Electrical Steels and Their Evaluation for Automobile Motors

Article Electrical Steels and Their Evaluation for Automobile Motors

Kunihiro Senda *, Masanori Uesaka, Soichiro Yoshizaki and Yoshihiko Oda

Steel Research Laboratory, JFE Steel Corporation, Kawasaki-dori 1-chome, Mizushima, Kurashiki, Okayama 712-8511, Japan; [email protected] (M.U.); [email protected] (S.Y.); [email protected] (Y.O.) * Correspondence: [email protected]

Received: 17 April 2019; Accepted: 20 May 2019; Published: 27 May 2019

Abstract: Achieving high efficiency and high torque is an important target in EV motors. This paper describes the effect of the magnetic properties of electrical steels used as core materials for synchronous motors with permanent magnets, which are commonly used as the EV traction motors. It was confirmed that electrical steels, which have high flux density and low iron loss properties can realize high motor efficiency and torque. When PWM excitation is considered, thinner electrical steels are advantageous to suppress increased loss due to higher harmonics. Based on these results, electrical steels having high flux densities and low iron losses at high frequencies were developed.

Keywords: electrical steel; motor eﬃciency; motor torque; iron loss; inverter; PWM excitation

1. Introduction Improvement of motor efficiency is an important issue for increasing the electric power efficiency of electric vehicles (EV). Generally, motor loss, which consists mainly of iron loss and copper loss is strongly influenced by the magnetic properties of the electrical steels [1,2] used in motor cores. In the case of EV, since traction motors are driven under a wide range of conditions [3], high efficiency or high torque is required in each driving condition. Under these circumstances, low iron loss and high flux density (high permeability) are demanded in electrical steels. Reduction of iron loss can contribute to downsizing of motors, which is effective for reducing motor weight through high revolution speed. It is known that improvement in the property of magnetization (flux density) in electrical steels can improve motor torque properties. (Hereinafter, the value of the flux density when a magnetic field of a given strength is applied is called “material flux density.” A notation such as B50 for the material flux density means the flux density at the magnetic field strength of 5000 A/m.) This means core materials having high flux density can realize motor size reduction under given torques. Equally, reduction of the motor current is achievable if the size of the motor and the torque are kept the same value, and this leads to a reduction of copper loss. As for the magnetic properties of electrical steels, since a trade-off relationship sometimes exists between iron loss and material flux density, it is difficult to realize both properties simultaneously. Therefore, choosing suitable core materials by considering the motor driving condition is important. At the same time, proper indicators and analysis methods are needed for appropriate selection of core materials. Previously, several researches have been carried out to investigate the influence of core materials on motors [4,5]. However, few research can be found on core materials focusing on basic magnetic properties systematically. Currently, synchronous motors with permanent magnets are commonly used as traction motors of EV and HEV because of their high efficiency. Hence, in this study, influences of core material’s magnetic properties on the motor performance were studied in order to establish proper indicators

World Electric Vehicle Journal 2019, 10, 31; doi:10.3390/wevj10020031 www.mdpi.com/journal/wevj World Electric Vehicle Journal 2019, 10, 31 2 of 10 of material and to contribute to new material development, using the SPM type brushless DC motor (BLDC motor). In this study, the eﬀects of the magnetic properties of electrical steels on motor loss and torque are discussed based on the measured results obtained with BLDC motors. Additionally, to understand the origin of the higher harmonic iron loss component in inverter-driven motors, which are commonly used in EVs, inverter-fed magnetic measurements were carried out, and the inﬂuence of the material thickness on core loss under PWM excitation conditions was evaluated. Finally, electrical steels suitable for EV motors are described based on the evaluation considering motor operation conditions.

2. Evaluation of Motor Properties

2.1. Experimental Procedure The influence of the magnetic properties of non-oriented electrical steels was evaluated by using a SPM brushless DC motor (8 poles, 12 slots, concentrated windings) [6]. The specifications of the motor are shown in Table1. In the evaluation, the tested motor was connected to a load motor through a torque sensor and tachometer, and the maximum efficiency when the applied torque was increased gradually from the no-load state at a revolution speed of 2100 rpm was taken as a candidate motor property. The maximum efficiencies were obtained around the revolution speed of 1500 rpm. The torque constants at 1500 rpm were calculated from the measured torques and currents.

Table 1. Speciﬁcation of tested motor.

Categories Speciﬁcations Motor type Surface permanent magnet brushless DC motor Rated power 300 W Input voltage 48 Vdc Driving voltage waveform 120◦ rectangular Stator dimensions odφ 178 mm, idφ 75 mm, h23 mm Number of slots 12 Rotor dimensions odφ 74 mm, h23 mm Number of poles 8 Winding 3-phase star connection, 4 coils/phase Carrier frequency 1.7 kHz

For the stator materials, several kinds of electrical steels were chosen from the commercial electrical steel products shown in Figure1 to evaluate the influence of magnetic properties on motor e fficiency, iron loss, and copper loss. In Figure1, the 35JN series and 50JN series are conventional non-oriented electrical steels with thicknesses of 0.35 mm and 0.50 mm, respectively. Here, the head figures of each series indicate the sheet thickness (e.g., 35JN indicates a thickness of 0.35 mm). JNA® series is a material which show good magnetic properties after stress relief annealing. The values of JNA in Figure1 are the magnetic properties measured after stress relief annealing in 750 ◦C for 2 h. The ® JNE series (35JNE and 50JNE) shows high material flux density B50 and low iron loss W15/50. Here, a notation such as W15/50 means iron loss at a flux density of 1.5 T and frequency of 50 Hz. Magnetic properties were measured by the Epstein method using the sample cut along rolling direction and the sample cut along the transverse direction toward rolling direction. World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 3 of 10

1.76 50JNA ] (T) 50JNE 50 1.74

B 35JNE 1.72 50JN series (Conventional) 1.70

1.68 JNA: Values obtained after stress relief annealing. 1.66

Flux density at 5000 A/m [ at 5000 A/m Flux density 35JN series (Conventional) 1.64 1 2 3 4 5 6 7 8 9 Iron loss at 1.5 T and 50 Hz [W15/50] (W/kg)

WorldFigure Electric Vehicle1. Map Journal of magnetic2019, 10, properties 31 of electrical steel products by iron loss W15/50 and material flux3 of 10 World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 3 of 10 density B50.

