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A SEMINAR ON LINEAR INDUCTION MOTOR

TALK FLOW 1. INTRODUCTION. 2. VARIABLE VOLTAGE

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A SEMINAR ON LINEAR INDUCTION MOTOR

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    1. A SEMINAR ON LINEAR INDUCTION MOTOR

    2. TALK FLOW 1. INTRODUCTION. 2. VARIABLE VOLTAGE & VARIABLE FREQUENCY LIM SYSTEM. 3. POSITION REGULATION USING SPEED SENSOR. 4. SENSORLESS SPEED REGULATION & SYNTHESIS. 5. ADVANTAGES OF LIM. 6. APPLICATIONS OF LIM. 7. CONCLUSION OF LIM.

    3. INTRODUCTION In a sector motor when the sector is laid out flat and a flat squirrel-cage winding is brought near to it ,we get a Linear Induction Motor.In practice,instead of a flat squirrel-cage winding ,an aluminium or copper or iron plate is used as a ‘Rotor’.The flat stator produces a flux that moves in a straight line from its one end to the other at a linear synchronous speed given by: Vs = 2Wf As the flux moves linearly,it drags to rotor plate along with it in the same direction . However, in many practical applications,the ‘Rotor’is stationary,while the stator moves . For eg: In high speed trains,which utilize magnetic leviations the rotor is composed of thick aluminium plate that is fixed to the Ground and extends over the full length of the track .

    4. VARIABLE VOLTAGE & VARIABLE FREQUENCY LIM SYSTEM Early developments in LIM have been done at EPFL.In the newer LIM applications, the existing motor is reused with variable voltage and variable frequency & this achieved with a modern IGBT PWM-Inverter fed through a diode rectifier from the AC Network. The output of the PWM-Inverter can be controlled measuring only the DC voltage, which needs only one voltage sensor.

    5. MOTOR(STATOR) The used LIM is a three-phase Linear Induction Motor with double inductor. In this utilisation, the neutral point isn’t connected, but a connector exists so it can be done if desired in the future. Its main characteristics are: · Sn= 36[kVA] : nominal apparent power · In=41[A] : nominal current · Un=165[V] : nominal phase voltage · p=1 : number of pole pairs · tp=129[mm] : pole length · vs=12 [m/s] : synchronous speed · Ns=58[turns/inductor] · Zn=12: number of slots · Star coupling, free neutral point · d=2[mm] : air gap

    6. RAIL(ROTOR) The motor rail is a simple V-shaped aluminium I-profile carrying and guiding the moving stator by rolls.Steep and reduced power climbing segments simulate conditions similar to a roller coaster, allowing high accelerations and fast movements but also stand-still under load conditions, as test-conditions for the control performances.

    7. CONTROL SYSTEM The control system is made of a floating-point architecture Digital Signal Processor, which facilitates the practical implementation of functionality by students.The DSP is completed by devices like FPGA for digital modulation of the PWM Inverter, and by fast AD converters for fast real time data acquisition. Because of its quickness, its large number of AD’s (14) and its multiple graphical tools, this dedicated card developed at EPFL/LEI is a very powerful and efficient development tool.

    8. COMPLETE SYSTEM Depending on the control strategy implemented, each of the sensors may or not be used. But they are anyway useful to dimension all the regulation parameters because they give interesting informations on the system behaviour.

    9. CHARACTERISTICS CURVES They are the same for the both regulation strategies.These curves characterise the motor when operated with constant stator flux. To assure this, the stator frequency must rise with the stator voltage.Along with these curves,the nominal stator voltage is set to 165[V] corresponding to the nominal stator frequency of 50[Hz]. The two important points on these curves are the nominal point (_rn=80[rad/s], In=41[A]) and the breakdown point (_rk=151[rad/s], Ik=52[A])). In comparison with classical rotating induction motors, the values of the slip during operation may be very high. Another particularity is the value of the magnetising current (_r=0) which is quite important (I0=29[A]).

