KAWASAKI STEEL TECHNICAL REPORT No. 48 March 2003 Electrical Steel for Motors of Electric and Hybrid Vehicles* Synopsis: Kawasaki Steel has found that there exists suitable electrical steel sheets as core materials depending on motor types such as induction, brushless DC, and reluc- tance motors, through the evaluation of core materials conducted by using several motor test machines. RP and RMA series of high flux density are suitable to induction motors. RMHE series of excellent punchability, low iron Atsuhito Honda Kunihiro Senda Kenichi Sadahiro loss and high density are suitable to brushless DC Dr. Eng., Senior Researcher, Senior Researcher, Senior Researcher, Electrical Steel Lab., Electrical Steel Lab., motors. B coating of self-adhesive type organic coating Electrical Steel Lab., Technical Res. Labs. Technical Res. Labs. was found to be effective to reduce the iron loss and the Technical Res. Labs. noise of motors. 1 Introduction Reluctance Brushless Since the second half of 20th century, the effective motor DC motor IPRM use of energy and environmental protection have been SynRM Low cost considered as the most important issues in the world. IPM SRM SPM The state of California has been especially advanced in dealing with these issues; for example it requires partial Recyclability High efficiency zero emission vehicles (PZEV) to account for at least Motor efficiency luduction 10% of all sales from 2003 onwards. This regulation by motor California has spurred the development of PZEV such Low High as an electric and hybrid vehicles which offer less Low High energy consumption and less pollution. Motor price Motors used for driving or other purposes in an auto- Fig. 1 Comparison of induction, reluctance and mobile, such as induction, brushless DC, and reluctance brushless DC motors motors, are shown in Fig. 1. Electric vehicles made in Europe and USA generally use induction motors, shows the relationship between iron loss and magnetic because they are more reliable than DC motors with flux density of non-oriented electrical steel produced by brushes and less expensive than brushless DC motors Kawasaki Steel.2) In addition to the RM series of con- that use permanent magnets. In contrast in Japan, brush- ventional electrical steels ranging from high grades of less DC motors have been mostly used in electric and high Si and low iron loss to low grades of low Si and hybrid vehicles, because of the advanced research and high iron loss, we have developed several electrical development of high quality and reliable magnets. steels which have higher magnetic flux density and the Unlike the full size or large cars in USA, cars in Japan same iron loss as conventional steels. are more compact, and thus have smaller motors. This This article describes the effects of electrical steel may be another reason for the use of brushless DC used as a core material on the properties of motors of motors. Moreover, the recent development of control different kinds, and proposes an optimum core material techniques has spurred the use of switched reluctance for individual motors. It also introduces a newly devel- motors (SRM) and synchronous reluctance motors oped electrical steel for reactors which is expected to (SynRM) which had been restricted in automobile use come into widespread use in automobiles in future. due to their noise and vibration.1) The properties of a motor are also greatly dependent on the electrical steel used as the core material. Figure 2 2 Effects of Core Materials on Motors 66666 Kawasaki Steel evaluated the properties of several * Originally published in Kawasaki Steel Giho, 34(2002)2, 85–89 kinds of motors, namely 600 W single phase induction 33 500 High Si content Low Si Total loss 1.80 400 50RP series RP1 Copper loss 300 RMA series RP2 1.76 35RMA300 RP3 (T) 50RMA350 200 50 50RMA500 B 1 000 Motor loss (W) 35RMHE Iron loss 800 100 series 300 1.72 700 600 0 250 50RM series 0 0.5 1.0 1.5 2.0 2.5 470 Si ϩ Al content (％) Flux density, 400 230 1.68 350 Fig. 4 Effect of Si ϩ Al content of core material on 310 iron loss, copper loss, and total loss of induc- 230 tion motor at high input voltage of 120 V 35RM200 1.64 12345678 Bm (T) Iron loss, W15/50 (W/kg) 00112 Fig. 2 Relation between iron loss and flux density of non-oriented electrical steel produced by R θ Kawasaki Steel motors, 400 W 3phase-6poles induction motors and 300 W 3phase-8poles brushless DC motors,3–5) and has determined the effects of electrical steel on the proper- ties. Presently we evaluate core material for SRM. We have also tried to evaluate core materials more precisely by using the magnetic field analysis software JMAG. Fig. 5 Flux density distribution in induction