Effects of Annealing on Magnetic Properties of Electrical ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014

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Effects of Annealing on Magnetic Properties of Electrical Steel and Performances of SRM After Punching Chao-Chien Chiang1 , Andrew M. Knight2 , Min-Fu Hsieh1,3 , Mu-Gong Tsai4 , Bernard Haochih Liu4 , In-Gann Chen4, Zwe-Lee Gaing5 , and Mi-Ching Tsai1,6 1 Electric

Motor Technology Research Center, National Cheng Kung University, Tainan 701, Taiwan of Electrical and Computer Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada 3 Department of Systems and Naval Mechatronic Engineering, National Cheng Kung University, Tainan 701, Taiwan 4 Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan 5 Department of Electrical Engineering, Kao Yuan University, Kaohsiung 821, Taiwan 6 Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan 2 Department

This paper compares the microstructure and magnetic properties of punched electrical steel before and after annealing. The effects of this manufacturing process on performance of a switched reluctance motor (SRM) are also investigated. The punching of laminations distorts the grains near the cut edges, and therefore an anneal process is used to recover the damage caused by punching. In this paper, samples are annealed at 750 °C for an hour under a N2 atmosphere. The damaged regions and corresponding recovery after annealing are observed under microscope as the measured core losses are improved up to 37%. The SRM performance is measured and it is found that the torque density can be enhanced when using the annealed electrical steel. Index Terms— Annealing, electric machine, electrical steel, punching, switched reluctance motor (SRM).

I. I NTRODUCTION

E

LECTRICAL steels are widely used in the production of electrical machines and devices for power applications. In recent years, the issue of reducing electrical consumption has taken a central role in efforts to reduce worldwide energy consumption [1], [2]. As a result, increased concern with energy conservation has made the control of electric power losses in ac applications more important. The magnetic hysteresis loop is the source of most information for electrical steels used in ac applications. The area enclosed by the loop corresponds to the dissipated energy density per cycle. The maximum flux density and switching behavior can also be obtained from the hysteresis loop. In general, the magnetic properties of cold-rolled nonoriented electrical steels mainly depend on their grain size and texture [3]. However, during the manufacturing processes, the microstructure of the electrical steel will change dramatically. These processes may cause significant dislocations, increased residual stress, cracking or deformation to the grains [4], [5]. To reduce the impact of the punching processes on the magnetic properties of electrical steel, annealing is often used to recover the damage due to cutting processes [6]–[8]. Annealing around 750 °C can eliminate shearing and residual stresses and reduce iron losses [7], [8]. These previous works focus on material properties but the effects on electromagnetic devices are still to be investigated. According to IEC60404-2, the standard measurement of electrical steel sheet should be conducted under conditions of no stress, uniform magnitude alternating field in a pre-

Manuscript received March 7, 2014; revised May 27, 2014; accepted June 4, 2014. Date of current version November 18, 2014. Corresponding author: C.-C. Chiang (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.2014.2329708

scribed direction, with a sinusoidal flux wave with very low harmonic content. However, the electrical steels in electric motors operate in very different conditions to those of standard measurement. The fabrication processes used in the manufacture of electrical motors also influence the material magnetic properties. The resulting performance of electrical steel in electric motors is usually different from that expected in the design stage. This may result in degraded machine performance. Previous researchers have made efforts to calculate the effects of fabrication and predict the performance of electric motors more accurately during design [9], [10]. However, the influences of magnetic properties on the performance of motors are still unclear and the literature on this topic is sparse. In this paper, the effects of annealing on the recovery of the punching damage, microstructure, magnetic properties, and the improvement of the switched reluctance motor (SRM) performance are studied. The resulting information is valuable to the electric machine industry for production of high-efficiency electric machines. II. D ESIGN OF E XPERIMENTS To understand the impact of punching and annealing, the experimental work is carried out using test samples to observe the microstructure and electromagnetic properties. Two fully constructed SRMs are also tested to observe the influence of the manufacture processes on motor performance. Two grades of electric steel, fabricated by China Steel Corporation, are used: models 50CS1300 and 50CS470 [11]. The samples are punched into 41.5 mm × 28 mm square ring shape with inner dimension of 28 mm × 14.5 mm. Stamping is carried out by a reputable company that works with China Steel Corporation. After punching the samples are annealed at 750 °C for 1 h under a N2 protection atmosphere following guidelines from the material provider. The temperature rise is set at a sufficiently slow rate of 5 °C/min.

