is rotor-fixed, there will be no induced voltage due to the rotation, so the same relation as for the stator must hold, but with “s → r”: r r s dψ v −R i − r r s dt r =0 (2.2) Figure.3 Let us now find a relation between the stator and rotor Here vrr, irr and ψrr are the rotor voltage, current, and flux flux linkages. The rotor winding is referred to the stator, i.e., space vectors respectively. But the rotor winding is short- the rotor winding is represented by coils in the α and β circuited, so vrr = 0. Now, let us transform irr and ψrr to directions. (Fig. 3). Assuming linear magnetic conditions, stationary coordinates. This is a αβ transformation using the the air gap flux ψαs can then be expressed as rotor position θr = ∫ ωr dt: s s s s s s j r s j r ψ = Lm i m , i = i +i (2.6) ir = e θ ir , ψ r = e θ ψ r r r α m s r (2.3) where Lm is the mutual inductance between the stator and Equation (2.2) is transformed as the rotor, which is also called the magnetizing inductance, and ims, is the magnetizing current. The stator flux is the sum of the air gap flux and the stator leakage flux, the latter d e θ rψ −j s which under linear magnetic conditions is proportional to 0 − Rr e θ r ir − −j r s the stator current only. Similar reasoning for the rotor flux =0⇒ yields dt d θ rψ −j s s e s r ψ =L i +L i s s − jθ r s − jθ r − Rr e ir − − j ω r e ψ =0⇒ s m m sl s (2.7) r dt s s s ψ =L i +L i r m m rl r s s s dψ where Lsl and Lrl are the stator and rotor leakage j ωrψ − Rr ir − r =0 inductances, respectively. The leakage inductances are r dt (2.4) typically 10% of Lm or less. Alternatively, with Ls = Lm + Lsl and Lr = Lm + Lrt as the stator and rotor self-inductances, The induction motor is thus described by the following respectively, the relations can be expressed as May Issue Page 50 of 53 ISSN 2229 5208 International Journal of Computer Information Systems, Vol. 2, No. 5, 2011 III. DIFFERENT METHODS OF SPEED CONTROL s s s OF INDUCTION MOTOR ψ =L i +L i s s s m r Due to advancement in power electronics, DSP and (2.8) s s s ASIC, various control techniques have been developed for ψ =L i +L i r m s r r many applications, namely Field oriented control or vector control, direct torque control, Sensorless vector control. Combining (2.6) with (2.8), assuming constant inductan- A. Direct Torque Control (DTC) ces, yields Define Figure.5 shows the basic DTC block diagram for s s AC machine [6]. s s d is d im v −R i −L s s s sl dt − Lm dt =0 s s s d ir d im jω ψ − R i − L r r r r s rl dt − Lm dt =0 (2.9) B. Power Performance The Figure.4 shows per phase equivalent circuit of polyphase induction machine [3]. Figure.5 Direct Torque and Flux Control (DTFC), also termed Direct Torque Control (DTC), has been developed by Figure.4 German and Japanese researchers for use in torque control of high power servo drives. DTC is a control philosophy Where: exploiting the torque and flux producing capabilities of ac U1 = stator terminal voltage machines when fed by a simple voltage source inverter that E1 = stator emf generated by resultant air-gap flux does not require current regulation loops, still attaining R1 = stator effective resistance similar performance to that obtained from a vector control drive. Three control techniques have been employed for X1 = stator leakage reactance implementing DTFC drives: The Switching Table (ST), the Rm = iron core-loss resistance Direct Self Control (DSC) and the Direct Vector Modulation Xm = magnetizing reactance Control (DVMC). R'2 = rotor effective resistance referred to stator X'2 = rotor leakage reactance referred to stator B. Field Oriented Control(FOC) urb = e.m.f due to the saturable iron bridges in the The Figure.6 shows the basic torque control scheme of rotor slots FOC for ac motor drives I0 = sum of magnetizing I0X and core-loss I0R current Field-oriented control enables control over both the components excitation flux-linkage and the torque-producing current in a I1 = stator current decoupled way. FOC can be implemented as indirect (feed- I´2 = rotor current referred to stator forward) or direct (feedback) depending on the method used for rotor flux identification. The direct FOC determines the Some of the important steady-state performance orientation of the air-gap