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

The aims of this project are 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.

Methodology

• 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 the 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 project 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.

In order to observe the model clearly, the exploded view of line-start axial-flux permanent-magnet motors can be shown below:

Results

Induction motors

Back-EMF

The three-phase induction motor generates a magnetic field inside the air gap. Based on Faraday’s law, a changing magnetic field will cause an induced EMF in the rotor that will attempt to prevent the magnetic field from being generated. Besides, the current in the rotor windings in turn creates a magnetic field in the rotor that reacts with the stator magnetic field. Based on the Lenz’s law, the direction of the resulting magnetic field will be opposite to the change in current through the rotor windings. As seen from the figure below, it can know that the root-mean-squared voltage at steady-state of Phase A is 303.84 V, that of Phase B is 283.15 V and that of Phase C is 302.43 V.

Magnetic fLux density and magnetic flux vector

The following figure shows the magnetic flux density and the flux linkage of the induction motor model.

The following figure shows the flux vector of the induction motor during no-load conditions.

Line-start radial-flux permanent-magnet motors

Back-EMF

Back-EMF is easily obtained from the windings. When winding A has been determined, winding B and C is obtained from winding A by 120° and 240° offset, respectively. The back-EMF in the software ANSYS Maxwell 2D can be found in the Winding section. In addition, the induced voltage depends on the magnetic flux and is given by the equation E_rms=4.44 Nfϕ_max, where N is the number of turns, f is the frequency and ϕ_max is peak magnitude of the flux density. As seen from the figure below, it can know that the root-mean-squared voltage at steady-state of Phase A is 212.012 V, that of Phase B is 213.1167 V and that of Phase C is 208.2742 V. Besides, it can observe that the steady-state period from 100 ms to 200 ms.

Efficiency versus torque load

Compare to the efficiency curve of induction motors. The efficiency of line-start radial-flux permanent-magnet motor is higher than that of the induction motor. Therefore, it means the line-start radial-flux permanent-magnet motors have an excellent performance of efficiency. The comparison of the two types of motor as shown below:

Power factor versus torque load

As seen from the figure below, it can observe that the red line represents line-start radial-flux permanent-magnet motors, while the red line represents induction motors. It also can know that comparison between line-start radial-flux permanent-magnet motors and induction motors for power factor. Besides, the power factor of line-start radial-flux permanent-magnet motors is higher than that of induction motors in the same load torque conditions. In full load condition, the power factor of line-start radial-flux permanent-magnet motors is 0.9756, while the power factor of induction motors is 0.8597.

Current versus torque load

As seen from the figure below, it can observe the relationship between current and load torque, which is a proportional relationship. As the load torque increases, the current also gradually increases.

Magnetic flux density and vector plots

The following figures show the magnetic flux density plot and magnetic flux density plot in terms of vector representation.

Synchronization test

The synchronization is an important characteristic of line-start radial-flux permanent-magnet motors. It means that the motor could reach the synchronous speed ω_s and keep constant. The ideal curve of rated speed and electromagnetic torque vs. time is shown as figure below. At this operation condition, the inertia J is 0.024kg m2 and the load torque T_l is zero.

When the inertia J increasing, the curves will have a difference. The increasing inertia curve of rated speed and electromagnetic torque vs. time. At this operation condition, the inertia J is 0.048kg m2 and the load torque T_l is zero. When the inertia J keep increasing, the line-start PM will be failed to synchronize. The increasing inertia curve of rated speed and electromagnetic torque vs. time is shown as figure below. At this operation condition, the inertia J is 0.12 kg m2 and the load torque T_l is zero.

Repeat the process of increasing the inertia, the synchronization test result for line-start radial-flux permanent-magnet motors could be summarised and the boundary of synchronization could be obtained. The figure can be shown below:

Line-start axial-flux permanent-magnet motors

Back-EMF and Flux linkage

It can observe the curve of induced voltage versus time. Line-start axial-flux permanent-magnet under open-circuit test, the value of back-EMF is usually 70%~90% of the rated voltage. In this design, the rated voltage is 415 V, the value of three-phase induced voltage is close to 300 V. Flux linkage performance in the steady-state also needs to be considered. Flux linkage loss is small. The flux linkage generated by the winding is proportional to the current in the winding. Therefore, the ratio of the flux linkage to the current that generates the flux linkage is a constant and is defined as the inductance of the winding.

Torque versus power angles

Setting power angle to change from 0 degrees to 160 degrees with a step size of 5 degrees. A total of 33 load torque versus time curves can be obtained. In the steady-state of the motor, the average torque load at different power angles is used to make the graph. It can see that when the power angle changes from 0 degrees to about 70 degrees, the load torque gradually increases, while the power angle changes from 70 degrees to 160 degrees, the load torque gradually decreases. Besides, when the power angle is 70 degrees, the value of torque load is maximum, which is 86.5607 Nm. When the power angle is 160 degrees, the value of torque load is minimum, which is about -12 Nm.

Copper loss versus power angles

The copper loss in the stator winding is strongly dependent on the frequency of the motor current. For high-speed operation, the resistance of the coil increases with speed due to proximity and skin effects. The coils of the stator must be properly designed to minimize the contribution of alternating current to copper losses. Using the same method of setting power angle to change from 0 degrees to 160 degrees with a step size of 5 degrees. It can see that the average copper loss increases with the increase of power angle. Besides, when the power angle changes from 0 degrees to 160 degrees, the average copper loss gradually increases. When the power angle is 0 degrees, the average copper loss is the lowest at 0.0361 kW. When the power angle is 160 degrees, the average copper loss is the highest at 18.5614 kW.

Current versus power angles

The phase current of line-start axial-flux permanent-magnet motors would be considered. Since the waveform of the three-phase current is a sine wave with equal amplitude, Phase A can be further analysed. Using the same method of setting power angle to change from 0 degrees to 160 degrees with a step size of 5 degrees. It can see that the RMS value of current increases as the power angle increases. RMS value of current is proportional to power angles. Besides, when the power angle is 0 degree, the RMS value of current for phase A is 2.3803 A. When the power angle is 160 degrees, the RMS value of current for phase A is maximum, which is 48.4546 A.

Conclusion

The results show that the power factor and efficiency of line-start radial-flux permanent-magnet synchronous motors are higher than that of induction motors. Using ANSYS Maxwell 3D to design the line-start axial-flux permanent-magnet motors, mainly considering the relevant characteristics of this type of motor running under steady-state condition. Under no-load condition, the back-EMF and flux linkage are mainly considered. Under the full-load condition, the simulation and analysis of the external circuit were used, mainly discussing the relationship between torque, iron loss, hysteresis loss, copper loss and current and time. Finally, consider the changes of the above parameters at different power angles. Through ANSYS simulation, the relationship between average torque and different power angles is obtained, which shows that with the increase of power angle, the average torque gradually increases and then gradually decreases. The average iron loss decreases as the power angle increases. The average eddy current loss, average copper loss and RMS value of current for phase A increase with the increase of power angles.

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