Energy-Saving Technologies for Inverter Air Conditioners

Transcription

1 TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING IEEJ Trans 28; 3: Published online in Wiley InterScience ( DOI:1.12/tee.2254 Review Paper Energy-Saving Technologies for Inverter Air Conditioners Kazunobu Ohyama a, Senior Member Toshinari Kondo, Member Almost all residential air conditioners in Japan are inverter air conditioners in which a permanent magnet synchronous motor (PMSM) is driven by a PWM inverter. The inverter technology can reduce the energy consumption to less than half that of air conditioners driven by a constant-speed induction motor (IM). This paper reviews the trends and the latest energy-efficient technologies for the motor and the power converter that achieve considerable energy saving. 28 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. Keywords: air conditioner, inverter, PM motor, IPMSM, sensorless Received 5 September 27; Revised 13 November Introduction Heat pump air conditioners have many advantages, such as high efficiency and their suitability for both heating and cooling. In the global market, air conditioning systems, which have a compressor driven by a constant-speed induction motor (IM) directly connected to the commercial power supply, are widely used [1]. This non-inverter system employs on off to the room temperature, and causes a large variation in room temperature and also a loss of energy owing to frequent on off cycles when the room temperature is close to the preset temperature. In Japan, as shown in Fig. 1, inverter systems account for 99% of air conditioners where a compressor is driven at variable speed by a permanent magnet synchronous motor (PMSM) fed by a PWM inverter [2]. The inverter contributes to comfort as a result of the stable room temperature, and the energy consumption of the air conditioning system can be reduced to less than half that of a conventional one. Owing to this significant energy-saving factor, air conditioners equipped with an inverter-fed PMSM have superseded almost all non-inverter ones in Japan. This paper reviews energy-saving technologies for inverter heat pump air conditioners. In sections 3 and 4, the technology trends and the latest energy-efficient technologies for compressor motors and power converters are described. In section 5, applications of the energy-saving technologies developed for residential air conditioners to other types of air conditioners, such as those for commercial and office buildings, are reported. a Correspondence to: Kazunobu Ohyama. kazunobu.ohyama@daikin.co.jp Daikin Industries, Ltd, 1-2 Ohtani Okamoto-cho Kusatsu, Shiga , Japan 2. Energy-saving Technologies for Air Conditioners Figure 2 shows a schematic diagram of a heat pump cycle in heating. The energy efficiency of air conditioners, the coefficient of performance (COP), is defined by the following equation: Heat released into inside air [kw] COP (in heating) = Input power to compressor [kw] Heat absorbed from outside air + Heat equivalent of compression work = Input power to compressor (1) A heat pump air conditioner exhibits a heating capacity which is five times larger than the input power to the compressor since it utilizes the heat absorbed from the outside air by the evaporator for heating, in addition to the heat equivalent of the compression work. An air conditioner with a heating capacity of approximately 3 kw, which is a typical capacity in Japan, is an example of considerably high energy efficiency. It has a rated power consumption of 5 W. The performance of this case corresponds to a COP of 6. A reduction of input power to the compressor and an improvement in efficiency of heat exchange are required to improve the energy efficiency. These two requirements can be achieved effectively by employing an interior permanent magnet synchronous motor (IPMSM) and an inverter for compressor motors. When the room temperature is close to a preset temperature, inverter air conditioners are driven by a compressor at low speed. Thus, usually, inverter air conditioners are operated at less than half the rated speed (under partial load conditions) for almost all of the operating time. Inverter-fed IMs, which were used for compressor motors, have a low efficiency at low speed and light loads. Therefore, it is possible to reduce the energy consumption under partial load conditions and to reduce the input power to the compressor by 28 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