1.76 2.2. Result of Motor Evaluation 50JNA ] (T) 50JNE 50 1.74

B The relationships between the 35JNEmaximum motor efficiency and magnetic properties of the stator 1.72 materials are shown in Figure 2. In Figure 2, the horizontal50JN axes series in (a), (b), and (c) are W15/50, W15/100, (Conventional) 10/400 W , and the correlation coefficients1.70 for the relationship in (a), (b), and (c) were 0.65, 0.80, and 0.94, respectively. The highest correlation was obtained when W10/400 was chosen as the material iron loss. 1.68 Material iron loss W15/50, which has been used conventionallyJNA: Values as obtained an ind aftericator for electrical steel, did not stress relief annealing. show a good correlation with 1.66 motor efficiency. This means that iron loss at low frequencies is not

suitable for prediction of the performance [ at 5000 A/m Flux density of35JN BLDC series motors.(Conventional) 1.64 In the BLDC motor used in this1 study,2 the3 peak4 flux5 density6 7 in the8 teeth9 of the stator core was about 1.6 T, and the fundamental frequencyIron loss at at maximum 1.5 T and 50 Hz efficiency [W15/50] (W/kg) was about 100 Hz. Therefore, the correlation with W15/100 should be the highest if the loss at the fundamental frequency was the largest FigureFigure 1. Map 1. Map of magnetic of magnetic properties properties of electrical of electrical steel steel products products by byiron iron loss loss W15/50W15 and/50 and material material flux flux part of motorB loss. However, the highest correlation was obtained when 400 Hz was chosen as the densitydensity B50. 50. material iron loss. This fact indicates that the influence of higher harmonics in the flux waveforms is 2.2.significant Result ofin MotorBLDC Evaluation motors, and the iron loss measured under high frequency conditions is important 2.2. Result of Motor Evaluation for core material evaluation. Here, the peak flux density at W10/400 differs from the peak flux density in The relationships between the maximum motor efficiency and magnetic properties of the stator theThe stator relationships core. The reasonbetween why the W maximum10/400 showed motor a good efficiency correlation and magneticwith the motorproperties efficiency of the is st thatator the materials are shown in Figure2. In Figure2, the horizontal axes in (a), (b), and (c) are W15/50, W15/100, materialsmaterial are iron shown loss aint 1.0 Figure T has 2. a Inlinear Figure relationship 2, the horizontal with the axes material in (a), iron (b), losses and (c)at higherare W 15/50flux, Wdensities15/100, W10/400, and the correlation coefficients for the relationship in (a), (b), and (c) were 0.65, 0.80, and 0.94, W10/400such, and as 1.6the T. correlation Moreover, coefficients measurement for theof materialrelationship iron in loss (a), at (b), high and flux (c) densitywere 0.65, such 0.80, as and1.6 T0.94, and a respectively. The highest correlation was obtained when W10/400 was chosen as the material iron loss. respectively.frequency The of 400highest Hz iscorrelation sometimes was difficult obtained because when W of 10/400 the was performan chosence as limitationsthe material of iron measuring loss. Material iron loss W15/50, which has been used conventionally as an indicator for electrical steel, did Materialapparatuses. iron loss Therefore, W15/50, which W10/400 has is beena useful used indicator conventionally for the evaluationas an indicator of materials for electrical used steel,in BLDC did notmotor not show a good correlation with motor efficiency. This means that iron loss at low frequencies is not showcores. a good correlation with motor efficiency. This means that iron loss at low frequencies is not suitable for prediction of the performance of BLDC motors. suitable for prediction of the performance of BLDC motors. In the BLDC motor used in this study, the peak flux density in the teeth of the stator core was about 92 92 92 1.6 T, and the fundamental frequency at maximum efficiency was about 100 Hz. Therefore, the correlation91 with W15/100 should be the highest91 if the loss at the fundamental91 frequency was the largest part of motor loss. However, the highest correlation was obtained when 400 Hz was chosen as the material90 iron loss. This fact indicates that90 the influence of higher harmonics90 in the flux waveforms is significant in BLDC motors, and the iron loss measured under high frequency conditions is important 89 89 89

for core material evaluation. Here, the peak flux density at W10/400 differs from the peak flux density in

