Estimation of Rotor Type Using Ferrite Magnet ... - IEEE Xplore

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This paper deals with the post-assembly magnetization process of motors ... concentrated flux spoke-type synchronous motor is analyzed with regard to the ...
IEEE TRANSACTIONS ON MAGNETICS, VOL. 52, NO. 3, MARCH 2016

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Estimation of Rotor Type Using Ferrite Magnet Considering the Magnetization Process Kyu-Seob Kim, Min-Ro Park, Hae-Joong Kim, Seung-Hee Chai, and Jung-Pyo Hong Department of Automotive Engineering, Hanyang University, Seoul 133-791, Korea This paper deals with the post-assembly magnetization process of motors using ferrite permanent magnets (PMs). In order to meet the needs of mass production, most motors are magnetized post assembly. However, increasing complex shapes are required to maximize the flux of PMs. As a result, certain locations in the magnet are not fully magnetized by the magnetizing fixture due to insufficient magnetomotive force. Therefore, an analysis concerning the post-assembly magnetization is needed. In this paper, the concentrated flux spoke-type synchronous motor is analyzed with regard to the magnetization process. Owing to more flux, this motor is designed as a V-shape PM. Therefore, it is not fully magnetized according to magnet shape. By identifying the effect of the magnet shape, the magnetization level is compared by the amount of flux linkage between post assembly and at full magnetization. Finally, the back electromotive force is estimated by the post-assembly magnetization method according to the magnet shape. Index Terms— Ferrite magnet, magnetization level, magnetization processes, magnetizing fixture, post-assembly magnetization.

I. I NTRODUCTION ECENTLY, environmental issues have worsened, and the depletion of fossil fuels has accelerated. To alleviate these problems, motors with much higher efficiency are required [1]–[3]. One particular type of motor, the interior permanent magnet synchronous motor (IPMSM), is widely used to meet the demand of high power density, with rare-earth PMs frequently being essentially incorporated [4]. However, the rare-earth PM is both costly and sparse. The rarity of the PM subsequently generates imbalances in supply and increases in price. Therefore, new types of motor have been proposed to eliminate the need for rareearth materials, without sacrificing performance. Concentrated flux spoke-type motors, using ferrite PMs, are particularly attractive. In this type motor, in order to increase the flux of the ferrite magnet, the magnet’s shape and arrangement become increasingly complex. To gain higher magnetic flux, there has been a great deal of research on flux concentrated motors and rotor structures of various shapes [5]. However, in the mass production system, the post-assembly magnetization method is used. As a result, certain locations in the magnet are partially magnetized. To obtain the motor performance as designed, magnet quality must be guaranteed. In this case, the motor performance may be lower when compared with the cases in which the PM is fully magnetized or when inserting a magnetized PM. Therefore, the magnet shape and arrangement are deemed important [6]–[9]. In this paper, the post-assembly magnetization method is analyzed with respect to the concentrated flux spoke-type motor, including the magnet shape. In addition, ferrite PM is used to analyze the magnetization. Unlike in NdFeB PM, the recoil permeability and residual induction (Br) according to the discharging are not linear in the case of ferrite PM. The process of post-assembly magnetization is introduced.

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Manuscript received July 6, 2015; revised August 28, 2015; accepted October 7, 2015. Date of publication October 13, 2015; date of current version February 17, 2016. Corresponding author: J.-P. Hong (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2015.2490281

Fig. 1. Magnetization curve of ferrite PMs during magnetization field intensity.

Finally, according to the magnet shape, the back electromotive force (EMF) and magnetization level are calculated according to the finite-element method (FEM). II. PM P ERFORMANCE ACCORDING TO M AGNETIC F IELD I NTENSITY The PMs can be either isotropic or anisotropic, depending on the production method. In general, the residual induction of anisotropic PMs is higher than that of isotropic PMs. In the anisotropic PM, the hard axis along a non-preferred direction is assumed to be linear. The preferred direction shows the non-linear performance in the easy axis. The PM materials along the easy axis have a non-linear magnetization curve, as shown in Fig. 1. In the magnetizing force H = 0 and flux density B = 0, this is the initial point of the non-magnetized magnet. During the magnetization process, when H increases, B will increase along the initial magnetization curve. When H decreases, B will decrease along the demagnetization curve. If H is not sufficient, B cannot reach the saturation point. Therefore, when H = 0, Br is lower than fully saturated. In addition, in this case, coercive force drops and recoil

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Fig. 2.