2 K. OHYAMA AND T. KONDO [Million] [Refrigeration year] 25 Refrigeration Year : one year between October in previous year and September in year concerned bar : Shipments of inverter air conditioners White bar : Shipments of air conditioners driven at constant speed (non-inverter air conditioners) Fig. 1 Shipments of residential air conditioners in Japan P (pressure) Heat available for heating Heat release Indoor unit Expansion valve Outdoor unit Heat absorption h (enthalpy) Heat equivalent of compression work Compressor Heat absorbed from outside air Fig. 2 Heat pump cycle in heating Annual energy consumption [kwh] : bar chart 3 8 energy-efficient technologies for compressor motors and power converters which enabled the reduction in power consumption are described respectively in the following sections. 3. Energy-saving Technologies for Compressor Motors [4,5] Table I compares the structures and features of IMs, surface permanent magnet synchronous motors (SPMSMs), IPMSMs, and switched reluctance motors (SRMs) for compressor drive applications. Figure 4 shows the efficiency characteristics of compressor motors for residential air conditioners. IPMSMs, in which magnets are embedded in the rotor, have advantages such as: 1. high efficiency and high torque owing to utilizing reluctance torque in addition to magnet torque; 2. wide constant power range as a result of flux weakening. Therefore, IPMSMs, which were produced in 1996 for the first time in the air conditioning industry, play an important role in improving the partial load efficiency because they have a considerably higher efficiency at low speed in comparison with IMs and SPMSMs, which were produced by Daikin Industries in 1994 and 1995, respectively. Furthermore, flux weakening of an IPMSM can increase the refrigerating capacity since it enables compressors Table I. Comparison of structures and features of compressor motors Motor type IM SPMSM IPMSM SRM Fig (IM) 96(IPM) (IPM) 7(IPM) Variation of annual energy consumption and average COP in cooling and heating COP : line graph Structure Efficiency at low speed Rated efficiency Flux weakening poor good excellent fair fair good excellent fair good poor good good employing PMSMs in which the excitation loss is eliminated. Further, the performance of a heat exchanger increases in inverse proportion to the flow rate of the refrigerant. Thus the efficiency of the heat exchanger can be improved and the heat can be effectively absorbed from the outside air if the compressor speed is reduced by the inverter under partial load conditions and the flow rate of the refrigerant is reduced. Thus, an inverter-fed PMSM can improve the energy efficiency of air conditioners, as the numerator of (1) becomes smaller and the denominator becomes larger. Figure 3 shows the annual energy consumption and COPs of air conditioners with a cooling capacity of 2.8 kw [3]. IPMSMs, which were employed for the first time in 1996, show a substantial energy saving. Since then, a number of developments have improved the energy-saving performance. The Efficiency [%] IPMSM( 96) 9 SPMSM( 95) 8 IM( 94) Speed [r/sec] Fig. 4 Comparison of efficiency of compressor motors 184 IEEJ Trans 3: (28)

3 ENERGY-SAVING TECHNOLOGIES FOR INVERTER AIR CONDITIONERS to be driven at a higher speed, whereas this is not easily applicable to an SPMSM. The torque T and the terminal voltage V a are given by (2) and (3) using the variables and parameters in the rotating d q coordinate system [6]. T = P n {ψ a i q + (L q L d )( i d )i q } (2) V a = (R a i d ωl q i q ) 2 +(R a i q +ωψ a ωl d ( i d )) 2 (3) where, i d,i q are the d and q-axis stator currents, ω is the electrical angular velocity, R a is the stator winding resistance, ψ a is the magnetic flux linkage, L d and L q are the d and q-axis inductances, and P n is the number of pole pairs. As shown in Table II, from the start of commercial production in 1996, Daikin Industries adopted a rotor structure with sintered rare-earth permanent magnets, NdFeB, embedded below the outer surface of the rotor. Thus, the proportion of reluctance torque increased since magnetic