motor 2.1 Effects of Core Materials on the Induction stator core measured by the stylus probe Motors method Figure 3 shows the measured losses for the single operation flux density. In order to investigate the effects phase induction motor using some of electrical steels of operational flux density, motor losses were measured shown in Fig. 2 as the core material.3) The copper loss when the operating flux density was increased by was more than twice as large as the iron loss in the increasing input voltage. Figure 4 shows the results.3) A induction motor. The use of high Si materials having low Si material having high saturation flux density low iron loss decreased motor iron loss, but increased decreased total loss of the motor, while a high Si mater- motor copper loss. This was because the increase of Si ial caused severe deterioration in copper loss because content in the core material reduced iron loss and the flux magnetic density nearly reached the saturation increased magnetizing current by reducing the magnetic point. flux density of the material in the saturation and high Figure 5 shows an example of local flux density dis- magnetic field regions. tribution in the induction motor measured by the stylus The downsizing of motors requires them to have high probe method.6–8) The flux densities of teeth and core back are both more than 1.5 T. Downsizing of the motor will make these areas approach the saturation point. 100 Total loss We concluded from these results that core materials 80 with low Si content are suitable for induction motors Copper loss having larger iron loss than copper loss. 60 2.2 Effect of Core Material on Brushless DC 40 Motor Properties Motor loss (W) Iron loss 20 Figure 6 shows the effects of core material on the 5) 0 maximum efficiencies of brushless DC motors. It was 0 0.5 1.0 1.5 2.0 2.5 found that the maximum efficiency of motor depended Si ϩ Al content (％) mainly on iron loss at 400 Hz regardless of stress relief Fig. 3 Effect of Si ϩ Al content of core material on annealing of the material after punching. The brushless iron loss, copper loss, and total loss of induc- DC motor requires smaller primary current because it tion motor at rated input voltage of 100 V has powerful magnets such as those of rare earth metal 34 KAWASAKI STEEL TECHNICAL REPORT ) B ϭ 1.67T ％ 50 90 Before SRA 90 After SRA ) 1.71 ％ 88 1.72 88 1.72 86 86 Motor efficiency ( 1.77 84 Maximum motor efficiency ( 20 40 60 100 200 300 400 Material iron loss, W (W/kg) 10/400 Output power (W) Fig. 6 Relation between material iron loss at high Fig. 8 Relationship between output power and frequency before and after stress relief motor efficiency annealing (SRA) and maximum motor effi- ciency At yoke At teeth 1.5T Fig. 7 Differential wave form of flux density in sta- tor core Fig. 9 Rotational flux distribution of brushless DC motor core calculated by JMAG and it requires no secondary current because there are no conductors in the rotor. Therefore the brushless DC Kawasaki Steel is conducting several studies on devel- motor has less copper loss and more iron loss than the oping techniques to analyze local iron loss. induction motor. This is the main reason the maximum 2.3 Effects of Core Materials on SRM efficiency of brushless DC motor shows a good correla- tion with material iron loss. Unlike induction or brushless DC motors, SRM con- The maximum efficiency is correlated very well with sisting of iron cores without conductors and magnets in material iron loss at 400 Hz despite the fact that the dri- rotor is inexpensive, tough, and superior in recyclability. ving condition is at a basic frequency of 100 Hz. This is Therefore it shows promise as a driving motor for cars.1) because the flux density waveform of the teeth and core Several papers have reported on the effects of core mate- back of the motors tested in this experiment included rials on SRM properties.9,10) Nakamura et al.9) prepared higher harmonics, as shown in Fig. 7. These higher har- SRMs using 50A1000 of low grade electrical steel and monics would be due to the magnetizing waveform of 35A290 of high grade low iron loss steel and tested PWM and the magnets in the rotor. them. They achieved more than 80% efficiency by using It can be suggested that materials having low iron loss 35A290 and by progress magnetization controlling at high frequency are suitable for brushless DC motors. method, which is 10% more efficient than 50A1000. It is also desirable to have high flux density because They concluded that the differences in motor efficiency high flux density material has higher efficiency in the were due to the differences in the iron loss of the core region over rated load shown in Fig.