Paper 10 International Journal of Computer Information by shilpabangalore


									                                                                    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

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
                                                                       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
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
                                                                                                                   = 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)
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.
                                                                                                                   = j ω rψ − Rr ir                 (rotor )
Hence, with vss as the stator voltage space vector, the                                                   dt                    r

following relation must hold:
                                                                                                        The Figure.3 shows induction machine

        s                   s
   v −R i −
        s              s 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                   s
   v −R i −
        r              r s
                                                     =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

                                                                                                     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
                                     d  θ rψ  
                                          −j  s
                                    s e
                                            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
                                   r      dt                                                                       s          s               s
                                                                                                             ψ =L i +L i
                                                                                                                     r       m m        rl r

              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
               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
                                                      − Lm
                                                                s             s
                        s                                  d ir             d im
         jω ψ − R i − L
                   r    r           r r
                                                                    − Lm
B. Power Performance
  The Figure.4 shows per phase equivalent circuit of
polyphase induction machine [3].


                                                                                                      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


                                                                            The Figure.9 shows the Stator voltage


  Figure.7 shows the simulink mathematical model of
Induction motor.


                                                                                 The figure.10 shows the speed characteristic


       May Issue                                            Page 52 of 53                                      ISSN 2229 5208
                                                                  International Journal of Computer Information Systems,
                                                                                                   Vol. 2, No. 5, 2011

       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.

    The figure.12 shows the rotor current characteristic


   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

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