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The Light Load Stability and Low Speed Performance Research of Speed Sensorless Control System of Induction Motor

Author: ChenWei
Tutor: XuDianGuo
School: Harbin Institute of Technology
Course: Power Electronics and Power Drives
Keywords: Ocsillation suppression Voltage vector control Speed adaptive fluxobserver Parameters estimation at standstill
CLC: TM343
Type: PhD thesis
Year: 2012
Downloads: 346
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Abstract


Energy saving is an eternal theme for a sustainable development in present society.Induction motor (IM) drive system plays an important role in the energy saving.However, with the continuous development of the modernization process in our country,the performance requirements for the IM drive system becomes higher and higher. Howto further improve the low speed performance of the IM drive system is one of the mostarduous technologies to be solved. Open loop variable voltage and variable frequency(V/f) control method and close loop speed sensorless vector control (SLVC) method aretwo popular methods for IM speed sensorless control. V/f control method possessesgood robustness and high reliability. However this method has poor low speedperformance and suffers from sustained oscillations under the light load conditions. Inaddition, the SLVC method is specialized by the good dynamic and static performanceand good low speed performance. But the drive systems controlled by this methodencounter instability in the regenerating mode at low speeds. Thorough study in IMspeed sensorless control algorithms and research on the solution for the core technologyis important for improving the applications of IM speed control technology in powerdrive systems.This thesis mainly studies the IM speed sensorless control methods, including theopen-loop V/f control method and speed sensorless vector control method, and focuseson solving the core technologies that involved in these methods. The contents includeseveral aspects as follows:In order to improve the stability of V/f controlled IM drive system in light loadconditions, a stabilizing control method is presented. Through simulation modeling, thesystem instability is analyzed in detail based on the simulation results. Then bytheoretical derivation, the reasons that result in the instability under light load conditionare studied, and the influences on system stability of various parameters are alsoanalyzed. Based on the relationship between the system oscillations and the fluctuationof reactive current the stabilizing method is implemented by adjusting the stator voltageamplitude according to the fluctuation of reactive current which is decoupled throughstator voltage oriented synchronous rotation frame transformation. The proposedmethod is parameter independent and robust. Experiments have proven that the systemhas a very stable operation over the whole speed range from0to50Hz with theproposed method. The oscillations have been suppressed effectively, and the systemstability in light load conditions is much improved.Based on the ideal model of IM, the motor speed is unobservable when the IM is operating in a very low speed range. In order to achieve speed control with load inextremely low speed range, on the basis of open loop V/f control method combing withthe vector control theory, a novel voltage vector control strategy is derived according tothe state equations of IM. The control scheme adopts a novel voltage boost method tocompensate the stator resistor voltage drop keeping the magnitude of the stator fluxconstant. The performance under very low speed conditions is greatly improved byboosting the torque and compensating the motor slip frequency. Different from speedsensorless vector control methods, all the compensation and regulation are based onstator voltage oriented synchronous rotation frame transformation, which avoidscomplicated flux linkage vector estimation, and is easy to implement, and has goodparameters robustness. The experimental results show that, by adopting the proposedmethod, the speed can be accurately controlled down to0.1Hz with rated load torque.The low speed performance and speed accuracy are much improved, and the systemunder no load condition is stabilized at the speed range from0to50Hz.In the speed-sensorless IM drive system adopting the speed adaptive flux observermethod, there exists an unstable region in the regenerating mode at low speeds. In orderto solve this problem, a novel speed estimation method is presented. Based on the smallsignal linearized model of IM, the transfer function of the speed adaptive estimationsystem is deduced, and a novel speed adaptive estimation method is developed byapplying Routh-Hurwitz criteria. The speed accuracy could be greatly improved byadding the magnetizing current estimation error to the general speed estimation law, andthe instability behavior in the low speed regenerating mode could be remedied byadjusting the adaptation gain on-line. Concerning the motor parameters error, therobustness of the proposed method is analyzed as well. The feasibility of the proposedscheme is verified by the experimental results of speed sensorless field-oriented vectorcontrolled7.5kW IM platform. Experimental results show that the speed estimationaccuracy and the low speed performance are greatly improved, and also a stableoperation is acquired in a very wide speed range.In the IM drive system, both the stator resistance voltage drop and slipcompensation in open loop V/f control algorithm, or the parameters design of variouscontrollers and flux observations in vector control algorithm, depend on motorparameters. The control performance is highly dependent on the accuracy of theparameters of IM. In order to obtain the parameters of IM accurately, a parameteridentification method at standstill is proposed. The nonlinearity of the inverter is takeninto account and compensated adaptively to improve the estimation precision. Inaddition, the influence due to the magnetic hysteresis phenomenon when calculating themutual inductance is also considered. The mutual inductance is estimated at the rated magnetizing level by superimposing a direct current into the traditional single phasealternating current. The feasibility of the proposed scheme is verified by theexperimental results which show a high estimation accuracy of IM parameters.

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CLC: > Industrial Technology > Electrotechnical > Motor > AC motor > Induction motor
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