2017 IEEE Transportation Electrification Conference and Expo, Asia-Pacific (ITEC Asia-Pacific) Research on new type electric variable transmission and its application in hybrid electric vehicle Qiwei Xu, Xiaobiao Jiang, Meng Zhao Shumei Cui, Xu Liu , Zhiyuan Wang State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University Chongqing, China [email protected] Harbin Institute of Technology Harbin University Harbin, China [email protected] Abstract?A new electric variable transmission in hybrid electric vehicle is analyzed. The equivalent magnetic circuit and finite element simulation model is established in this paper for analysis of internal magnetic field coupling law. As for the strong coupling relation in the internal magnetic field of electrical transmission, magnetic flux-insulation ring is the electrical the applied into the electric variable transmission in hybrid electric vehicle, and its mathematical model is also built. Then, the working modes of hybrid electric vehicle are analyzed. Power output of engine and electric variable transmission is depended on the power follower strategy. At last, the simulations and experiments have verified that the electric variable transmission can be used in hybrid electric vehicle, thus exploring a new way for the study of hybrid electric vehicles. Keywords?Electrical variable transmission; hybrid electrical vehicle; magnetic field coupling; power follower strategy; I. INTRODUCTION Professor Martin Hoeijmakers of Holland Dyer Fu Delft University proposed electrical variable transmission in 2004 (Electric Variable, Transmission, EVT) concept, and its design idea comes from the induction motor cascade speed control. EVT has the advantages of fast dynamic response, high mechanical integration, strong overload capacity, etc. Under the condition of limited working space, it can make full use of the space and volume of the motor to improve the performance of the motor [1, 2]. In practical application, EVT can be used as a new electronically controlled continuously variable transmission device used in hybrid electric vehicle (hybrid electric vehicle, HEV) , that is, its function is equivalent to the TOYOTA Prius series hybrid vehicle continuously variable transmission (continuously variable transmission, CVT) Based on planetary gear. The reasonable control of engine can guarantee the engine always works in the zone of high EVT, which is of important significance to improve the vehicle's power, fuel economy, and emission reduction [4, 5]. At present, due to the specific characteristics of its structure, there are still some problems in the design and control of EVT. In the body design, there is no good way to design EVT in large. Still internal and external motor are separately designed in a traditional way, without considering the magnetic coupling effect. So EVT will be split into inside and outside the two induction motor design . In the control aspect, if still using the traditional mathematical model and control strategy of motor for analysis, it is not only hard to improve the performance of motor control through magnetic coupling EVT, but also cause the output torque fluctuation, severe fever and the efficiency of the system reduction . In the light of the problem of EVT applied in HEV, a model of equivalent circuit of EVT is established. By finite element simulation and analysis on the internal magnetic field distribution, this paper puts forward using magnetic fluxinsulation ring in EVT. Based on the working mode of EVT in HEV, Power output of engine and electric variable transmission is depended on the power follower strategy . At last, the simulations and experiments have verified that the electric variable transmission can be used in hybrid electric vehicle, thus exploring a new way for the study of hybrid electric vehicles. II. INTERNAL MAGNETIC FIELD DISTRIBUTION OF ELECTRIC TRANSMISSION A. Equivalent magnetic circuit model The EVT architecture is shown in Fig.1, with two mechanical ports and two electrical ports, comprised of the inner rotor, the middle rotor squirrel cage structure and independent double winding stator. The rotor winding in the shaft receives three collector ring through the brush out. The inner rotor named EM1, while the outer rotor named EM2. It?s easy to analyze the EVT when splitting the inner motor and the outer motor. It is shown in the Fig.1. Fig.1 Split diagram of EVT In order to study the coupling mechanism of the internal magnetic field of EVT, the equivalent magnetic circuit is established as shown in Fig.2. This work is Project No. 51507021 supported by the National Natural Science Foundation of China. 