Design of spoke type motor and magnetizer for improving efficiency based rare-earth-free permanent-magnet motor

Performance efficiency and production costs in interior permanent magnet synchronous motors (IPMSM) can be improved by using rare earth permanent magnets. However, rare earth magnets contain materials such as neodymium, the cost of which has increased over the last few years.^1,2 To solve this cost problem, cheaper ferrite magnets can be used as a substitute. This substitution results not only in a lower production cost, but also in a motor that has an output equal to or greater than that of IPMSM. The ferrite magnet is disposed in a spoke-type motor in a radial direction. Because this motor has both reluctance torque and a concentrated magnetic flux, the torque density increases.^3,4 However, the cogging torque of the spoke-type motor is larger than that of IPMSM and thus generates large differences in magnetic reluctance on the air gap. This cogging torque causes noise and vibration, which has an adverse harmonic effect on motor components.⁵ In addition, a problem exists with the assembly of the permanent magnet, as it is magnetized by the large repulsive force within the suction force in the permanent magnet. To solve this problem, we can apply an optimum designed motor and magnetizer system for the spoke-type motor using ferrite magnets. This not only improves performance, but also reduces manufacturing costs. Our criteria are verified using a finite element method (FEM) together with a response surface method (RSM). These methods are used to optimize the stator and rotor shape as well as the magnetization system. These methods allow us to produce an efficient system for mass-production of the motor.

To reduce the cogging torque, a spoke-type motor design method is proposed. The shape and design parameters of the model are shown in fig. 1 and specifications in Table I. Where, B_s0 is slot opening width, B_s1 is slot width, H_s0 and H_s1 are teeth tip and Y_s is yoke width are design variables of stator. Also, M_w is magnet length, M_t is magnet width and G_a is rotor arc. These parameters are designed considering the manufacturability and saturation of the iron core. Based on this initial model, the stator and rotor are designed to reduce cogging torque. In addition, a notch and rotor arc are applied to reduce cogging torque and to obtain a sinusoidal air-gap flux density.

We further reduce the cogging torque by applying the notch and rotor arc to the stator and rotor respectively. Applying notch generates an opposite direction of cogging torque, thereby canceling the existing cogging torque. Whereas applying rotor arc changes the air-gap magnetic-flux distribution to the spherical-routine air-gap flux distribution. This method also reduces the cogging torque. Fig. 2 show optimal shape and a cogging torque waveform of the optimized model in spoke-type motor. Although permanent magnet volumes of the proposed and initial models are the same (205mm²), cogging torque declined from 0.77 to 0.12 [N·m].

Compared to other methods, magnetization that uses capacitor discharge consumes less power and can be obtained from a simple electrical circuit. Electrical circuit analysis of the magnetizer system using condenser as shown in fig. 3(a) is relatively simple. The series circuit consist of a resistance, inductance and condenser. The analysis of the impulse current using transient theory is possible. If circuit constant is the R, L and C a change in current i about the time t is established by the following equation.

The result of equation (1) varies with circuit conditions. Having a minimal winding resistance of the magnetizer designed to flow high current is advantageous. Therefore, the condition of $R<2L/C$ by current waveform is one of vibration damping.

In the design of a magnetizer, the charging voltage and capacitance are the main factors for generating magnetization flux. These are thus the most essential aspects of the design process. Fig. 3(b) shows the magnetic properties of permanent magnets in the magnetization. The initial state "a" of the permanent magnet is H = 0, B = 0. A straight line in fig. 3(b) is a load line that indicates the permanence coefficient of the external magnetic circuit. To magnetize the permanent magnet completely, we must shed i_max or additional current must be applied to a magnetic field of more than H_max. The energy required to magnetize the permanent magnets of the volume V_m is 1/2B_satH_satV_m, which means that magnetizer with a capacity of 1/2CV² is required.

The capacity of the magnetizer (charging voltage and capacitance) generates a magnetomotive force to magnetize the permanent magnets sufficiently. The charging voltage and capacitance are set to the same value, and we determined the magnetomotive force necessary for magnetization. As shown in fig. 4, when the capacitance and charging voltage increase, magnetizing of the magnetomotive force increases to nearly a constant value. To become the intrinsic coercive force of the permanent magnet and the magnetomotive force are each 420kA/m, 160kA-turn, the capacity of the magnetizer (capacitance and charging voltage) should be higher than 1900uF-1900V. Therefore, as a result of using finite element analysis, 1900uF and 1900V were selected as the capacity of the magnetizer (capacitance and charging voltage, respectively).

The magnet must be magnetized in the direction H_x. A ferrite magnet (12E) requires an intrinsic coercive force of 360∼420kA/m for magnetization. Therefore, for sufficient magnetization, a coercive force of at least 420kA/m in the direction of H_x is required. The flux line of the ferrite magnet flows in the H_x direction, as shown in fig. 5(a). Fig. 5(b) shows the strength of the magnetic field required to magnetize a ferrite magnet according to each magnetization direction. The red portion of fig. 5(b) denotes a magnetic field with a strength of 420kA/m or higher. This means that in this direction, the ferrite magnet (12E) will be fully magnetized. Based on the design variable described previously, we can optimally design the magnetizer based on the shape and specifications shown in fig. 5(c).

This study proposed a design method that uses rare earth magnets in a system to replace the IPMSM. To verify our optimal design, torque characteristics were analyzed using FEM and RSM. Excellent results were achieved regarding the reduction of cogging torque and torque ripple. In addition, our design of the magnetizer enables a cost-effective mass-production system for the motor.

This work was supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (NRF-2015H1C1A1033580).