Yukio Honda a, Member Yoji Takeda, Member

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1 TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING IEEJ Trans 27; 2: Published online in Wiley InterScience ( DOI:1.12/tee.2118 Review Technical Evolution of Permanent Magnet Synchronous Motors for Home Appliances Yukio Honda a, Member Yoji Takeda, Member This paper reports on the highly efficient motor technologies used in home appliances in Japan. The permanent magnet synchronous motor (PMSM) is especially suitable because the use of permanent magnets does not require any extra current to produce magnetic power in the rotor, or any other kind of energy. In Japan, there has been a rapid shift from induction motors to PMSMs, and in this paper we will show several examples of PMSMs as applied to the home appliance field. It can be seen that great improvements have been made in high-efficiency motor technologies. 27 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. Keywords: permanent magnet synchronous motor, surface permanent magnet synchronous motor, interior permanent magnet synchronous motor, double layer permanent magnet, sinusoidal current drive, air-conditioner, washing machine, energy saving, variable speed operation Received 25 October 26; Revised 1 November Introduction In the past few years, the awareness of environmental problems has grown dramatically worldwide, and tremendous interest is shown in developing highly efficient motor technologies and their applications. In particular, energy saving programs concerning the consumption of electricity have become very important issues in realizing a sustainable society. Figure 1 shows the consumption of electricity in Japan. Electric motors consume 5% or more of the total electricity. Improving the efficiency of each electric motor by only 1% could eliminate the need for a nuclear power plant with an output capability as high as approximately 5 kwh. Therefore improving motor efficiency is a very important issue from the points of view of preventing an energy crisis and preserving the global environment. On the other hand, Figure 2 shows the household electric power consumption in Japan. Air-conditioners and refrigerators use up to 46% of the household electric power consumption. They need compressors and fans that use electric motors and also usually use induction motors. But recently there has a Correspondence to: Yukio Honda. Honda.yukio@jp.panasonic.com Matsushita Electric Industrial Company, Limited, 16 Kadoma, Kadoma City, Osaka , Japan Osaka Prefectural College of Technology, 26-12, Saiwaicho, Neyagawa, Osaka , Japan been a dramatic change from induction motors to permanent magnet synchronous motors (PMSMs) because using permanent magnets does not require any extra current to produce magnetic power in the rotor. Other home appliances also use a large number of electric motors. Figure 3 shows the motor performance in the home appliance field and the shaded rectangles show the operating area. As can be seen, many motors require variable speed operation and this is why we can achieve energy savings. We have found that the PMSM is also suitable for variable speed operation [1]. In some cases, surface permanent magnet synchronous motors (SPMSMs) are more suitable, and in other cases interior permanent magnet synchronous motors (IPMSMs) would be suitable. In this paper, we will report on examples of the highly efficient PMSM technologies for home appliances. 2. Technical Evolution of Compressor Motors for Air-conditioners A PMSM is capable of highly efficient operation because it does not require any extra current to produce magnetic power in the rotor, owing to the fact that permanent magnets generate the magnetic field. Compressor motors use SPMSM with an SUS pipe wherein the magnets are fixed. In compressor motors for air-conditioners and refrigerators, materials that might melt in the refrigerant are not allowed to be used because that would affect the reliability. Unfortunately, the SUS pipe produces an 27 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

2 MOTOR TECHNOLOGIES FOR HOME APPLIANCES Others % % Stator q-axis d-axis Heating % 91 bkwh Lighting % Motors Fig. 1 Consumption of electricity in Japan Magnet Rotor Fig. 4 Definition of d- andq-axes q-axis Others 26% Air-Conditioner 29.6% R a I a V a β v q ψ L d i d Electric Carpet 2.6% TV 9.6% 2B kwh Lighting 15.8% Refrigerator 16.4% ωψ v d I a δ i d i q ψ a L q i q d-axis Fig. 2 Household electric power consumption in Japan Fig. 5 Vector diagram 1, A/C Fan motor Air-conditioner & Refrigerator Compressor Motor magnets that permeates the vacant spaces was assumed to be sinusoidal, and the high harmonic component of the armature current was ignored [1,2]. Motor output (W) Washing machine motor Vacuum cleaner motor T = P n {ψ a i q + (L d L q )i d i q } = P n ψ a I a cos β (L q L d )I 2 a sin 2β (1) V a = (R a i d ωl q i q ) 2 +(R a i q + ωl d i d + ωψ a ) 2 (2) 1K 2.5K 5K 3K Motor Speed (min 1 ) 5K Fig. 3 Motor characteristics of home appliances eddy current loss. Since the IPMSM does not need any SUS pipe because it has a permanent magnet embedded in the rotor itself, it is suitable for use in compressor motors. Figure 4 shows a definition of d- andq-axes. Figure 5 plots the vector diagram of an IPMSM at the steady state. The torque and terminal voltage of the IPMSM are computed by plugging the d- and q-coordinates from this vector diagram into (1) and (2). For the sake of simplicity, the distribution flux from the permanent In the above, T = output torque ψ a = armature interlinkage flux from permanent magnets expressed in terms of d q coordinates V a = terminal voltage L d = d-axis inductance L q = q-axis inductance I a = armature current amplitude expressed in terms of d q coordinates i d,i q = armature current d- andq-axis components β = armature current lead angle from the q-axis expressed in terms of d q coordinates R a = armature resistance per armature winding P n = number of poles ω = electric angle speed. 119 IEEJ Trans 2: (27)