flux by use of a hall-effect sensor, characteristics of a polyphase induction motor include the search coil or other measurement techniques. The goal of variation of current, speed, and losses as the load-torque FOC is to maintain the amplitude of the rotor flux linkage requirements change, and the starting and maximum torque. Ψr at a fixed value, except for field-weakening operation or Performance calculations can be made from the equivalent flux optimization, and only modify a torque-producing circuit. All calculations can be made on a per-phase basis, current component in order to control the torque of the ac assuming balanced operation of the machine. machine. This control strategy is based on projections. Electromagnetic torque is produced by the interaction of May Issue Page 51 of 53 ISSN 2229 5208 International Journal of Computer Information Systems, Vol. 2, No. 5, 2011 stator flux linkages and stator currents (or rotor flux and A three-phase induction motor rated at 20hp, 460V, rotor current), and can be expressed as a complex product of 4pole, 60Hz is chosen for our model. The following are the the flux and current space phasors. In order to gain a parameters taken for the model. complete decoupling of torque and flux, the current phasor is Stator resistance Rs = 0.01 is transformed into two components of a rotating reference Rotor resistance Rr = 0.02 frame: A flux producing component id, aligned with the d- Stator leakage inductance Lsl = 0.1 axis representing the direction of the rotor flux phasor, and a Rotor leakage inductance Lrl = 0.1 torque-producing component iq, aligned with the q-axis Magnetizing inductance Lm = 4.5 perpendicular to the rotor flux. In this way, a linear relation Inertia Constant H(s) = 0.3 between torque and torque producing current is achieved Stator voltage vs = 1.0 and the torque in the ac machine could be expressed as Tel = Stator frequency ωs = 1.0 c Ψr iq. Thus, the electromagnetic torque generated by the Base frequency = 60 motor can be controlled by controlling the q-axis current. Speed ωm = 0.98 This is equivalent to the torque control of a separately excited dc machine. Figure.8 shows the smulation result of stator current Figure.8 The Figure.9 shows the Stator voltage Figure.6 IV. STEADY STATE CHARECTERSTICS OF INDUCTION MOTOR Figure.7 shows the simulink mathematical model of Induction motor. Figure.9 The figure.10 shows the speed characteristic Figure.7 Figure.10 May Issue Page 52 of 53 ISSN 2229 5208 International Journal of Computer Information Systems, Vol. 2, No. 5, 2011 REFERENCES The figure.11 shows the torque characteristic [1] Sadarangani, C., “Electrical Machines”, Royal Institute of Technology, Stockholm, Sweden,(2000) [2] Mohan N, Undeland, T. M. and Robbins, W. P., “Power Electronics”, John Wiley & Sons Inc., USA, (2003) [3] El-Hawary, M. E, “Principled of Electric Machines with Power Electronic App.”, John Wiley & Sons Inc, USA, (2002) [4] Simulink model of direct torque control of induction machine, American journal of applied sciences, 2008 publication [5] DSP solution for AC induction motor, BPRA043, Application note, Texas instruments. [6] Fatiha Zidani, Rachid Nait Said, “ Direct Torque Control of induction motor with FUZZY minimization torque ripple”, Journal of Electrical Engineering, Algeria, Vol.56, No. 7-8, 2005, pp. 183 – 188. [7] Fatiha Zidani, D. Diallo, M. E. H. Benbouzid, Rachid Nait Said, “Direct Torque Control of Induction motor with Fuzzy stator resistance adaptation”, IEEE Transaction on Energy Conversion, Vol.21, June-2006, pp. 619-621. Figure.11 The figure.12 shows the rotor current characteristic Figure.12 CONCLUSION The Analog components raise tolerance issues and upgrades are difficult as the design is hardwired. Digital systems offer improvements over analog designs. Digital Signal Processors go on further to provide high speed, high resolution and sensor less algorithms in order to reduce system costs. In future work digital Signal Processors are to be used for the speed control of induction motor. It may be the best solution providing high speed, high resolution and sensor less algorithms in order to reduce system costs. Upgrades can easily be made in software. May Issue Page 53 of 53 ISSN 2229 5208