Maximum efficiency Maximumefficiency (%) Maximum efficiency Maximumefficiency (%) Maximum efficiency Maximumefficiency (%) the stator88 core. The reason why W10/400 showed88 a good correlation with the88 motor efficiency is that the 2 3 4 5 5 7 9 11 15 20 25 30 35 40 material iron Ironloss loss, at W1.015/50 T(W/kg) has a linear relationshipIron withloss, W 15/100the material(W/kg) iron losses atIron higher loss, W flux10/400 densities(W/kg) (a) (b) (c) such as 1.6 T. Moreover, measurement of material iron loss at high flux density such as 1.6 T and a frequency of 400 Hz is sometimes difficult because of the performance limitations of measuring Figure 2. Relationship between maximum motor efficiency and iron loss W15/50, W15/100, W10/400. apparatuses. Therefore, W10/400 is a useful indicator for the evaluation of materials used in BLDC motor (Figurea) Relationship 2. Relationship between between maximum maximum motor motor efficiency efficiency and and material iron loss iron W loss15/50, W at15/100B =, W1.510/400 T,. 50 Hz; cores. ((ba) Relationship relationship between maximum maximum motor motor efficiency efficiency and and material material iron iron loss loss at B at = 1.5B = T,1.5 50 T,Hz 100; Hz; ((cb)) relationshiprelationship betweenbetween maximummaximum motormotor eefficiencyfficiency and and material material iron iron loss loss at atB B= =1.0 1.5 T,T, 400 100 Hz. Hz;

92 (c) relationship between maximum92 motor efficiency and material iron92 loss at B = 1.0 T, 400 Hz. In the BLDC motor used in this study, the peak ﬂux density in the teeth of the stator core was

about91 1.6 T, and the fundamental frequency91 at maximum efficiency91 was about 100 Hz. Therefore, the correlation with W15/100 should be the highest if the loss at the fundamental frequency was the largest part90 of motor loss. However, the90 highest correlation was obtained90 when 400 Hz was chosen as the material iron loss. This fact indicates that the influence of higher harmonics in the flux waveforms is 89 89 89

signiﬁcant in BLDC motors, and the iron loss measured under high frequency conditions is important

Maximum efficiency Maximumefficiency (%) Maximum efficiency Maximumefficiency (%) Maximum efficiency Maximumefficiency (%) for88 core material evaluation. Here,88 the peak flux density at W di88 ffers from the peak flux density 2 3 4 5 5 7 9 11 10/400 15 20 25 30 35 40 in the statorIron loss, core. W15/50 The(W/kg) reason why W10/Iron400 loss,showed W15/100 a(W/kg) good correlation withIron loss, the W motor10/400 (W/kg) efficiency is (a) (b) (c) that the material iron loss at 1.0 T has a linear relationship with the material iron losses at higher flux densities such as 1.6 T. Moreover, measurement of material iron loss at high flux density such as 1.6 T

andFigure a frequency 2. Relationship of 400 between Hz is sometimes maximum dimotorfficult efficiency because and of theiron performance loss W15/50, W15/100 limitations, W10/400. of measuring apparatuses.(a) Relationship Therefore, between maximumW10/400 is motor a useful efficiency indicator and formaterial the evaluationiron loss at B of = 1.5 materials T, 50 Hz used; in BLDC motor(b) relationship cores. between maximum motor efficiency and material iron loss at B = 1.5 T, 100 Hz; (c) relationship between maximum motor efficiency and material iron loss at B = 1.0 T, 400 Hz. World Electric Vehicle JournalJournal 20192019,, 1010,, 31x FOR PEER REVIEW 4 of 10 World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 4 of 10 Frequency components higher than the fundamental frequency are contained in the flux density waveformsFrequencyFrequency of the components stator core higher due tothan armature the fundamental mmf [7] andfrequency the carrier are contained frequency in the of the flux flux inverter. density waveformsAlthough the of frequencies the sstatortator coreof these due higher to armature harmonics mmf are [ [7 ]higher] and thethan carrier 400 Hz, freq frequency theuency material of of the the iron inverter. inverter. loss at Although400 Hz has thet hea roughly frequencies linear of relationship these higher with harmonics the losses are at higher higher than frequencies 400 Hz, the over material 400 Hz. iron From loss this at 400 Hz has a roughly linear relationship with the losses at higher frequencies over 400 Hz. From this viewpoint as well, W10/400 is a good indicator for evaluation of core materials for BLDC motors. viewpoint as well, W1010/400/400 isis a agood good indicator indicator for for evaluation evaluation of of core core materials materials for for BLDC BLDC motors. motors. Figure 3 shows the relationship between the material flux density B50 and the motor torque B 50 constant.Figure As 3 3 canshows shows be seen the the in relationship relationship the graph, the between between torque the theconstants material material increased flux flux density density linearly B50 with and increasing the motor mat torqueerial constant. As can can be be seen seen in in the the graph, graph, the the torque torque constants constants increased increased linearly linearly with with increasing increasing mat materialerial flux density B50. This result shows that electrical steels having high B50 are effective for obtaining high torque.fluxflux density In the BB 50motor50. .This This usedresult result in shows this shows study, that that e lectricalthe electrical flux steelsdensity steels having in having the high stator high B50 yoke areB50 effective arewas e almostffective for obtaining1.6 for T, obtaining and high this hightorque. torque. In the In motor the motor used usedin this in study, this study, the flux the density flux density in the in stator the stator yokeyoke was almost was almost 1.6 T, 1.6 and T, andthis value was roughly close to the material flux density B50. This means B50 is a good indicator for thisvalue value was was roughly roughly close close to theto the materia materiall flux flux density density B50B. This. This means means BB50 isis a a good good indicat indicatoror for predicting the motor torque. However, if the peak flux density50 in a motor core50 is much lower, B50 is predictingnot a good theindicator motor torque.because However,the flux density if the peak level fluxflux is different density infrom a motormotor the excitation corecore isis muchmuch condition lower,lower, of BB50 50the is motornot a good core. indicator because the flux flux density level is different different from the excitation condition of the motorFigure core. 4 shows the relationship between the torque constant and maximum efficiency. As can be Figure 44 shows shows thethe relationshiprelationship betweenbetween thethe torquetorque constantconstant andand maximummaximum eefficiency.fficiency. As can be seen in this graph, higher efficiencies are achieved in the JNE® series which has higher flux densities B50 ® ® whileseen in keeping this this graph, graph, low higher higher iron loss.efficiencies effi ciencies In a com are areparison achieved achieved of in products in the the JNE JNE having seriesseries which the which same has has higher sheet higher thickness, flux flux densities densities higher B50 Befficiencieswhile50 while keeping keeping are low achieved low iron iron loss. in loss. JNE In In athan com a comparison inparison conventional of of products products material having having JN for the the the same same same sheet sheet levels thickness, of the to higherrque constant.eefficienciesfficiencies The are advantage achieved of in the JNE JNE than series in conventionalin the torque property material shown JN JN for in the Figure same 4 levels is based of onthe its torqueto rquehigh constant. The The advantage advantage of of the the JNE JNE series series in inthe the torque torque property property shown shown in Figure in Figure 4 is 4based is based on its on high its B50 without deterioration of iron loss. For EV traction motors, high torque and low loss must be achieved athighB50 the withoutB same50 without deteriorationtime. Therefore, deterioration of iron improvement ofloss. iron For loss. EV of Fortraction material EV tractionmotors, flux density motors,high torque as high it achieved and torque low loss andin JNE must low is loss effectivebe achieved must for be improvingachievedat the same at the time. the total same Therefore, performance time. Therefore,improvement of EV improvementmotors. of material offlux material density flux as it density achieved as in it achievedJNE is effective in JNE for is eimprovingffective for the improving total performance the total performanceof EV motors. of EV motors. 0.25 0.25

