Influence of Stress-Relief Annealing on Magnetic Properties of Motor Cores Using Induced Current Heating Method

Transcription

1 APSAEM14 Journal of the Japan Society of Applied Electromagnetics and Mechanics Vol.23, No.3 (2015) Regular Paper Influence of Stress-Relief Annealing on Magnetic Properties of Motor Cores Using Induced Current Heating Method Yuichiro KAI *1, Shogo YOSHIDA *1 and Masato ENOKIZONO *1, 2 This paper presents the effect of stress-relief annealing on magnetic properties using an induced current heating(ich) method in order to reduce the residual stress in the motor cores. The magnetic properties of a motor core annealed using the ICH method were first compared with those of a core annealed using a conventional electric furnace. When the hold time in each method was the same, the magnetic properties using the ICH method were improved over those using the EF method. The effects of varying the annealing temperature and time for the ICH process were then investigated. It was found that the values of the maximum permeability increased and the hys-teresis loss decreased with increasing annealing temperature and time. The total time of the stress-relief annealing could be achieved in a short period of about one hour. Keywords: stress-relief annealing, induced current heating method, motor core, magnetic power loss. (Received: 24 July 2014) 1. Introduction The magnetic properties of motor cores deteriorated due to the presence of residual stress that is introduced during the manufacturing process [1,2]. Therefore, it is necessary to reduce this stress by stress-relief annealing when producing electrical machines which require low loss and high efficiency. The conventional method involves the use of electric furnaces to anneal electrical steel sheets and actual motor cores in order to improve their magnetic properties and reduce the residual stress [3-7]. However, this process takes a long time and a significant cost is incurred in running the furnaces. In order to solve this issue, we have developed a new stress-relief annealing technique based on the induced current in a transformer. We call this the induced current heating (ICH) method. This paper presents the effect of stress-relief annealing using the ICH method on the magnetic properties of annealed with using ICH were compared with those of motor cores. The magnetic properties of a motor core annealed with using ICH were compared with those of a core annealed using a conventional electric furnace (EF). The effects of varying the annealing temperature and time in the ICH process were also investigated. 2. Stress-Relief Annealing Device using Induced Current Method Fig. 1 shows the structure of the heating coil and yoke in the stress-relief annealing device using the ICH method. When the magnetic flux lines generated by the Correspondence: Y. KAI, Faculty of Engineering, Oita University, 700 Dannoharu, Oita , Japan ykai@oita-u.ac.jp *1 Oita University *2 Vector Magnetic Characteristic Technical Laboratory Screw bolt Yoke Coil Heat insulation material Motor core Sample stage Screw bolt Yoke Coil (a) Over view Water cooling tube (b) Side view Fig. 1. Structure of the stress-relief annealing device by using the ICH technique. 469