saturation can be avoided and the saliency ratio can be increased (L q L d becomes larger than zero in (2)). The magnetic flux can be produced in the opposite direction to ψ a by applying a relatively small current i d (flux weakening ), since the magnets are thin because of the high coercive force of NdFeB magnets and then L d is large in (3). As a result, it is possible to widen the constant power operation range. These advantages were realized by both the development of the rotor structure and the increase in the remanent flux density and coercive force of the permanent magnets, as shown in Fig. 5. Table II. Structure Comparison of structures and features of compressor motors In addition, the use of a concentrated winding, in which coils are wound directly around the stator teeth, has become the main technique for the stator winding since around 2. It shortens the length of the end-windings, and, hence reduces the copper loss. Generally, however, a concentrated winding increases the acoustic noise, although the motor efficiency is improved. Thus, the rotor structure design, e.g. the arrangement of slits and the enlargement of the air gap in the q-axis direction, as shown in Fig. 6, have been developed to smooth the magnetic flux variation in the air gap [7]. In addition, the acoustic noise can be reduced by using sinusoidal waveforms rather than rectangular ones as the PWM inverter waveforms. In addition to the improvement of motor electromagnetic structures, progress in both permanent magnets, as shown in Fig. 5, and silicon steel sheets, as shown in Fig. 7, has been an important factor to improve the motor efficiency. PM motors can reduce the copper loss more compared to IMs as the excitation loss is eliminated. Since the iron loss is the dominant loss component in IPMSMs, which utilize the reluctance torque, a further decrease in the iron loss of silicon steel sheets is desirable. At the early stage of the development of IPMSMs, the level of saturation flux density of silicon steel sheets tended to be sacrificed to reduce the iron loss. Recently, however, an increase in the saturation flux density has been demanded because new designs of motor have a stator with concentrated winding and a rotor in which the flux density is high as a result of magnetic flux focusing. Consequently, practical use of silicon steel sheets which exhibit both a low iron loss and a high-saturation flux density has been extensively developed. Figure 8 shows a rotor and a stator of the latest model of an IPMSM, whereas Fig. 9 compares the efficiency of the latest 27 model of IPMSM with that of the 1996 model. By applying a variety of energy-efficient technologies as mentioned above, the efficiency can be improved by 8% under partial load conditions, which is the most important for saving energy in the air conditioners. Rotor Rare earth Rare earth Rare earth magnet magnet magnet magnet Stator Distributed Distributed Concentrated winding Driving Rectangular Sinusoidal Sinusoidal waveform Fig. 6 (a) Manufacturer A (b) Manufacturer B (c) Manufacturer C Rotor structures produced by air conditioner manufacturers Remanent flux density [T] IPMSM IP MSM.8 Alnico Sintered NdFeB Bonded NdFeB.4, 95 SPMSM Ferrite Coercive force [ka/m] Fig. 5 Development trend of permanent magnets Saturation flux density B5 [T] IPMSM 95 SPMSM 94 IM Iron loss W15/5 [W/kg] Fig. 7 Development trend of silicon steel sheets 185 IEEJ Trans 3: (28)

4 K. OHYAMA AND T. KONDO (a) PWM boost chopper (b) Partial switching Fig. 11 Circuit configurations of PFC converter Fig. 9 Efficiency [%] Fig. 8 Latest model of IPMSM (27) IPMSM with concentrated winding IPMSM with distributed winding Speed [r/sec] Efficiency characteristics of IPMSMs with distributed and concentrated windings 4. Energy-saving Technologies for Power Converters to Drive Compressor Motor 4.1. Converter circuit Figure 1 shows the progress which has been made in converter circuit configurations, s, and power devices for inverter air conditioners every ten years. Initial power converters, which supplied IMs, Trend Circuit Voltage doubler rectifier reactor for improving power factor In addition to the above, simplifying gate drive circuit Harmonic restraint high power factor converter Harmonic restraint loss reduction Control 8-bit microcomputer IM : V/f constant 8/16-bit microcomputer PM : analogue sensorless 32-bit microcomputer PM : digital sensorless PM motor sensorless Motor Power device Power transistor module IGBT module Trench gate IGBT module Loss reduction small sizing Fig. 1 Transition of power converters for air conditioners consisted of a voltage doubler rectifier comprising a diode bridge and an integrated inverter module consisting of six transistors, and employed constant V/f by an 8-bit single-chip microcomputer and sinusoidal PWM. The voltage doubler rectifier was used to reduce the current capacity of the power devices by utilizing the doubled DC voltage, since many of the power supplies for domestic use in Japan are single phase 1 V. Harmonic currents were restrained by connecting a reactor to the input terminal of the rectifier circuit to improve the power factor. IGBTs were introduced in the 199s because of the progress of power devices. As a result, it allowed the PWM carrier frequency to be increased to around 1 khz. Furthermore, it was achieved to reduce the acoustic noise, to reduce the motor harmonic loss, and to simplify the gate drive circuit. In recent years, the loss in the main circuit of the inverter has been reduced significantly, since trench gate IGBTs have superseded planar ones. As for the rectifier circuit, in the late 199s, DC link voltage was required to be higher because it was required to drive an SPMSM at high speed despite the difficulty of applying flux weakening. Recently, it has been required to reduce the input current by improving the power factor, and to reduce the harmonic current. As for unity power factor AC DC conversion, power factor correction (PFC) converters using a PWM boost chopper are well known, as shown in Fig. 11(a). However, a reduction in the conversion efficiency occurs owing to the high switching frequency. To overcome this problem, a method was applied in which the device switches once or more times during every half cycle of the applied voltage in the same configuration of the main circuit shown in Fig. 11(a) [8]. More recently, in the circuit configuration in Fig. 11(b), a partial switching method has been used in which the device switches several times in each cycle of the applied voltage [9]. These methods are important for energy-saving technologies owing to their high conversion efficiency, and they conform to the Japanese guideline for reduction of harmonic emission and IEC Standards. A number of patterns have been developed since the optimization of the timing and number of switching events of the device enable a significant reduction in the circuit loss Position sensorless technology for permanent magnet motors It is necessary to detect the magnetic pole positions to drive synchronous motors. However, it is difficult to install position sensors inside an air conditioner compressor owing to the high temperature and pressure. Therefore, many sensorless position methods have been developed. At the early stage of the development of PM motors, 12 electrical-degree conduction periods sensorless for motors fed by voltage source inverter was adopted using the circuit shown in Fig. 12 [1,11], and afterwards a part of the signal 186 IEEJ Trans 3: (28)

5 ENERGY-SAVING TECHNOLOGIES FOR INVERTER AIR CONDITIONERS Microcomputer su vfu v vfu-v su Fig. 12 Back-EMF position sensing method Speed command Advance angle i q * Speed i d * + - i q Three phase PWM Current Current v q * Estimated speed Position speed estimator i q i d Estimated position i d + - v d * d -q 3 f 3 f d -q Fig. 14 Model-based position sensing method 1% Efficiency of power converter Loss of PFC converter Loss of inverter 98% s3 Microcomputer Integrator v n M 96% 94% 92% v nm v nm dt s3 Fig. 13 Sensing method using third harmonic of back-emf processing was digitized. These methods utilize the back-emf which is induced during non-conduction periods of IGBTs to detect the pole positions of the permanent magnets. They have an advantage that sensorless can be achieved by an 8- bit single-chip microcomputer since the magnetic pole positions can be detected by a simple calculation. On the other hand, it is essential that the current vector of IPMSMs is led in a wide angle range to utilize the reluctance torque and flux weakening. However, the advantages of IPMSMs