(Nm/A)

K 0.24 0.24

0.23 0.23

1.65 1.70 1.75 1.80 Motor torque constant constant torque Motor 1.65 1.70 1.75 1.80 Motor torque constant constant torque Motor Material flux density B (T) Material flux density B50 (T) 50 Figure 3. Relationship between motor torque constant and material flux density B50. Magnetic Figure 3. Relationship between motor torque constant and material ﬂux density B . Magnetic propertiesFigure 3. Relationshipwere measured between by ring motorcore samples. torque constant and material flux density B5050. Magnetic properties were measuredmeasured byby ringring corecore samples.samples. 92 92 JNE B50=1.69-1.70 T JNE B50=1.69-1.70 T 91 JN 91 JN B50=1.66 T Thickness: B50=1.66 T JNE Thickness:0.35mm 90 JNE 0.35mm 90 JN JN B50=1.70-1.73 T 89 B50=1.70-1.73 T 89 Thickness: B50=1.66-1.69 T Thickness:0.50 mm B50=1.66-1.69 T 88 0.50 mm

Maximum motor efficiency (%) efficiency motor Maximum 880.230 0.235 0.240 0.245 Maximum motor efficiency (%) efficiency motor Maximum 0.230Motor torque0.235 constant0.240 K (Nm/A)0.245 Motor torque constant KT (Nm/A) T Figure 4. Relationship between torque constant and maximum effic eﬃciencyiency in comparison of material Figure 4. Relationship between torque constant and maximum efficiency in comparison of material fluxﬂux density B50.. flux density B50. World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 5 of 10

3. Material Evaluation under Inverter Excitation Condition

3.1. Experimental Procedure Figure 5 shows the experimental system for PWM excitation using an inverter. In the inverter part, a single phase Si-N channel IGBT was used. This system also has a linear amplifier excitation system for comparison with the usual iron loss in the sinusoidal condition. In both the measurements with PWM excitation of inverter and with sinusoidal excitation, the fundamental frequency was 50 Hz and the maximum flux density was 1.5 T. In the inverter excitation measurement, the carrier frequency was changed between 500 Hz and 20 kHz, and the modulation index was changed between 0.1 and 1.0. World Electric Vehicle Journal 2019, 10, 31 5 of 10 As samples for magnetic measurement, ring core samples having an outer diameter of 80 mm and inner diameter of 60 mm were prepared. The turn numbers of the primary and secondary 3. Materialwindings Evaluation were 150 and under 100, Inverterrespectively. Excitation The flux Conditiondensity (B) of the core was measured by the output voltage of the secondary coil, and the strength of the magnetic field (H) was obtained by the current 3.1. Experimentalmeasured by Procedurethe current sensor. Iron losses were derived by calculating the inner area of the B-H loops. FigureElectrical5 shows steels the containing experimental 3% silicon system having for sheet PWM thicknesses excitation of using0.50 mm, an 0.35 inverter. mm, and In 0.25 the invertermm part,were a single prepared phase inSi-N the laboratory. channel IGBTRing cores was were used. prepared This system using these also electrical has a linear steels ampliﬁer by wire cutting excitation systemfollowed for comparison by stacking with so that the the usual thickness iron lossof the in core the was sinusoidal 7 mm. condition.

Inverter part Primary current Oscilloscope Ring core V dc SW I

V2 V1

Wave Linear Secondary winding generator Amplifier Primary winding Primary voltage Figure 5. Measurement system using inverter excitation and linear ampliﬁer excitation. Figure 5. Measurement system using inverter excitation and linear amplifier excitation.

