Projects:2019s2-25401 Line-Start Axial-Flux Permanent-Magnet Motor

The aims of this project is to design line-start axial-flux permanent-magnet synchronous motors using the Finite Element Method (FEM), simulate radial-flux and axial-flux permanent-magnet synchronous motors using ANSYS software, analyse and compare the steady-state and transient-state performance of conventional radial-flux permanent-magnet synchronous motors with axial-flux permanent-magnet synchronous motors.

Introduction

The project is related to the Axial Flux Permanent Magnet Synchronous Motor (AF-PMSM), which has recently undergone significant work based on the development of magnet and motor technology. In this study, a novel AF-PMSM is designed analytically through Finite Element Method (FEM) which can be started by connecting to a line such as an asynchronous motor in a transient-state and can operate with high efficiency and power factor after synchronization in steady-state without the need for an expensive motor drive.

Project team

Project students

• Cheng Zhang
• Hengyi Zhang
• Seetharaman Kavasseri Sankaranarayanan

Supervisors

• Prof. Wen Soong
• Dr Solmaz Kahourzade

Objectives

The line-start axial-flux permanent-magnet synchronous motor has a higher torque density than that of the radial-flux motor, resulting in lower cost. Besides, it is more efficient than standard induction motors at rated output power from 0.55 kW to 7.5 kW. It also adds a squirrel cage to the rotor of an axial-flux permanent-magnet motor for providing self-starting capability, which could improve start-up performance and high efficiency in the steady-state operation. Furthermore, this type of model can reduce the cost more compared with the permanent-magnet synchronous motors.

Background

Induction motors

Induction motors are widely used in a variety of industrial, commercial and domestic applications because they have many advantages, which are self-starting, economical and reliable [1]. The induction motor works on Faraday’s law of electromagnetic induction. When a three-phase power source is supplied to the stator, a rotating magnetic field is generated that operates at a synchronous speed [1]. This field is cut by the rotor conductors and back-EMF is induced in the rotor winding. As the rotor circuit is closed, current flows through the winding and mechanical force act on the conductors. Besides, In the stator and the rotor of the induction motor, the current will cause a large mechanical force, which will break the insulation or the conductor. This mechanical force produces a torque which tends to move the rotor in the same direction as the rotating field [2].

Permanent-magnet synchronous motors

Permanent magnet synchronous motors are commonly used in industrial applications that require rapid torque response and high-performance operation, such as automation for traction, robotics and aerospace [3]. The permanent magnet synchronous motor is an AC synchronous motor whose field excitation is provided by permanent magnets and has a sinusoidal back-EMF waveform. Besides, it has a permanent magnet rotor and windings on the stator. However, the structure of the stator with windings configured to produce a sinusoidal magnetic flux density in the air gap of the motor resembles that of an induction motor. Compared with the conventional synchronous motor, for the stator, it is symmetrical three-phase windings, but for the rotor, it uses a unique shape of rare-earth permanent magnet instead of the excitation winding [4]. This means that the permanent magnet synchronous motor has the characteristics of simple structure, small size, lightweight and high overload capacity, which means the motor is compact, efficient, and has high torque density and high dynamic performance [5]. Furthermore, permanent magnet synchronous motors must operate with a drive because they require a drive to operate. The drive uses the current-switching technique to control the motor torque and the mathematically intensive conversion between one coordinate system and another coordinate system to control the torque and flux current simultaneously [6].

Line-start permanent-magnet synchronous motors

The name of line-start permanent magnet synchronous motors comes from its ability to start directly when connected to the source. This type of motor has permanent magnets on its rotor, and a squirrel cage starting winding. To be specific, it means that line-start permanent-magnet synchronous motors have permanent-magnet with induction rings for self-starting to avoid inverter fed mechanism. Besides, this type of motor combines the high efficiency and ease of use of permanent magnet synchronous motors with design simplicity and high starting capacities of induction motors [7]. However, due to the braking torque caused by permanent magnets, the starting torque of the line-start permanent magnet motors can be very poor [8] [9].

Comparison between radial-flux and axial-flux permanent-magnet motors

As seen from the left figure, the arrow shows the direction of the magnetic flux in each motor. For radial-flux permanent-magnet motors, flux is produced radially along the sideways of the rotor. As seen from the right figure, for axial-flux motors, flux is produced axially along the axis of the rotor. Axial-flux permanent-magnet motors offer priority features to replace conventional field winding motor. As it employs permanent magnets losses in field excitation are completed avoided resulting in high efficiency and output power. Another advantage it offers is the requirement of less core material in the construction of such machines which in turn provides high torque to weight ratio reducing rotor losses significantly.

Methodolody

• MATLAB / Simulink
• ANSYS Maxwell

Design

Design of induction motors

Equivalent circuit analysis method

A large number of mathematical models for three-phase induction motors have been intensively studied in the past decades. In this project, MATLAB / Simulink has been chosen for the implementation of the induction motor. MATLAB / Simulink offers a convenient graphical user interface to implement continuous and discrete systems in the input-state-output form. In addition, MATLAB / Simulink can be expediently used to set up a suitable solution method and analyse results.

ANSYS simulation model of induction motors

The induction motor as shown above. This model is presented in 2D appearance and has been drawn in master-slave representation in a 90-degree surface. For this 90-degree surface, the structure of this model consists of five parts, including an outer stator, an inner rotor, a shaft inside of the inner rotor, stator windings and rotor windings. For the stator part, it has 9 slots in a quarter of the motor, 36 slots of full-cycle in total. The windings are distributed from a three-phase excitation on the motor operation. Furthermore, the excitation of this model is very significant and defines the excitation of three-phase. Specifically, phase A are distributed among the windings in the stator of the motor and is located in the three windings on the right. Phase B are distributed among the windings in the stator of the motor and is located in the three windings on the left, while Phase C are located in the middle of windings.

Design of line-start radial-flux permanent-magnet motors

Simulink model of line-start radial-flux permanent-magnet motors

using above equation to transfer the three phase voltage to d-q axis voltage. In this model, the θ will be the value of integration of magnetization speed ω_e, which will be the value of ω_e times pole pairs P.

ANSYS simulation model of line-start radial-flux permanent-magnet motors

The line-start radial-flux permanent-magnet motor as shown in Figure above. For this 90-degree surface, the structure of this model is very similar to induction motors. The differences between induction motors and line-stat radial-flux permanent-magnet motors, which the radial-flux model has a permanent magnet inside of the inner rotor and the size of windings have been reduced for putting the suitable size of permanent-magnets.

Design of line-start axial-flux permanent-magnet motors

ANSYS simulation model of line-start axial-flux permanent-magnet motors

The design method of line-start axial-flux permanent-magnet motors is to add a squirrel cage to the rotor for providing the line-start capability like an induction motor during the start-up and high efficiency in the steady-state operation. This design avoids the need to use variable speed drives to drive permanent magnet synchronous motors. This thesis is mainly to design line-start axial-flux permanent-magnet motors with output power of 5.5kW and rated voltage of 415V. Combine the component of stator, windings, induction rings, permanent magnets and rotor, then set the boundary of the model, the whole model is built. The following figure shows a half cross-sectional view of line-start axial-flux permanent-magnet motors.