Influence of Soft Magnetic Material Type in Fixture Components on the Magnetization of Bonded Neo Magnet and Motor Performance

Abstract

The advantages like higher magnetic properties than ferrite, near net shape magnet production, and no use of heavy rare earth elements makes the bonded neo magnet very attractive in motors used for automotive accessory, home appliance and office automation. The isotropic nature of bonded neo magnets offers a feasibility to obtain wide range of magnetization profiles. The magnetization of the magnet influences the air-gap flux distribution and hence the motor performance 1. Magnetizing fixture comprising of copper coils embedded in soft magnetic material is used to magnetize the magnet. When radial magnetization profile is desired, a back iron made up of soft magnetic material is also used to reduce the amount of magnetizing energy needed to saturate the magnet. Laminated steel is the preferred material for the magnetizing fixture as well as back iron. We have observed that at times solid steel is used in place of laminated steel. This paper presents the effects of using solid steel in place of laminated steel on the magnetization and motor performance. A magnetizing fixture is designed using the 2-D finite element analysis (FEA). The designed fixture is fabricated with fixture core and back iron made of laminated steel (LCLB). The magnetization performance of the designed fixture is evaluated using 2-D FEA and validated by performing magnetization of the magnets. To evaluate the influence of solid steel as a soft magnetic material two more combinations; (i) fixture core is of laminated steel but the back iron is of solid steel (LCSB) and (iii) both fixture core and back iron made of solid steel (SCSB) are simulated and evaluated. To ensure the full saturation in a bonded neo magnet a magnetizing field of 3T is desired through the thickness of the magnet 2. From the simulation it is observed that to achieve 3T magnetizing field for LCLB combination the magnetizing energy required is 5.44 kJ. The required energy increases to 34.02 kJ and 68.28 kJ for LCSB and SCSB combinations respectively. The increase in energy is due to the generation of eddy currents in solid steel components which counteract the applied field, reducing the available field for magnetization. The energy needed to achieve full magnet saturation in LCSB and SCSB combination exceeds the capability of most of the commercially available magnetizers also the increase in magnetization energy requirement will lead to higher thermal stress and reduced fixture reliability. Based on the capability of the available magnetizer we applied up to 6 kJ for LCSB and SCSB combinations. We have also measured the corresponding applied field at the back of the magnet as 1.6 T and 1.4 T respectively. The less than desired applied field led to the partial saturation of the magnet. Figure 1 shows the measured mid airgap closed circuit flux density waveforms for magnets magnetized using various combinations. From this figure it is observed that the use of solid steel component makes the airgap flux density waveform less radial. The flux integral is reduced by 11% and 6.6% for LCSB and SCSB combinations compared to LCLB combination. The magnets magnetized using various combinations are assembled in a motor for performance measurement, the results of which are summarized in Table I. The presence of solid steel component during magnetization leads to partial magnetization of the magnet, resulting in higher no-load speed and lower stall torque. It is also observed that the motor with the magnets magnetized using LCSB and SCSB combinations has 64% and 60% lower cogging torque compared to LCLB combination. This is due to partial magnetization and hence lower flux offered by the magnets from these combinations.