An exact model and simulation of a permanent magnet synchronous motor system when driving a light load at low speed

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An exact model and simulation of a permanent magnet synchronous motor system when driving a light load at low speed

 

Author: Ho, Ping-kwong
Title: An exact model and simulation of a permanent magnet synchronous motor system when driving a light load at low speed
Year: 2000
Subject: Permanent magnet motors
Electric motors, Synchronous
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Electronic and Information Engineering
Pages: xi, 131 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1535396
URI: http://theses.lib.polyu.edu.hk/handle/200/4364
Abstract: The prime concern of this research is to improve the precision of Permanent Magnet Synchronous Motor (PMSM) speed control system when the motor is driving a light load at low speed. Ripple content in the speed and the torque cannot be ignored when a motor operates under this condition. The switching sequence of motor phases for conduction explains the cause of ripples. This project has developed a novel model for the PMSM, which can describe exactly its behavior when driving different loads either high or low speed. Also, with the knowledge derived from this model, the performance of the motor controller is thus improved. In this project, the PMSM is operated under the discontinuous current mode. This mode avoids the faulty simultaneous conduction of two transistors in the same phase. Currently, a two-mode operation model: - the conduction mode and the commutation mode, is used for describing its behavior with a heavy load at high-speed [22]. However, when the PMSM is driving a light load at low speed, this two-mode operation model cannot fully describe its behavior. The novel model proposed here consists of 5 operation modes and their combinations depend on the conditions applied to the PMSM. Most of the other's works make the assumption that the motor speed is constant within the whole motor cycle, and with such an assumption, the PMSM system is linearized in the qd-frame by using Park's transformation [30]. However, this does not bring us any advantage due to the no-constant motor speed I our model. Also, Howe's model [46] requires the computation of the conduction time for the freewheeling diode; while our approach eliminates this step. The novel model is developed in the abc-frame. The PMSM system is treated as a control system that is described with several sets of differential equations; whereas the appropriate parameters of these sets of differential equations are from 2 lookup tables. These sets of differential equations of:- a) 2 sets for the electrical part; b) 1 set for the electromagnetic torque; and c) 1 set for the mechanical part. Furthermore, we employ two approaches for simulating the PMSM system. In the first approach, we describe the system with four sets of differential equations and simulate it with the numerical package, MATLAB. In the second approach, the system is modeled with circuit elements in PSPICE. Here, equivalent electrical circuits are used to model the motor; while the non-linear behavior of the motor is handled by using the macro-modeling technique. The reason for us to develop two different approaches for simulation is that we want to take the full advantages from them. For example, the first approach offers us a convenient way to simulate the controlled PMSM system. Also, it provides us an easy way to manipulate the numerical simulation results. The second approach offers us no need to realize the details of the different operation modes. Furthermore, it is used to support us to use experimental results to verify the MATLAB simulation results. It is because some signals cannot be measured physically in the experimental setup, like the neutral voltage of the stator motor phases. Thus, the validity of our model is established by comparing the results in different combinations of operation modes form the two simulations with the experimental result. Some previous works have also brought out the effect of the phase-advanced angle on the PMSM. This effect will result in a higher mean torque and speed, but the ripple components in the torque and the speed become more significant, which is known as the flux-weakening effect. Our model can also simulate this effect.

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