cannot be utilized sufficiently by using a back-emf-based sensorless method due to the limitation of the range of the current phase angle. Thus, when the production of IPMSMs started in 1996, the sensorless using the neutral point voltage of a stator winding was adopted, as shown in Fig % Cooling intermediate Heating intermediate Cooling rated Fig. 15 Efficiency of power converter Heating rated Theoretically, there are no limitations on the range of the phase angle in this method since the non-conduction period of an inverter is not required. Thus, it allows the inverter to use sinusoidal PWM for modulation, since switching is available over a range of 36 electrical degrees owing to the elimination of the non-conduction period [12]. At present, as one of the most advanced sensorless methods, a rotor position estimation method to detect the magnetic pole position has been developed for practical use [13]. This method uses the information of voltage and current supplied to the actual motor and its mathematical model to estimate the magnetic pole positions. A typical configuration for the magnetic pole position estimation method is shown in Fig Efficiency of power converter Figure 15 indicates the efficiency of power converters for the latest models of an inverter air conditioner under rated cooling, rated heating, intermediate cooling, and intermediate heating conditions. 187 IEEJ Trans 3: (28)

6 K. OHYAMA AND T. KONDO The unshaded portions show the efficiency of power converters and the other portions show the loss of PFC converters and inverters, respectively. The latest model of an inverter air conditioner employs technologies as mentioned above, such as the partial switching method in a PFC converter, a trench gate IGBT module for the inverter switching devices, and a magnetic pole position estimation method for the motor. The efficiency under the rated heating condition is 96%, whereas the efficiency of the inverter is 98.6%. It has significantly high efficiency of power conversion. In addition, the efficiency under intermediate heating conditions has reached 94.5%, which is the most important consideration to reduce annual energy consumption, whereas the efficiency of the inverter has achieved 97.2%. Thus, a sufficiently high efficiency has now been achieved. 5. Further Applications of Inverter Air Conditioners and IPMSMs IPMSMs, which were employed in residential air conditioners for the first time in 1996, were applied to commercial air conditioners with a cooling capacity from 4 to 14 kw in Figure 16 shows the appearance of the outdoor unit of a commercial air conditioner, which achieved a third of the energy consumption compared with a conventional one. A half of the energy consumption of a conventional one is saved by a motor and inverter system. With the upsizing of motors, it is required further to improve efficiency of IPMSMs and to reduce the acoustic noise. Figure 17 shows a rotor and a stator of an IPMSM in which the reduction of acoustic noise is achieved by employing a stepwise skewed rotor. The stepwise skewed rotor, in which the magnets are subdivided in the axial direction, allows rectangular magnets to be used to reduce the production cost of motors. The rotor structures, one of which is not skewed, are divided into two or four segments, and cogging torque waveforms are as shown in Fig. 18 [14]. By employing the stepwise skewed rotor, the third harmonic component, which is mostly caused by the stator slotting, can be reduced to less than a fifth. The low acoustic noise motors are used for a VRV (variable refrigerant volume) system, which is an air conditioning system with a Torque [Nm] (a) Stator core and coil (b) Rotor core and magnets Fig. 17 Rotor and stator of IPMSM (i) No skewed model (ii) Skewed model A (iii) Skewed model B No skewed model Skewed model B Skewed model A.4.6 Rotation angle [deg] Fig. 18 Rotor structures and cogging torque waveforms x z y Fig. 16 Sensorless inverter sinusoidal PWM (Energy saving 17%) BLDC fan motor (Energy saving 6%) IPMSM NdFeB magnet Skewed rotor (Energy saving 2%) Appearance of outdoor unit for commercial air conditioners larger cooling capacity, and has made a major contribution to energy saving for air conditioning in office buildings. In recent years, heat pump systems have been used for hot water heaters and floor heating systems. Similar to air conditioners, in these systems, IPMSMs are used for the compressor motors. This has made a substantial contribution to the energy saving, and the expansion in the use of IPMSMs can be expected in the near future. 6. Conclusions From the global environmental point of view, there is a strong expectation that CO 2 will be reduced by using heat pump air conditioners. In Japan, almost 1% of residential air conditioners are operated by an inverter. The air conditioners 188 IEEJ Trans 3: (28)