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TABLE I M AIN D IMENSIONS AND D ESIGN OF THE SRM

Fig. 2.

Dynamometer used to measure motor performance.

Fig. 3. Optical microscope as-cut images of (a) 50CS1300 and (b) 50CS470. Images are taken from the cut edges. The arrows indicate the punching direction and red squares illustrate the distorted area.

Fig. 1.

(a) 12/8 three-phase SRM. (b) Driver circuit.

The microstructure of the specimens at the cut edge is then observed using an optical microscope. The magnetic properties are measured with sinusoidal magnetic induction at 400 Hz (a typical frequency in the SRM operating range). The core losses are measured at a range of flux densities, described in Section IV. The two SRMs with an identical design are fabricated using model 50CS350 electric steel (different from the samples due to availability) to study the annealing effect on macroscale motor performance. The eight-pole 12-slot SRMs are designed for a specification of 48 V, 2400 r/min rated speed, and 7.0 N m rated torque. The main dimensions of the SRMs are given in Table I. Fig. 1 shows the designed 12/8 SRM and the associated electronic drive, which is of the typical asymmetrical half-bridge type. After stamping and stacking the laminations into rotors and stators, one set of rotor and stator was directly assembled into an SRM while the other was annealed at 750 °C for 1 h with N2 protection, an identical condition to those used for the ring samples. The postannealing rotor and stator was then assembled into the other SRM with the windings and frame. Note that these two motors are identical except that the electrical steel in one of the motors was annealed. The motor performances are measured by the dynamometer shown in Fig. 2.

Fig. 4. After annealed optical microscope images of (a) 50CS1300 and (b) 50CS470. Images are taken from the cutting edges. (c) Image of the near edge area and (d) center part of 50CS1300.

III. E FFECTS OF A NNEALING ON G RAIN S IZE As stated, the ring samples before and after annealing are observed using an optical microscope and the images of their cut edges are shown in Figs. 3 and 4, respectively. The punching directions are indicated by the arrows. The condition before annealing is shown in Fig. 3, where a distorted area can be observed near the cut edge of both types of laminations. The marked region represents the distorted area where the aspect ratio of the grains is greater than two. The width of the distorted area perpendicular to the punching edge is larger in sample 50CS470 (97 µm) than in sample

CHIANG et al.: EFFECTS OF ANNEALING ON MAGNETIC PROPERTIES

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TABLE II AVERAGE G RAIN S IZE OF T WO K INDS OF S AMPLES B EFORE AND A FTER A NNEALING

Fig. 6. Variation of core loss with flux density at 400 Hz for samples before and after annealing. TABLE III I RON L OSS OF T WO K INDS OF S AMPLES B EFORE AND A FTER A NNEALING Fig. 5. Hysteresis loops before (black line) and after (red line) annealing. (a) 50CS470. (b) 50CS1300.

50CS1300 (52 µm). This may be due to the fact that the average grain size of 50CS470 steel is larger (33.8 ± 15.1 µm) compared with 50CS1300 steel (12.2 ± 4.4 µm). The larger the grain size is, the easier it is for the dislocations to form and move. Therefore, for the electrical steel with a larger grain size, the damaged zone after punching appears to be larger than that with the smaller grain size [12]. After annealing, the distorted area recovers and the grain size becomes larger, as shown in Fig. 4. The grain size of the sample using 50CS470 steel grows from 33.8 ± 15.1µm 51.6 ± 41.3 µm. However, it is observed that there are two different zones on samples using 50CS1300 steel. As shown in Fig. 4(a) and Table II, the grain size near the cut edge [shown in Fig. 4(c)] is larger than the size in the middle [shown in Fig. 4(d)] after annealing. The acceleration of the grain growth is due to the strain energy stored by the punching process [12]. This implies that the defect density is larger at the cut edge for sample using 50CS1300 steel than that using 50CS470. IV. E FFECTS OF A NNEALING ON M AGNETIC P ERFORMANCE The hysteresis loops for both samples, before and after annealing, are measured with the results shown in Fig. 5. It is clear that both samples present a trend toward a more rectangular hysteresis loop after annealing and become much closer to the original property given by the manufacturer [11]. For the sample using 50CS470 steel [Fig. 5(a)], the increased rectangularity implies that the magnetic switching is more coherent. The microstructure of the sample also confirms that the annealing process eliminates the cutting damage and, therefore, makes the switching homogenous. On the other hand, the hysteresis loop of the sample using 50CS1300 steel [Fig. 5(b)] indicates a two-step switching process. As described above, there are two different areas of grains observed in the postannealing sample and these are the cause of the magnetic switching a two-step pattern. The core losses for each sample are obtained by sinusoidal magnetic induction. Measurements are taken at 400 Hz for