In3.2. both Results the of measurements Measurement under with Inverter PWM Excitation excitation Condition of inverter and with sinusoidal excitation, the fundamental frequency was 50 Hz and the maximum flux density was 1.5 T. In the inverter excitation Figure 6 shows the waveforms of the applied voltage, excitation current, and flux density. Under measurement, the carrier frequency was changed between 500 Hz and 20 kHz, and the modulation the inverter excitation condition, the rectangular voltage waveform generated by the inverter caused index was changed between 0.1 and 1.0. sharp changes in current, resulting in high frequency components in the flux density waveform. Here, Asthe sampleshigh frequency for magnetic components measurement, were twicering the carrier core samples frequency. having an outer diameter of 80 mm and inner diameter of 60 mm were prepared. The turn numbers of the primary and secondary windings were 150 and 100, respectively. The flux density (B) of the core was measured by the output voltage of the secondary coil, and the strength of the magnetic field (H) was obtained by the current measured by the current sensor. Iron losses were derived by calculating the inner area of the B-H loops. Electrical steels containing 3% silicon having sheet thicknesses of 0.50 mm, 0.35 mm, and 0.25 mm were prepared in the laboratory. Ring cores were prepared using these electrical steels by wire cutting followed by stacking so that the thickness of the core was 7 mm.

3.2. Results of Measurement under Inverter Excitation Condition Figure6 shows the waveforms of the applied voltage, excitation current, and flux density. Under the inverter excitation condition, the rectangular voltage waveform generated by the inverter caused sharp changes in current, resulting in high frequency components in the flux density waveform. Here, the high frequency components were twice the carrier frequency. In the voltage waveform (a), ON-voltage [8,9], which is the opposite voltage component to the applied voltage can be recognized. As the ON-voltage keeps constant values at different applied voltages, the ratio of the ON-voltage with respect to the applied voltage is larger in cores with smaller cross-sectional areas [10]. For this reason, the heights of the tested ring cores were controlled to be the same in this study. Figure7 shows the dependence of iron loss on the sheet thickness under the usual sinusoidal excitation and inverter PWM excitation. As it is well known, iron loss increases with sheet thickness under sinusoidal excitation. In the inverter PWM excitation, the iron losses were larger than the losses in the sinusoidal condition, and the increasing tendency of iron loss with respect to the sheet thickness was prominent compared to that under sinusoidal excitation. From this result, under actual inverter-fed conditions, thinner gauge materials are more advantageous than thicker materials to suppress motor loss increase. Figure8 shows the dependences of iron loss on the carrier frequency under the inverter PWM excitation (modulation index: 0.4) for the materials with different thicknesses. World Electric Vehicle Journal 2019, 10, 31 6 of 10 World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 6 of 10

20 Applied voltage 15 (a) 10 5 0 0 5 10 15 20 -5 -10 On-voltage

Primary voltage (V) -15 -20 1.5 (b) Sinusoidal excitation 1.0 Inverter PWM excitation 0.5 0.0 0 5 10 15 20 -0.5 -1.0 -1.5

Primary current (A) 2.0 1.5 (c) 1.0 0.5 0.0 0 5 10 15 20 -0.5 -1.0

Flux density Flux density (T) -1.5 World Electric Vehicle Journal 2019, 10-2.0, x FOR PEER REVIEW 7 of 10 Time (ms) in Figure 11 as a function of the modulation index. In the measurements to obtain the data for Figure in FigureFigureFigure 11 6. as WaveformsWaveforms a function of of (a the) voltage, modulation ((bb)) current,current, index. andand In ((cc )the) ﬂuxflux measurem density, density at, atents maximum maximum to obtain ﬂux flux densitythe density data of offor 1.5 1.5 Figure T, 10, theTfundamental, fundamentalpeak flux frequencydensity frequency was of of 50 co 50 Hz,ntrolled Hz, carrier carrier to frequency be frequency 1.5 T ofby of 1 changing kHz, 1 kHz, and and modulationthe modulation DC voltage index index of of the 0.4.of 0.4. inverter.

5 In the voltage waveform (a), ON-voltage [8,9], which is the opposite voltage component to the Inverter PWM applied voltage can be recognized.4 As the ON-voltage keeps constant values at different applied voltages, the ratio of the ON-voltage with respect to the applied voltage is larger in cores with smaller cross-sectional areas [10]. For this3 reason, the heights of the tested ring cores were controlled to be the same in this study. 2 Figure 7 shows the dependence of iron loss on theSinusoidal sheet thickness under the usual sinusoidal

Iron loss (W/kg) excitation and inverter PWM excitation.Iron loss (W/kg) As it is well known, iron loss increases with sheet thickness 1 under sinusoidal excitation. In the inverter PWM excitation, the iron losses were larger than the losses in the sinusoidal condition, and the0 increasing tendency of iron loss with respect to the sheet thickness 0.1 0.2 0.3 0.4 0.5 0.6 was prominent compared to that under sinusoidalSheet thickness excitation. (mm) From this result, under actual inverter- Sheet thickness (mm) fed conditions, thinner gauge materials are more advantageous than thicker materials to suppress Figure 7. Dependence of iron loss on sheet thickness comparing sinusoidal and inverter excitation. motorFigure loss increase. 7. Dependence of iron loss on sheet thickness comparing sinusoidal and inverter excitation. Maximum flux density: 1.5 T, fundamental frequency: 50 Hz, carrier frequency: 1 kHz, modulation FigureMaximum 8 shows ﬂuxflux density: the dependences 1.5 T, fundamentalfundamental of iron frequency: loss on the 50 carrier Hz, carrier frequency frequency: under 1 kHz, the modulationmodul inverteration PWM index: 0.4. excitationindex: (modulation 0.4. 0.4. index: 0.4) for the materials with different thicknesses.