2 heating coil go along the yoke in the central leg, a current is induced inside the motor core. The upper yoke is fixed by the screw bolt after inserting the ring-shaped motor core through the yoke in the central leg. The motor core is covered with an insulation material to prevent heat from radiating from the motor core. In addition, thermocouples are attached to the motor core, and the heating temperature is maintained with a PID controller. Fig. 2 shows a schematic illustration of the ring core. In this measurement, the non-oriented electrical steel sheet (35A350 in Japanese industry standards) is used for the motor cores. Ten sheets are laminated after the punching process. The thermocouples inside the motor core is used to measure the temperature of the motor core and the thermocouple outside the motor core was used to control the heating temperature. 3. System for Measuring Magnetic Properties of Motor Core Fig. 3 shows the system for investigating the effect of stress-relief annealing on the magnetic properties of the motor cores. The excitation coil and -coil are wound around the ring motor core. The motor core is magnetized using the excitation coil. Then, the magnetic flux density is calculated by measuring the induced voltage V in the -coil. Also, the magnetic field strength H is evaluated by measuring the excitation current I ex. The voltage applied to the excitation coil is the amplified output of a D/A converter using a power amplifier. Also, the induced voltage in the -coil and the terminal voltage of the shunt resistance are recorded on a personal computer using an A/D converter. The magnetic flux density waveform is controlled to be sinusoidal. The magnetic flux density is calculated as follows, 1 Vdt N S (1) where N is the winding number of the -coil, and S is the cross-sectional area of the sample, respectively. The magnetic field strength H is calculated as follows, H N V H ex (2) RLm where N H is the winding number of the exciting coil, L m is the magnetic path length, and R H is the shunt resistance. 4. Effect of Stress-relief Annealing using Induced Current Method Fig. 4 shows the temperature profiles for the EF and ICH method. The annealing temperature for the ICH method was set to be 750 o C and held for 1 hour, and the samples were cooled in atmosphere. The heating frequency was 500 Hz. On the other hand, the annealing temperature of the electro furnace was set to be 750 o C and kept for 1 hour, and then the samples were cooled in an argon atmosphere. The total time at about 8 hour in the electro furnace became about 3 the annealing temperature for the EF was set to 750 o C and held for 1 hour, and the samples were then cooled in an argonatmosphere. For the EF and ICH methods, the total D/A converter Power amplifier V ex Exciting coil PC A/D converter Motor core -coil 60 Thermo couple (Measurement) Thermo couple (Control) Fig. 2. Ring specimen. 40 I ex V Fig. 3. System for measuring the magnetic property of ring core. Fig. 4. Temperature profiles for the EF and ICH methods. 470

3 heating times were about 8 hours and about 2 hours, respectively. Fig. 5 shows the -H hysteresis loops for the ring cores annealed using the EF and ICH methods. For both annealing methods, the area enclosed by the loop was smaller than that for the non-annealed material because of the decrease in residual stress. Fig. 6 shows the magnetic properties of the motor cores. Here, max is calculated using the following equation, max max (3) 0Hmax where, max is the maximum magnetic flux density, H max is the maximum magnetic field strength and 0 is the permeability of a vacuum. The max values for the annealed motor core increased by using the EF and ICH methods, as shown Fig. 6(a). In particular, the values of max for the motor cores annealed using the ICH method increased in comparison with those for the core annealed in the E.F. The magnetic power loss W m is calculated by the following equation, 1 T d W H dt (4) m T 0 dt where T is one period of excitation, is the material density. Also, the hysteresis loss W h and the eddy current loss W e is separated by measuring the magnetic power loss at 25, 50, 75, and 100 Hz. The values of W m for the annealed motor core decrease by using the EF and ICH methods, as shown in Fig. 6(a). In addition, the eddy current loss hardly changed by annealing. On the other hand, the value of W h decreased by annealing. Non ICH I.C.H E.F. EF Fig. 5. Hysteresis loops for the motor core using the EF and the ICH method. Thus, it was clarified that the magnetic properties of the motor cores were improved by using the ICH method. When the hold time in each method was the same, the magnetic properties using the ICH method were improved over those using the EF method. 5. Stress-Relief Annealing Conditions for ICH Method 5.1 Annealing temperature The annealing conditions (the annealing temperature and hold time) for the ICH method were next examined. Fig. 7 shows the temperature profiles for different annealing temperatures. The annealing temperature was set to 400, 500, 600, 700, 750 and 800 o C, held for 1 hour and the samples were then cooled in atmosphere. Fig. 8 shows the magnetic properties of the ring core for the different annealing temperatures. As the annealing temperature increased, max increased, whereas W h decreased. From these results, it was clarified that the magnetic properties of the motor cores were improved by increasing the temperature of the ICH method. In addition, the effect of the stress-relief annealing became small above 750 o C. (a) Permeability max Fig. 6. Magnetic properties of the motor cores using the EF and the ICH. 471