    10. CHARACTERISTICS CURVES OF LIM SYSTEM

    11. POSITION REGULATION USING A SPEED SENSOR REGULATION STRATEGY: A simpler strategy is followed for the set up of the test track, which is particularly easy to implement.First the drive is regulated in speed, then an additional loop for regulating the position will be added. The external loop is a speed regulation. Depending on the speed error, the regulation system sets the motor force by acting on the slip frequency (block 9). The stator frequency is obtained by summing the desired slip frequency and the measured mechanical speed Each phase current, corresponding to the slip frequency The phase difference between the three phase currents is 2_/3.

    12. REALISATION When the frequency is high, the regulation must be very efficient. The regulation sampling period is set to 150[s]. If symmetric current is imposed, more voltage is needed in the central phase. The reason is that the others phases surround this one. This confers to it a greater self-inductance. In this case the stator frequency is 19[Hz]. As the current regulation is working well, we can expect to use the drive efficiently, with good dynamic performances. If the rail is short, as the motor can be slowed down very well, a high set value of 3[m/s] can be chosen for the constant speed.

    13. SPEED PROFILE The graph shows the chosen speed profile depending on the position error. It means that as long as the motor is far enough from the desired position (>1.2[m]), the regulation sets the speed reference to a constant value of 3[m/s] towards the desired position. As soon as the motor is within 1.2 meter from the final position, the speed reference is diminishing linearly with the position error.

    14. SPEED & POSITION REGULATION This figure shows the results recorded on the real track.Position 0 corresponds to the lowest part of the track in the hollow of the rail in V .The speed reference is ramped.Ramping the reference avoids the integrator regulating the slip frequency to saturate and disturb the dynamic. Both speed and position regulations are working well and there is no position overshoot. The excellent current regulation makes it possible to use the motor in an efficient manner.

    15. SENSORLESS SPEED REGULATION Control strategy: Because the motor model is not accurate in every case, speed estimation will not be perfect, especially during transient behaviour. Consequently the same strategy than above will not be used because its efficiency and its stability depend on a precise measuring of the instantaneous speed. Rather than setting the currents, the regulation sets the voltage and always imposes constant and rated stator flux. This way the motor constantly has a maximum of force. To achieve that, the magnitude of the phase voltage must vary proportionally with the frequency [3]. The phase difference between the 3 phase voltages is constant and equals 2_/3.

    16. RESISTIVE FIELD LOSSES COMPENSATION A typical model of the asynchronous motor in steady state (Fig. 11) is used. This representation does not to take into account the difference between the three phases nor the specific behaviours due to the linearity of the motor.

    17. COMPLEX REPRESENTATION OF VECTORS The magnitude of Em can be written: Em^2=Us^2+(IsRs)^2-2UsIsRscos(F) Let Em0 be the value of Em at rated stator frequency fsn. The magnitude of Em must vary linearly with the stator frequency (fs)

    18. POWER FLUX IN THE MOTOR Pel [W] = Stator electric Power Pcus [W] = Stator resistive losses Pm [W] = Iron losses Pcur [W] = Rotor resistive losses Pa= Air gap power Pmec[W] = mechanical power Pf+v[W]= friction + fanning losses Pshaft[W] = Power at the shaft. Pa = Pel-Pcus-Pm If the parameters of the motor are unknown, the model can be simplified by choosing Rm=0

    19. RESULTS & COMMENTS The speed response in three different cases is showed. At the beginning of each test, the motor is in the hollow of the rail. Then the speed reference is set to a constant value.When the motor has travelled over 8 meters, the reference is set back to zero, so that the motor stands still on the steep rail.

    20. REFERENCE & MEASURED SPEED To show the behaviour of the motor in more general case, the speed reference is varied with a potentiometer. Fig. 16 shows that when the reference is not varied to quickly and for lower commanded speeds, the speed response is quite good, with only little oscillations.