It is noted that an increase5 in iron loss occurs at low carrier frequencies, and this tendency is obvious in thicker materials. The differences between materials0.50 having mm different thicknesses are significant at lower carrier frequencies. Figure 9 shows the ratios0.35 of themm losses between the materials 0.25 mm with different sheet thicknesses.4 Here, W(0.25), W(0.35), and W(0.50) mean the losses measured with materials having thicknesses of 0.25 mm, 0.35 mm, and 0.50 mm, respectively. As can be seen in Figure 9, the advantage of the 0.25 mm thick material in comparison with the 0.50 mm material was large, especially at low carrier3 frequencies. However, this tendency is not so apparent in the

Iron loss (W/kg) comparison between the 0.35 Iron loss (W/kg) mm thick material and the 0.25 mm thick material. Figure 8 also shows the iron losses under sinusoidal condition. The carrier frequency in the inverter-driven condition at 20 2kHz is so high that the appearance of the waveform looks close to the 0 5 10 15 20 Sinu- waveform in the sinusoidal condition. However, the iron losses at the soidalcarrier frequency of 20 kHz are Carrier frequency (kHz) considerably higher than the losses under the sinusoidal condition. This means the increment of iron loss dueFigure to inverter8. DependenceDependence excitation of iron iron should loss loss on on becarrier carrier considered frequency frequency in underunder loss evaluations PWM PWM excitation. excitation. of EV Maximum Maximum motors. fluxﬂux density: density: The1.5 T, dependence fundamentalfundamental frequency:frequency: of iron loss 50 Hz,Hz, on modulationthemodulation modulation index:index: index 0.4. is shown in Figure 10 for the carrier frequency of 1.0 kHz. The ratios of the iron losses of materials with different thicknesses are shown 1.7 W(0.35)/W(0.25) 1.6 W(0.50)/W(0.25) 1.5 1.4 1.3 1.2

Ratio of iron loss Ratio iron of

Ratio of iron loss Ratio iron of 1.1 1.0 0 5 10 15 20 Carrier frequency (kHz) Carrier frequency (kHz) Figure 9. Ratio of iron loss of different thickness materials with respect to carrier frequency. Measurement conditions are the same as in Figure 8. World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 7 of 10

in Figure 11 as a function of the modulation index. In the measurements to obtain the data for Figure 10, the peak flux density was controlled to be 1.5 T by changing the DC voltage of the inverter.

5 Inverter PWM 4

2 Sinusoidal Iron loss (W/kg) 1

0 0.1 0.2 0.3 0.4 0.5 0.6 Sheet thickness (mm) Figure 7. Dependence of iron loss on sheet thickness comparing sinusoidal and inverter excitation. Maximum flux density: 1.5 T, fundamental frequency: 50 Hz, carrier frequency: 1 kHz, modulation index: 0.4.

5 0.50 mm 0.35 mm 0.25 mm World Electric Vehicle Journal 2019, 10, 314 7 of 10

It is noted that an increase3 in iron loss occurs at low carrier frequencies, and this tendency is

obvious in thicker materials.Iron loss (W/kg) The differences between materials having different thicknesses are significant at lower carrier frequencies. Figure9 shows the ratios of the losses between the materials with different sheet thicknesses.2 Here, W(0.25), W(0.35), and W(0.50) mean the losses measured with 0 5 10 15 20 Sinu- materials having thicknesses of 0.25 mm, 0.35 mm, and 0.50 mm, respectively. As can be seen in Carrier frequency (kHz) soidal Figure9, the advantage of the 0.25 mm thick material in comparison with the 0.50 mm material was large,Figure especially 8. Dependence at low carrier of iron frequencies. loss on carrier However, frequency this under tendency PWM isexcitation. not so apparent Maximum in theflux comparisondensity: between1.5 T, the fundamental 0.35 mm thick frequency: material 50 Hz, and modulation the 0.25 mm index: thick 0.4. material.

1.7 W(0.35)/W(0.25) 1.6 W(0.50)/W(0.25) 1.5 1.4 1.3 1.2

Ratio of iron loss Ratio iron of 1.1 1.0 0 5 10 15 20 Carrier frequency (kHz) FigureFigure 9. 9. Ratio of iron loss of didifferentﬀerent thickness thickness materials materials with with respect respect toto carrier carrier frequency. frequency. MeasurementMeasurement conditions conditions are are the the same same as as in in Figure Figure8. 8.

Figure8 also shows the iron losses under sinusoidal condition. The carrier frequency in the inverter-driven condition at 20 kHz is so high that the appearance of the waveform looks close to the waveform in the sinusoidal condition. However, the iron losses at the carrier frequency of 20 kHz are considerably higher than the losses under the sinusoidal condition. This means the increment of iron loss due to inverter excitation should be considered in loss evaluations of EV motors. The dependence of iron loss on the modulation index is shown in Figure 10 for the carrier frequency of 1.0 kHz. The ratios of the iron losses of materials with diﬀerent thicknesses are shown in Figure 11 as a function of the modulation index. In the measurements to obtain the data for Figure 10,

Worldthe peak Electric ﬂux Vehicle density Journal was 2019 controlled, 10, x FOR PEER to be REVIEW 1.5 T by changing the DC voltage of the inverter. 8 of 10

13 0.50 mm 11 0.35 mm 0.25 mm 9

Iron loss (W/kg) 3

1 0 0.2 0.4 0.6 0.8 1.0 Sinu- soidal Modulation index Figure 10. DependenceDependence of of iron iron loss on modulation index under PWM excitation. excitation. Maximum Maximum flux ﬂux density: 1.5 1.5 T, T, fundamental frequency: 50 50 Hz, Hz, carrier carrier frequency: frequency: 1.0 1.0 kH kHz.z.

1.8 1.7 W(0.35)/W(0.25) 1.6 W(0.50)/W(0.25) 1.5 1.4 1.3 1.2

Ratio of iron loss iron of Ratio 1.1 1.0 0.9 0 0.2 0.4 0.6 0.8 1.0 Modulation index Figure 11. Ratio of iron loss of different thickness materials with respect to modulation index. Measurement conditions are the same as in Figure 10.