7 ENERGY-SAVING TECHNOLOGIES FOR INVERTER AIR CONDITIONERS equipped with a sinusoidal PWM inverter and an IPMSM can achieve the highest efficiency. This paper has described the trends and the latest energyefficient technologies for motors and power converters, the core technologies of inverter air conditioners. The technologies, which were initially applied to residential air conditioners and showed considerable energy saving, have been widely used for commercial and office building applications. It is expected that the technologies developed in Japan will be used widely across the whole world, and the heat pump technology using an inverter-fed IPMSM will make a significant contribution to global environmental conservation. References (1) Donlon J, Achhammer J, Iwamoto H, Iwasaki M. Power modules for appliance motor. IEEE Industry Applications Magazine 22; 8(4): (2) statistic.html, Shipments of Air Conditioning and Refrigeration Equipment, The Japan Refrigeration and Air Conditioning Industry Association. (3) Environmental Report 23, Daikin Industries Ltd. (4) Ohyama K, Kosaka M. High efficiency for interior permanent magnet synchronous motor. In Proceedings of IPEC- Tokyo2, Tokyo, Japan, 2; (5) Ohyama K. Recent advances of reluctance torque assisted motors. IEEJ Transactions on Industry Applications 23; 123(2): (6) Morimoto S. Trend of permanent magnet synchronous machines. IEEJ Transactions 27; 2: (7) Ohyama K, Enomoto Y, Higuchi T, Yoshikawa Y. Influence of reluctance torque assisted motors on constant torque load drive system and the recent applications. In The 27 Annual Meeting Record I.E.E. Japan, Toyama, Japan, 27; S16 3. (8) Suga I, Kimata M, Uchida R. A simple switching method for an improved power factor type single phase converter. IEEJ Transactions on Industry Applications 1996; 116-D(4): (9) Uesugi M, Kanazawa H, Hiruma A, Miyazaki H, Kanbe T. Single-phase twice voltage PFC converter for air conditioner. IEEJ Transactions on Industry Applications 1999; 119- D(5): (1) Endo T, Tajima F, Okuda H, Iizuka K, Kawaguchi Y, Uzuhashi H, Okada Y. Microcomputer-led brushless motor without a shaft-mounted position sensor. In Proceedings of IPEC- Tokyo1983, Tokyo, Japan, 1983; (11) Paul PA, Watson JF. Review of position-sensorless operation of brushless permanent-magnet machines. IEEE Transactions on Industrial Electronics 26; 53(2): (12) Matsuno S, Taniguchi T, Mizobe H, Ohyama K. Rotor position sensorless using neutral point signal of IPMSM. In IEEJ the Papers of Technical Meeting on Rotating Machinery, RM-2-48, Tokyo, Japan, 22; (13) Fukumoto T, Ohyama K. Technical trends of AC motor drive systems in home appliance. In The 26 Annual Meeting Record I.E.E. Japan, S17-5, Yokohama, Japan, 26. (14) Yamagiwa A, Nishijima K, Sanga Y, Kawase Y, Yamaguchi T, Yano T. Reduction of motor vibration by stepwise skewed rotor. In Proceedings of the 25 Japan Industry Applications Society Conference, Vol.III, Fukui, Japan, 25; Kazunobu Ohyama (Senior Member) Kazunobu Ohyama received the B.E. and M.E. degrees from Tokyo Institute of Technology, Tokyo, Japan in 1979 and 1981, respectively. He was a Research Associate at Nagoya Institute of Technology, Nagoya, Japan, during He joined Daikin Industries in 1986 and is currently an Associate Officer. He has been engaged in research and development on inverter and motor drive systems. Kazunobu Ohyama is a member of the IEEE and the Japan Institute of Power Electronics. Toshinari Kondo (Member) Toshinari Kondo received the B.E. and M.E. degrees from Okayama University, Okayama, Japan in 1995 and 1997, respectively. He joined Daikin Industries in 24. He has been engaged in research and development on electromagnetic design of permanent magnet motors. 189 IEEJ Trans 3: (28)