3 Y. HONDA AND Y. TAKEDA The first term in (1) represents magnetic torque from the permanent magnets, whereas the second term represents the reluctance torque derived from the difference in d- and q-axis inductance. From this equation, it is evident that in order to increase the torque generated by a set electric current, it should be sufficient either to increase the interlinkage flux from the permanent magnets ψ a or to increase the difference parameter (L q L d ) between the d- andq-axis inductances L d and L q.however, because these parameters are also functions of the terminal voltage V a from (2), if the inverter capacity is limited, simply increasing these parameters will keep the possible operating range under constant torque at a minimum. This reluctance torque is an advantage compared to SPMSM with the SUS (stainless steel) pipe. To enable air-conditioners to operate in a wide range of capacities, compressor motors require high efficiency over a wide operating range. The IPMSM is suitable for use in this area. In allowing the use of reluctance torque, IPMSM demonstrated a lower copper loss. In addition, it does not require SUS pipes to fix the magnets, which means one need not worry about the SUS pipe s iron loss. Figure 6 shows the structure of a scroll compressor. The scroll compression section efficiently compresses the refrigerant by the rotating rotor of the IPMSM. Figure 7 shows two rotor structures of the IPMSM as applied to an air-conditioner s compressor. One uses a ferrite magnet, the other a rare earth magnet. The magnetic power of a ferrite magnet is smaller than that of the rare earth one. Consequently we developed the double layer structure IPMSM using ferrite magnets. The double layer structure tends to increase the L q inductance. The efficiency of the double layer IPMSM using ferrite magnets is the same as that of a single layer IPMSM using NeFeB magnets. The double layer IPMSM can produce more reluctance torque than the single layer IPMSM. As a result, as shown in Inhalation Pipe Scroll Compression Section Stator Rotor of IPMSM Magnetic Wire Fig. 6 Structure of a scroll compressor Discharge Pipe Ferrite magnets (red&blue colors) 1 (%) 95 Motor Efficiency Fig Double layer Single layer Fig. 7 Rotor structures of IPMSM Induction Motor SPMSM IPMSM Motor Efficiency 86 Energy Consumption NeFeB magnets 2 1 (W) Energy Consumption History of improvement of the compressor motor s efficiency Figure 8, motor efficiency improved sharply and resulted in a large energy saving [3]. 3. Technical Improvements on the Fan Motors of Air-conditioners The IPMSM has a number of advantages. However, it also tends to suffer from sound and vibration problems due to the buried permanent magnets in the rotor. For the IPMSM, the outer periphery of the rotor facing the inner periphery of the stator is usually covered with a steel layer of high magnetic permeability. The permeance of the air gap therefore tends to increase, causing higher vibration and noise than the SPMSM. So airconditioner fan motors usually use SPMSM, especially those requiring very low noise levels. Since SPMSM requires a higher efficiency, the winding method and sinusoidal current drive systems have been improved. Figure 9 shows a highly efficient SPMSM fan motor using the new winding method, which we call the armadillo winding. In Section 4, we will show how the armadillo winding can make a difference in washing machines. Figure 1 compares a conventional winding with the armadillo winding. As a result of using a 12 IEEJ Trans 2: (27)