As can been in Figure 10, iron loss under PWM excitation increased markedly with the decrease in the modulation index. From Figure 11, it is obvious that thinner materials are more advantageous at lower modulation indexes. Under actual EV driving conditions, the output voltage (effective value) of the inverter is controlled by changing the modulation index. On the other hand, in the measurements of this study, the DC voltage was changed to achieve the targeted maximum flux density. Therefore, the data in Figure 11 do not show individual influence of the modulation index, but indicate the contribution of the modulation index when the DC voltage is adjusted cooperatively to obtain certain torque values. The increase in iron loss under inverter excitation is attributed to the fact that the flux density waveforms in the core contain higher frequency components. Therefore, the increment of loss due to inverter excitation mainly originates from eddy current loss, resulting in the improvement of loss by thinner materials. Although the frequencies of the harmonic components are low under low carrier frequencies, the loss increases are significant because the amplitude of the harmonics is high. Under low modulation index conditions, higher voltages are output in a short time, resulting in a larger amplitude of higher harmonics. This section has described the results of an analysis of iron loss under inverter excitation. It was found that iron loss increased more significantly in thicker materials under inverter excitation than under conventional sinusoidal waveform conditions. The increase of iron loss becomes larger at lower carrier frequencies and lower modulation indexes. Considering these results, thinner materials are advantageous for improving motor efficiency in EV motors driven by inverters.

4. Electrical Steels for EV Traction Motors As described above, high material flux density while keeping low iron loss is needed to achieve high torque and high efficiency in EV traction motors. Furthermore, for lower iron losses in high World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 8 of 10

13 0.50 mm 11 0.35 mm 0.25 mm 9

Iron loss (W/kg) 3

1 0 0.2 0.4 0.6 0.8 1.0 Sinu- soidal Modulation index WorldFigure Electric Vehicle 10. Dependence Journal 2019 , of10 ,iron 31 loss on modulation index under PWM excitation. Maximum flux8 of 10 density: 1.5 T, fundamental frequency: 50 Hz, carrier frequency: 1.0 kHz.

1.8 1.7 W(0.35)/W(0.25) 1.6 W(0.50)/W(0.25) 1.5 1.4 1.3 1.2

Ratio of iron loss iron of Ratio 1.1 1.0 0.9 0 0.2 0.4 0.6 0.8 1.0 Modulation index FigureFigure 11. RatioRatio of of iron iron loss loss of of different diﬀerent thickness thickness materials materials with with respect respect to to modulation modulation index. index. MeasurementMeasurement conditions are the same as in Figure 10.10.

AAss can can been been in in Figure Figure 10,10, iron iron loss loss under under PWM PWM excitation excitation increased increased markedly markedly with with the the decrease decrease inin the the modulation index.index. From FigureFigure 1111,, itit isis obviousobvious that that thinner thinner materials materials are are more more advantageous advantageous at atlower lower modulation modulation indexes. indexes. UnderUnder actual actual EV EV driving driving conditions, conditions, the outputthe output voltage voltage (effective (effective value) of value) the inverter of the is inverter controlled is controlledby changing by thechanging modulation the modulation index. On index. the other On the hand, other in hand, the measurements in the measurements of this study,of this thestudy, DC thevoltage DC wasvoltage changed was changed to achieve to the achieve targeted the maximum targeted maximum flux density. flux Therefore, density. theTherefore, data inFigure the data 11 doin Figurenot show 11 individualdo not show influence individual of the influence modulation of the index, modulation but indicate index, the but contribution indicate the of contribution the modulation of theindex modulation when the index DC voltage when isthe adjusted DC voltage cooperatively is adjusted to cooperatively obtain certain to torque obtain values. certain torque values. TheThe increase increase in in iron iron loss under inverter inverter excitation excitation is is attributed attributed to to the the fact fact that that the the flux flux density density waveforms in the core contain higher frequency components. Therefore, Therefore, the the increment increment of of loss loss due due to to inverterinverter excitation excitation mainly mainly originates originates from from eddy eddy current current loss, loss, resulting resulting in in the the improvement improvement of of loss loss by by thinnerthinner materials. materials. Although Although the the frequencies frequencies of of the the harmonic harmonic components components are are low low under under low low carrier carrier frequencies,frequencies, the the loss loss increa increasesses are significant significant because the amplitude of the harmonics is high. Under Under lowlow modulation modulation index index conditions, conditions, higher higher voltages voltages are are output output in in a a short short time, time, resulting resulting in in a a larger larger amplitudeamplitude of higher harmonics. ThisThis section section has described described the results results of an analysi analysiss of iron loss under inverter excitation. It It was was foundfound that that iron iron loss increased more significantly significantly in in thicker materials under under inverter inverter excitation excitation than than underunder conventional sinusoidal sinusoidal waveform waveform conditions. conditions. The The increase increase of iron of iron loss lossbecomes becomes larger larger at lower at lowercarrier carr frequenciesier frequencies and lower and lower modulation modulation indexes. indexes. Considering Considering these these results, results, thinner thinner materials materials are areadvantageous advantageous for for improving improving motor motor effi ciencyefficiency in EV in motorsEV motors driven driven by inverters.by inverters.