100 mT intervals from 100 to 500 mT for all the four samples. The applied flux density is limited by the test equipment. The resulting core losses are plotted as a function of peak flux density, as shown in Fig. 6. It is clear that the core losses of both samples are reduced significantly by the annealing process. The core losses at 400 Hz, 500 mT are tabulated in Table III. The core loss after annealing is reduced by 37% for the samples using 50CS470 steel and 23% for those using 50CS1300 steel. It can be observed that the core loss improves more significantly on the higher grade steel (50CS470) than the lower grade steel (50CS1300). This is because the damaged zone after punching for the 50CS470 steel sample is wider than that using 50CS1300 steel. Punching creates more defects in the higher grade steel than the lower grade steel, and therefore, annealing the 50CS470 sample (higher grade steel) results in a higher recovery from the punching process, because there is more damage from which to recover. This result implies that annealing may be more necessary for high-performance electric machine production using high-quality laminations. V. SRM P ERFORMANCE As previously stated, one set of rotor and stator was annealed at 750 °C for an hour after the stamping process, and the other was not. The annealing process is the only intentional difference in manufacture of the two motors. The SRMs are tested using the same driver and test bench (Fig. 2) to measure the output torque and current under different rotating speeds. The motor driver is a voltage-controlled system—the dc supply voltage is switched to the three phases depending on the rotor position and current is not controlled directly. The motors are initially energized with 48 V and operated at no-load condition. The load is gradually increased until the torque reaches approximately 160% of rated torque. The measured

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current and speed, which correspond to a lower flux and higher output power. It should be noted that the driver is included in the efficiency calculation for all the measurements. VI. C ONCLUSION

Fig. 7. Variation of motor output torque as a function of the rotor speed with and without annealing.

The influences of the punching and annealing processes have been demonstrated with regard to microstructure, magnetic properties, and assembled SRM performance. Punching causes grain distortions near the cutting and thus changes the magnetic switching behavior and increases core loss. Annealing recovers the damage and increases the grain size, resulting in a reduction in iron losses and increase in permeability after annealing. The measured performance of the two SRMs demonstrates that the motor with the annealed steel has higher torque capability and power density, with increased efficiency at high torque loads. ACKNOWLEDGMENT

Fig. 8.

Efficiency as a function of (a) torque and (b) speed.

This work was supported in part by the Taiwan National Science Council and in part by the Ministry of Economic Affairs under Contract NSC102-2622-E-006-032 and Contract 102-EC-17-A-05-S1-192. The prototype design was completed using JMAG provided by JSOL, whose support is highly appreciated.

TABLE IV C OMPARISON OF T WO SRMs AT 5.0 Nm N OMINAL T ORQUE

R EFERENCES

torque–speed curves are shown in Fig. 7. As can be observed, the output torque of the annealed motor is higher than that of the nonannealed motor at the same rotating speed. Considering efficiency, plotted in Fig. 8, it is clear that the annealed motor operates with higher efficiency at higher torques, but with reduced efficiency at lower torque. Fig. 8(b) plots the efficiency as a function of speed. The efficiency at high speed is lower for the annealed motor and very similar between cases at low speed. It should be noted that Figs. 7 and 8 are from the same test data, but with different presentations. The change in material properties makes testing under identical conditions impossible, due to the different torque–speed characteristics. In general, it may be observed that the torque density of the motor is increased and high-torque operation becomes more efficient, while efficiency is reduced at low load. One possible explanation for this observation is the increase in permeability at low flux density. At high speeds, the flux density is low and the nonaligned inductance of the annealed motor will be higher than that of the nonannealed motor. At high torques, the motors are highly saturated and the sharp saturation knee of the annealed motor may result in a larger area for the flux– current loop. The net effect of these changes will be higher current and lower efficiency at low load, with increased torque capability and efficiency at higher loads. The efficiency curves cross at about 5 Nm, and detailed performance data at this load is presented in Table IV. The annealed motor has higher

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