4 MOTOR TECHNOLOGIES FOR HOME APPLIANCES Fan motor Armadillo winding Torque (Nm) 6. 79% % 5. 83% 85% % 89% 91% Fig. 9 Fan motor with the armadillo winding 2. Conventional Armadillo Winding Speed (min 1 ) Fig. 12 Map of the efficiency of the armadillo fan motor Slot fill factor 45% Slot fill factor 7% (a) Low speed region Input Voltage Back EMF Winding Current Fig. 1 Comparison of winding methods Torque (Nm) 6. 75% 77% % 81% 5. 83% % 87% 89% Speed (min 1 ) Fig. 11 Map of the efficiency of the conventional fan motor (b) High speed region Input Voltage Back EMF Winding Current Fig. 13 Relationship between a current waveform and the back- EMF at the low- and high-speed regions Common Bias Current separated stator core, armadillo winding can improve the slot-fill factor. Also, by increasing the slot-fill factor, copper loss is decreased, and then the high-efficiency driving area is also increased. Figure 11 is a map of the efficiency of a conventional fan motor. The highest efficiency is 89%. Figure 12 is a map of the efficiency of the armadillo fan motor, and here we achieved approximately 91% efficiency. By comparing the efficiency areas of the two motors, we can see the difference in efficiency. One of the major problems in the design of fan motor is to keep the noise level down while not sacrificing the high efficiency. A problem of the utmost importance is to make the phase of winding current and back-emf in agreement for an efficient motor drive. In a sinusoidal current drive, this becomes much more important. Figure 13 shows this relationship between a current waveform and the back- EMF waveform. As can be seen in Figure 13(a), the U-phase Current W-phase Current (inversion) V-phase Current W-phase Current Fig. 14 Relationship between U-, V -, and W -phase current and common bias current phase of the back-emf can match the winding current automatically at the low-speed range. However, at the high-speed range, the phase of the back-emf cannot match the winding current without a phase adjustment because of the influence of the inductance (Figure 13(b)). Figures 13(a) and (b) describe these conditions. It is very 121 IEEJ Trans 2: (27)

5 Y. HONDA AND Y. TAKEDA Supply Voltage Cont Control Circuit ui t P opower r Circuit ui t Winding Current Control Supply Voltage Speed Control Command Rotation speed output PWMM Cont Controller le r Cur Currentren t Phase Controller C o ntr o lle r Motor r Current Sense Resistor Common Current Fig. 15 Diagram of a sinusoidal current drive system difficult to match the phase of the back-emf with the winding current phase considering the cost of the electro circuits. In this case, expensive current sensors would be needed. So we have developed a new phase adjustment algorithm without the need for expensive current sensors. We use an over-current sense resistor that is normally used in a fan motor drive circuit. As shown in Figure 14 (top and bottom), at the common current waveform, this up and down waveform is buried in the two phase current waveforms. The common current is detected by the overcurrent sensor and we can distinguish two phases, and by using the current phase controller we can distinguish the other phase. Figure 15 shows the block diagram of a new current drive system. One can see the current sense resistor at the common current line. Figure 16 shows a real circuit for the drive system of an air-conditioner fan motor. Figure 17 shows the result of the improvement in the current angle adjustment algorithm [1,4 6]. The new current angle adjustment algorithm can maintain the efficiency even at the high-speed arange. 4. Technical Improvement in the Motor of the Washing Machine By using the IPMSM technique, washing machines also showed an improvement in energy savings. We Motor Efficiency (%) Fig Output Torque (Nm) with Current Phase s Control t without Current Phase Control Rotation Speed (min -1 ) Test results of the improvement of the current angle adjustment algorithm Washing mode Low rotation speed / High torque Spin-dry mode High rotation speed / lower torque Power MOSFET Level shift IC 5 Rotation speed (min 1 ) Fig. 18 Requirements of a washing machine s motor Sine wave drive IC Fig. 16 IC circuit of a sinusoidal current drive system changed the drive motor from a conventional motor with a reduction gear to a direct drive motor using the IPMSM with a concentrated winding. Figure 18 shows the requirements of a washing machine s motor. A wide driving range is required and, furthermore, a high torque is necessary during low-speed rotation. We found that the IPMSM is suitable for use in this application. Figure IEEJ Trans 2: (27)