IEEE TRANSACTIONS ON MAGNETICS, VOL. 52, NO. 3, MARCH 2016

Fig. 3.

Magnetizing fixture circuit with capacitor bank.

Fig. 4.

Block diagram of magnetizing fixture circuit.

Post-assembly magnetization process.

permeability grows. All of these various effects ultimately influence the motor’s performance. Therefore, during the postassembly magnetization procedure, proper magnet shape and arrangement need for sufficient magnetic field intensity. III. P OST-A SSEMBLY M AGNETIZATION P ROCESS In the mass production, productivity and quality are influenced by product state. Because the complex PM shape may be not fully magnetized, the motor productivity and quality decrease. In the post-assembly magnetization, the manufacturing process of IPMSM flows thus according to the following. 1) Insert non-magnetized PMs into the rotor. 2) Combine the rotor and magnetizing yoke. 3) Magnetize the PMs. 4) Separate the rotor and magnetizing yoke, assemble the rotor and stator at the same time. 5) Finish the assembly. In this paper, this process is introduced and is applied to the concentrated flux spoke-type synchronous motor. The magnet’s shape is an important factor that is related to the motor’s performance. In this paper, the effect that PM shape has on the magnetization level and back EMF is studied in the post-assembly magnetization. Fig. 2 shows the post-assembly magnetization process. The process is composed of three sections: 1) the calculation of discharging current; 2) the PM magnetization; and 3) the analysis of motor parameter. A. Calculation of Discharging Current To magnetize the PMs, a magnetizing fixture is needed. The magnetizing fixture is combined with a capacitor bank that supplies sufficient voltage [10], [11]. Fig. 3 shows the magnetizing fixture circuit with the capacitor bank. If the switch is turned ON, discharging current flows into the magnetizing fixture, which is then combined with the rotor. The diode prevents current from flowing counterclockwise. The circuit

is governed by the following equations: 1 di (t) d 2 i (t) + i (t) = 0 (1) +R dt 2 dt C i (0) = 0 (2) V0 di (0) = (3) dt L where L = L p + L m , L p is the parasite inductance, L m is the inductance of the magnetizing fixture with rotor, R = R p + Rm , R p is the parasite resistance, Rm is the resistance of the magnetizing fixture, C is the capacitance of the capacitor bank, i (t) is the current flowing into the magnetizing fixture, and V0 is the initial voltage on the capacitor. L is calculated by lookup table because of the variation of the discharging current in the transient analysis. Equations (2) and (3) are the initial conditions for solving the differential equation (1). MATLAB SIMULINK is used to solve the differential equation (1). Fig. 4 shows the block diagram of the differential equation of the magnetizing fixture circuit in the transient analysis. The input values are R, L, C, and V0 , while the output value is the discharging current i (t). In particular, the inductance, L, is given in the table which is composed of discharging current and inductance. Therefore, the discharging current is calculated by reflecting the load condition. Fig. 5 shows the inductance according to the discharging current and the output discharging current results of solving the block diagram using SIMULINK. Therefore, the discharging current flows into the magnetizing fixture coil. L

B. PM Magnetization For the analysis of the PM magnetization using the FEM, discharging current and material data (which are covered

KIM et al.: ESTIMATION OF ROTOR TYPE USING FERRITE MAGNET CONSIDERING THE MAGNETIZATION PROCESS

Fig. 7.

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Vector M of the magnetized PM.

Fig. 5. Input and output parameter. (a) Inductance according to discharging current. (b) Discharging current.

Fig. 8. FEM modeling. (a) Rectangular magnet shape. (b) Hexagon magnet shape.

Fig. 6. Determination of the Br according to discharging current in each PM element.

with the PM initial magnetization curve) are needed. The residual induction, recoil permeability, and coercive force of the PM element are calculated by the PM’s magnetization curve after the FEM is conducted. In Fig. 2, Step (B) shows the input and output data for PM magnetization. Fig. 6 shows the numerical calculation process for determining the Br according to the discharging current in each PM element. On the load condition according to the charging current, B k is determined and H k is also calculated by H k = B k /µk . This process is repeated until the error is