    21. SYNTHESIS The thoughts and measures above make it possible to draw the following conclusions concerning a sensorless regulation: Speed estimation and regulation algorithm are stable. This makes it possible to guaranty a stable behaviour in any operating case. Most of the speed oscillations are due to the varying force due to the supply cable. In the future, it will be avoided by using an integrated supply from on board batteries. Speed estimation depends strongly of the rail temperature. If the rail temperature raises, the rotor resistance raises too. Consequently the breakdown slip (_rk) raises too (linearly with Rr). This strategy directly uses _rk to estimate the slip, it is quite sensitive to the variations of Rr. Because the rotor (rail) is linear, a constant rotor resistance for every position can not be expected.

    22. ADVANTAGES Low initial cost and low maintenance cost due to absence of rotating parts. Simplicity in construction due to linear shape. Elimination of overheating of rotor because of motor continuous movement over cool rotor plate leaving behind rotor portion. In rotary motor traction,co-efficient of adhesion decreases with the increase in speed. There is no limitations of maximum speed due to centrifugal forces and absence of rotating parts. Higher power or weight ratio is obtained,as the rotor is not carried on the traction unit.

    23. APPLICATIONS Linear induction motors can be used on trolley cars for the internal transport in workshops. Linear induction motors can be used as booster accelerator for moving heavy trains from rest or up the inclines. LIMs are used as propulsion unit in marshalling yards in place of shunting locomotives. LIM has superiority over conventional rotary motors for speeds above 200km/hr. LIM is also used in magnetically suspended trains where conventional motor cannot be used.

    24. CONCLUSION A drive using a linear asynchronous motor has been realised.First the system is regulated in a “classical” way by using a speed sensor. The current regulation is excellent and allows use to use the drive in a very demonstrative manner. In the next step, the same motor is regulated without any speed sensor. But the speed estimation is not perfect. Indeed the varying electrical characteristics of the rail with temperature and position have not been taken into account. It is important to remind that the first regulation is totally unstable if the sensor breakdowns. In case it is desired to guaranty a greater availability than dynamic precision, a sensorless regulation might be more indicated. In case very high dynamic performances are desired, a better model will be needed in order to take care of the characteristic properties of linear motors.

    25. REFERENCES [1] E=TeM2, “Tomorrow’s education in electrical technologies: revisited methods & tools for reneved motivation”, Liège, Belgium 14-16 March 2001. [2] Morizane T., Rufer A., Tanigucci K., “Sensorless control of linear induction motor considering its asymmetric parameter”, MAGLEV 2000, 16th international conference on magnetically levitated systems and drives, Rio de Janeiro, Brasil. [3]H. Bühler, Convertisseurs statiques, Presse Polytechniques et universitaires romandes, Lausanne, Suisse, 1991. [4] N. Wavre, “Etudes harmoniques tridimensionnelles des moteurs linéaires asynchrones à bobinages polyphasés quelconques“, Thèse EPFL, Lausanne, Suisse, 1975. [5] T. Seki, ”The development of HSST-100L 14th International Conference on magnetically Levitated systems, Hotel Maritim, Bremen, 1995. [6] A. Munoz-Garcia, T.A. Lipo & D.W. Novotny, “A new induction motor open-loop speed control capable of low frequency operation”, IEEE Industry Applications Society, New Orleans, Louisiana, USA, 1997 [7] http://leiwww.epfl.ch/sharc/sld003.htm [8] M. Jufer, Electromécanique, traité d’électricité (Vol. IX), presses polytechniques et universitaires romandes, Lausanne, Suisse, 1995. [9] H. Bühler, Théorie du réglage de l’électronique de puissance, notes de cours EPF-Lausanne, Suisse, 1994. [10] B. Kawkabani, Contrôle vectoriel des machines asynchrones, notes de cours, EPF-Lausanne, Suisse, 1995. [11] F. Gardiol, Electromagnétisme, traité d’électricité (Vol. III), presses polytechniques et universitaires romandes, Lausanne, Suisse, 1996.

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