6 MOTOR TECHNOLOGIES FOR HOME APPLIANCES Armadillo core U-phase nozzle V-phase nozzle Winding direction Stator tooth W-phase nozzle Nozzle attachment of winding (a) Armadillo winding method. countermeasure magnet Fig. 2 Stator core shape for reducing the cogging torque molding stator (b) After winding. molding IPMSM rotor Fig. 21 Molding core shapes of the armadillo IPMSM (c) Finished core. Fig. 19 Armadillo winding method shows the armadillo winding method and their structures. Figure 19(a) shows the winding method. Three nozzles are wound as illustrated U-, V -andw -phase against the stator core. Figures 19 (b) and (c) show the stator core after winding and a finished winding core, respectively. As mentioned earlier, the IPMSM has the disadvantages of high noise and vibration levels, especially when used with a concentrated winding. Figure 2 shows the countermeasure we applied to reduce the cogging torque. The front edge of each stator tooth is kept away from the magnet embedded in the rotor. This new stator shape has been successful in reducing the cogging torque. It should be noted that this motor requires a waterproof structure and a molding made of a resin. Figure 21 shows the IPMSM molding rotor and the molded stator core. Figure 22 shows the test results conducted to reduce the cogging torque. As can be seen, the cogging torque was sharply reduced from 4.5 Nm to 1 Nm. Consequently, the electric power loss of the motor in the washing and spin-dry modes, which had been 32 W conventionally, was sharply reduced to 7 W [7]. 5. Conclusions This paper was prepared to report on increasing the efficiency of motor technologies with regard to the home Cogging torque (Nm) Cogging torque (Nm) Rotor rotation angle (deg.) (a) Before improvement Rotor rotation angle (deg.) (b) After improvement Fig. 22 Test results concerning the reduction in cogging torque appliance field in Japan. In particular, it was shown that the use of PMSMs is a suitable alternative because it does not require any extra current to produce magnetic power in the rotor, or other forms of energy. In Japan, there has been a rapid change over from induction motors to PMSMs. This report illustrates many 123 IEEJ Trans 2: (27)

7 Y. HONDA AND Y. TAKEDA examples of PMSMs as applied to the home appliance field. Nowadays, all the compressor motors for air-conditioners have use IPMSM. This is because the IPMSM excels when compared to all other motors in respect of performance and operation in the variable speed mode. Through the evolution of IC technology, the sinusoidal current drive has been put to practical use. Also, from the findings contained in this report, it can be reasonably expected that the ongoing replacement of conventional induction motors by highly efficient SPMSM and IPMSM will be accelerated. The improvement in motor efficiency is leading to substantial reduction in energy consumption. With the advent of IPMSM and the sinusoidal drive, the evolution of motor structures and drive methods is rapidly reaching its full potential. So from now on, we will have to expend more energy on improving the performance of materials used in the construction of motors. References (1) Takaeda Y, Matsui N, Morimoto S, Honda Y. Interior Permanent Magnet Synchronous Motor. Ohmsha, Ltd.: Tokyo, Japan, 21. (2) Morimoto S, Tong Y, Takeda Y, Hirasa T. Loss minimization control of permanent magnet synchronous motor drives. IEEE Transactions on Industry Electronics 1994; IE-41(5): (3) Honda Y, Higaki T, Morimoto S, Takeda Y. Rotor design optimization of a multi-layer interior permanent-magnet synchronous motor. IEE Proceedings of Electric Power Applications 1998; 145(2): (4) Jahns TM. Flux-weakening regime operation of an interior permanent-magnet synchronous motor drive. IEEE Transactions on Industry Applications 1987; IA-23(4): (5) Miller TJE. Brushless Permanent-Magnet and Reluctance Motor Drives. Oxford Science Publications: New York, USA, (6) Sugiura K, Yasohara M, Yamamoto M. High performance brushless fan motor for air conditioner applications. Matsushita Technical Journal 25; 51(1): (7) Kai T, Ikami T. Direct drive brushless motor for electric drum-type washer dryer applications. Matsushita Technical Journal 25; 51(1):3 33. Dr Yukio Honda was born in Osaka, Japan, in He received his B.E. and Ph.D. degrees from Kobe University and Osaka Prefectural University, respectively. He worked at Denso Co. Ltd, for 1 years carrying out small motor design for automotive applications. Then he joined the Matsushita Electric Industrial Co. Ltd, in 1989, and recently he has been working at the Robotics Development Office specializing in small motors for life-assist robots. His research interests are motor design, motor control, and life-assist robots. Dr Honda is a member of the Institute of Electrical Engineers of Japan and the Institute of Electrical and Electronics Engineers, Inc. Dr Yoji Takeda was born in Osaka, Japan, in He received his B.E., M.E., and Ph.D. degrees from Osaka Prefectural University, Japan, in 1966, 1968, and 1977, respectively. He joined the College of Engineering at Osaka Prefectural University in 1968, and became a professor in Since 26, he has been the President of Osaka Prefectural College of Technology. His main areas of research interest are PMSMs, linear motors, and their control systems. Dr Takeda is a member of the Institute of Electrical Engineers of Japan, the Institute of Systems, Control and Information Engineers, and the Japan Institute of Power Electronics. 124 IEEJ Trans 2: (27)