4.4. Electrical Electrical Steels Steels for for EV EV Traction Traction Motors AsAs described described above, above, high high material material flux flux density density while while keeping keeping low low iron iron loss loss is is needed needed to to achieve achieve highhigh torque torque and high efficiency efficiency in EV traction motors. Furthermore, Furthermore, for for lower lower iron iron losses losses in in high high frequency excitation conditions, especially when an inverter driven system is used, thinner gauge materials are advantageous because the eddy current loss can be suppressed effectively. As mentioned earlier, the advantage of the high flux density material was confirmed in the motor evaluation using the JNE®, which has higher material flux density than conventional materials for the stator core of BLDC. Based on these results, a new material having higher flux density than the JNE series, which was named JNP®, were developed by using a texture (grain orientation distribution) control technique. As thinner gauge materials which are advantageous under inverter driven conditions, 30JNE, 25JNE, and 20JNEH, whose thicknesses are 0.30 mm, 0.25 mm, and 0.20 mm, respectively, have been developed by applying the texture control techniques developed in JNE. Figure 12 shows a map of the magnetic properties by iron loss W10/400 and material flux density B50. Here, the loss at 400 Hz, which was confirmed to be a suitable indicator for material evaluation World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 9 of 10 frequency excitation conditions, especially when an inverter driven system is used, thinner gauge materials are advantageous because the eddy current loss can be suppressed effectively. As mentioned earlier, the advantage of the high flux density material was confirmed in the motor evaluation using the JNE® , which has higher material flux density than conventional materials for the stator core of BLDC. Based on these results, a new material having higher flux density than the JNE series, which was named JNP® , were developed by using a texture (grain orientation distribution) control technique. As thinner gauge materials which are advantageous under inverter driven conditions, 30JNE, 25JNE, and 20JNEH, whose thicknesses are 0.30 mm, 0.25 mm, and 0.20 mm, respectively, have been developedWorld Electric by Vehicleapplying Journal the2019 texture, 10, 31 control techniques developed in JNE. 9 of 10 Figure 12 shows a map of the magnetic properties by iron loss W10/400 and material flux density B50. Here, the loss at 400 Hz, which was confirmed to be a suitable indicator for material evaluation for EV motors, was chosen for the horizontal axis. As can be seen in Figure 12, the newly developed for EV motors, was chosen for the horizontal axis. As can be seen in Figure 12, the newly developed JNP [2,11] and thinner gauge materials (30JNE, 25JNE, 20JNEH [2]) have expanded the variety of JNP [2,11] and thinner gauge materials (30JNE, 25JNE, 20JNEH [2]) have expanded the variety of possible choice of core materials. Traction motors are designed so that the motor attains the traveling possible choice of core materials. Traction motors are designed so that the motor attains the traveling performance demanded in EV. These newly developed electrical steels, which cover a wide range of performance demanded in EV. These newly developed electrical steels, which cover a wide range of improved properties can contribute to improvement of EV performance through appropriate material improvedselection properties for each motor can design. contribute to improvement of EV performance through appropriate material selection for each motor design. 1.72 35JNE series ] (T) 35JNP

B 1.70 35JN series (Conventional)

1.68 25JNE 30JNE

1.66 20JNEH

Flux density at 5000 [ A/m 1.64 10 15 20 25 Iron loss at 1.0 T and 400 Hz [W10/400] (W/kg)

Figure 12. Map of magnetic properties by iron loss W10/400 and material flux density B50. Figure 12. Map of magnetic properties by iron loss W10/400 and material flux density B50. 5. Conclusions 5. Conclusion The influence of the magnetic properties of electrical steels on the motor performance was investigatedThe influence by using of the laboratory magnetic BLDC properties motor andof electrical the material steels evaluation on the motor under PWM performance excitation. was investigatedThe following by using conclusions laboratory were obtained.BLDC motor and the material evaluation under PWM excitation. The foll(1)owing Higher conclusions frequency were iron loss obtained. than fundamental frequency of motors, such as 400 Hz in this study, is(1) a dominant Higher frequency iron loss which iron decidesloss than effi fundamentalciencies of BLDC frequency motors. of motors, such as 400 Hz in this study, is(2) a dominant Core materials iron havingloss which higher decide fluxs densities efficiencies are of advantageous BLDC motors. for achieving higher torque ® constants.(2) Core Electricalmaterials steels having that higher show high flux flux densities density are and advantageous low iron loss, such for asachieving JNE , realizes higher higher torque constants.efficiencies Electrical while keepingsteels that higher show toques. high flux density and low iron loss, such as JNE®, realizes higher efficiencies(3) Iron while losses keeping are significantly higher toques. high in thicker materials under the PWM excitation with inverter. Iron losses increase with the decrease in carrier frequencies under a constant DC voltage, and this (3) Iron losses are significantly high in thicker materials under the PWM excitation with inverter. tendency is significant in thicker materials. Iron losses increase with the decrease in carrier frequencies under a constant DC voltage, and this (4) A remarkable increase of iron loss appears in lower modulation under a variable DC voltage tendency is significant in thicker materials. condition, and this tendency is marked in thicker materials. (4) TheA remarkable results mentioned increase above of iron shows loss appear that thes materialsin lower modulation having higher under flux a density variable and DC thinner voltage condition,gauge are and advantageous this tendency to achieving is marked high in performancesthicker materials. in motors. Based on this fact, high flux density electricalThe results steels mentioned named JNP above®, and shows thinner that gauge the electrical material steelss having such higher as 30JNE, flux 25JNE, density and and 20JNEH, thinner gauinge which are advantageous texture control to techniques achieving were high applied, performance were developed.s in motors. These Based materials on this can contributefact, high to flux densityimproved electrical performances steels named of EV JNP traction®, and motors thinner through gauge appropriate electrical coresteels material such as selection. 30JNE, 25JNE, and

Author Contributions: Investigation, K.S., M.U., S.Y. and Y.O. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest. World Electric Vehicle Journal 2019, 10, 31 10 of 10

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