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Research on Synchronous Motor Excitation Control System

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Tutor: GaoQiang
School: Harbin Institute of Technology
Course: Electrical Engineering
Keywords: Synchronous Motor,Asynchronous Starting,Exciting Current Adjusting,DSP,CPLD SCR
CLC: TM341
Type: Master's thesis
Year:  2008
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Abstract:
Synchronous motor is widely used in industrial and agricultural fixed-speed system, with features in free-conditioning power factor, constant speed, hard load characteristic and so on. Excitation system, an important part of the synchronous motor, has a direct impact on the operation of the motor. Two problems about the synchronous motor excitation control have to be solved; one is how to control the exciting, the other is how to adjust the exciting current after the synchronization. This thesis focuses on the exciting control system in these two problems.When comes to exerting exciting, a large part of this thesis is dedicated to the asynchronous starting method. Because the rotor-inductive voltage becomes rather weak when synchronous motor¡¯s speed approaches synchronous speed in rotor-electricity exerting exciting method, a sensorless stator-electricity exerting exciting method is presented in this paper based on the elaborate analysis of the synchronous motor starting and exerting exciting process. Simulations and experiments show that this method is reliable, short synchronized time, low impact to the grid, easy to synchronize, which to a large extent solves the exerting existing problems in synchronous motor asynchronous starting.As far as exciting adjust is concerned, a power factor detecting circuit was designed, which not only provides the fixed exciting current running mode but also the constant power factor running mode. Because the most processors currently used in digital exciting system are MCUs, with limited computing speed and accuracy, which impact the control system¡¯s response speed and accuracy. In order to meet the developing requirements of the excitation control system, DSP, taking advantage of its powerful computing capabilities and rich hardware and software resources, is used as the core of excitation controller. The hardware and software designs showed in this paper are based on this platform.In addition, the phase-shifting trigger circuit is the core of the three-phase thyristor circuit. For the distributed analog pulse trigger device parameters of the traditional SCR rectifier system, it is inconvenient to debug and use and with the problem of poor pulse symmetry. In view of the strong logic and timing functions of CPLD, as well as in-system programming, convenient debugging, it is designed to generate the double pulse sequence digital phase-shift trigger. With its simplicity and reliablility, pulse symmetry, and anti-jamming ability, the phase-shift trigger greatly reduces the burden on the processors, and has a broad prospect in the field of electrical excitation control.
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