4 Fig. 7. Temperature profiles for different ICH annealing temperatures. Fig. 9. Temperature characteristic by using the electric furnace and induced current heating method. (a) Permeability max Fig. 8. Magnetic properties for different ICH annealing temperatures 5.2 Hold time Fig. 9 shows temperature profiles for different ICH annealing times. The annealing temperature was set to be 750 o C and held for 0, 0.5 and 1 hour. Fig. 10 shows the magnetic properties of the ring cores for the different annealing times. Here, max increased with annealing time as shown in Fig. 10(a). Also, W h decreased with increasing the annealing time, as shown in Fig. 10(b). (a) Permeability max Fig. 10. Magnetic properties of the motor cores depending on the annealing time. From these results, it was possible to recover the magnetic properties of the motor cores by increasing the annealing time. 5.3 Short-time stress-relief annealing We attempted to determine the optimum annealing time and temperature based on these results. Fig. 11 shows the annealing conditions. The annealing temperature was set to 750 and 800 o C, held for 0 hours, and the samples were then cooled in atmosphere. 472

5 Fig. 12 shows the magnetic properties of the motor cores. Here, max decreased with increasing annealing temperature, as shown in Fig. 12(a). In addition, W h also decreased with increasing annealing temperature, as shown in Fig. 12(b). It is possible to improve the magnetic properties of the motor core by increasing the annealing temperature during stress-relief annealing for about 1 hour. Fig. 11. Temperature profiles for two different annealing temperatures when the hold time is 0 hours. (a) Permeability max Fig. 12. Magnetic properties of the motor cores for different annealing temperatures when the hold time is 0 hours. From these results, a difference in the magnetic properties of the motor core was obtained by changing the annealing temperature and time in the ICH method. In particular, it was possible to improve the magnetic property by annealing the motor core for a short period of about 1 hours. 6. Conclusion This paper presents the effect of stress-relief annealing on the magnetic properties of motor cores using the ICH method. The magnetic properties of the annealed motor core using the ICH method improved in comparison with those of the core annealed using the EF. It can be concluded that the proposed stress-relief annealing technique using the ICH method is useful for recovering the magnetic properties of the core material. In addition, the effect of the annealing time and temperature in the ICH method was examined. The results indicated that the maximum permeability increased and the hysteresis loss decreased with increasing annealing temperature and annealing time. Also, stress relief could be achieved in a short period of about 1 hour. Therefore, the ICH method is very useful for effectively annealing motor cores. References [1] Yousuke Kurosaki, Hisashi Mogi, Hiroyasu Fujii, Takeshi Kubota, Morio Shiozaki, Importance of Punching and Workability in Non-oriented Electrical Steel Sheets, Journal of Magnetism and Magnetic Materials, Vol. 320, Issue 20, pp , [2] A. Schoppa, J. Schneider, C.-D. Wuppermann, Influence of the Manufacturing Process on the Magnetic Properties of Non-oriented Electrical Steel, Journal of Magnetism and Magnetic Materials, Volumes , pp , [3] Zodenko Godec, Effect of Various Stress-Relief Annealing Treatments on Permeability and Aging of Grain- Oriented Electrical Steel Strips, IEEE Trans. on Magnetics, Vol. 14, No.1, pp. 4 8, [4] Sebastiao C. Palinelli, Marco A. da Cunha, Effect of Stress Relief Annealing Temperature and Atmosphere on the Magnetic Properties of Silicon Steel, Journal of Magnetism and Magnetic Materials, 304, pp.e599-e601, [5] M. Takezawa, K. Kitajima, Y. Morimoto, J. Yamasaki, and C. Kaido, Effect of Strain by Mechanical Punching on Nonoriented Si-Fe Electrical Sheets for a Nine-Slot Motor Core, IEEE Trans. on Magnetics, Vol. 42, No.10, pp , [6] H. Yashiki, T. Kaneko, Effect of Hot-band Annealing on Anisotropy of Magnetic Properties in Low-Si Semiprocessed Electrical Steels, Journal of Magnetism and Magnetic Materials, Vol.112, pp , [7] A. eglietti, A. Cavagnino, L. Ferraries, M. Lazzari, The Annealing Influence onto the Magnetic and Energetic Properties in Soft Magnetic Material after Punching Process, Electric Machines and Drives Conference, 2003, Vol. 1, pp ,