978-1-5386-2894-2/17/$31.00 �17 IEEE 978-1-5386-2894-2/17/$31.00 �17 IEEE 2017 IEEE Transportation Electrification Conference and Expo, Asia-Pacific (ITEC Asia-Pacific) Fig.2 Equivalent flux paths of the EVT As is shown in Fig.2??1 is inner rotor yoke permeance ??2 is stator yoke permeance, ?o and ?i are inner and outer air gap and adjacent alveolar permeance, ?c is outer rotor yoke radial permeance??o?1 and ?o?2 are tangential permeance of outer rotor yoke???1 is internal rotor winding flux???2 is flux supplied by stator winding outward??o?1 is the flux of the inner rotor winding and the outer rotor yoke tangential part ??o?2 is the flux of the stator winding and the tangential part of the outer rotor yoke ?c is radial flux of outer rotor winding? B. Analysis of internal magnetic field distribution With the different working states of the motor, the angle ? between the inner rotor magnetic potential and the stator magnetic potential will change continuously in the range from 0 degree to 360 degrees. Fig.3 shows that when the ? is separately equal to 0 degree, 90 degrees, 180 degrees and 270 degrees of electrical angle and EVT internal and external windings are introduced into the three-phase 30A current, the finite element simulation results of the internal magnetic field distribution are presented. (a) IEM1=30A?IEM2=30A? ? =0� (c) IEM1=30A?IEM2=30A? ? =180� (d) IEM1=30A?IEM2=30A? ? =270� Fig.3 Magnetic field distribution of the EVT From the simulation results, when the ?=0 degree, the majority of the EVT internal magnetic field is through the outer rotor magnetic yoke, while the tangential flux is very small, as shown in Fig.3 (a). At ?=180�, the external rotor tangential flux is added and prone to saturation, as shown in Fig. 3 (c). When the ? changes in the range from 0 degree to 180 degrees, or in the range from 180 degrees to 360 degrees, as shown in Fig.3 (b) and 3 (d), the distribution of the EVT magnetic circuit varies between the above two magnetic states. When the ?=180 degrees, the outer rotor yoke center tangential flux density reached 2.2T, and ?=0 degree, the outer rotor yoke tangential flux density is less than 0.2T, however the maximum radial magnetic flux density is only 0.6T. Therefore, the ?=180 degrees, the outer rotor yoke is easy to saturation. III. INTERNAL MAGNETIC FIELD DISTRIBUTION OF VET WITH MAGNETIC FLUX-INSULATION RING A. Analysis Equivalent magnetic circuit model ? Inner rotor magnetic potential F1 couples with inner ? squirrel cage. Outer rotor magnetic potential F2 couples with external squirrel cage . The equivalent circuit is shown in the Fig.4. (b) IEM1=30A?IEM2=30A? ? =90� Fig.4 Equivalent flux paths of the EVT with magnetic isolation ring 2017 IEEE Transportation Electrification Conference and Expo, Asia-Pacific (ITEC Asia-Pacific) ?c1 is the magnetic permeance between Inner rotor winding and radial direction of external rotor yoke; ?c2 is the magnetic permeance between stator winding and radial direction of outer rotor yoke; ?c1 is the Radial flux between outer rotor winding and Inner rotor winding; ?c2 is the Radial flux between outer rotor winding and stator winding. B. Analysis of internal magnetic field distribution Included angle between inner rotor magnetic potential ? ? F1 and outer rotor magnetic potential F2 is also changing in the range of 0皛360�. As shown in the Fig.5, it is finite element simulation results of internal magnetic field distribution when the inner and outer windings are respectively connected with the three-phase 30A current and the electrical angles ? were 0�90�180� and 270�. (d) IEM1=30A?IEM2=30A? ? =270� Fig.5 Magnetic field distribution of the EVT with magnetic isolation ring As can be seen from Fig.5, because of magnetic fluxinsulation ring using inner rotor, magnetic circuits of internal and external motor are isolated. If the radial size is the same as in Fig.4, the outer rotor yoke is easy to be saturated. IV. ANALYSIS OF OPERATING MODE BASED ON HYBRID ELECTRIC VEHICLE (a) IEM1=30A?IEM2=30A? ? =0� EVT can get more energy conversion mode, it is very suitable for the application of electromechanical energy conversion in a wide variety of hybrid vehicles as a power separation device. EVT can be used to replace the traditional automobile CVT, starter, generator, clutch and other devices, achieving the function of CVT. EVT is shown in Fig.6. (b) IEM1=30A?IEM2=30A? ? =90� Fig.6 HEV structure based on the EVT (1) CVT mode (c) IEM1=30A?IEM2=30A? ? =180� The main working mode of the VET is the continuously variable transmission (CVT) mode, which is traditional vehicle CVT function. The mechanical energy output of the engine PICE is divided into two parts. A part of the energy is directly transferred to the stator of EM2 in the form of electrical energy passed from EM1, through collector ring, brush, rectifier, inverters. On the other hand, the other power is in form of magnetic field passed to output shaft. It can be shown in the equation (7). PICE=PEM1+Pd (7) The electromagnetic torque of EM1 TEM1 is balanced with the output torque of motor TICE. Thus, the power equations can be expressed as follow: 2017 IEEE Transportation Electrification Conference and Expo, Asia-Pacific (ITEC Asia-Pacific) ? PICE = TICE?ICE ? ? PEM1 = TEM1 (?ICE ? ?EM2 ) = TICE (?ICE ? ?EM2 ) ?P = P ? P = T ? ? d ICE EM1 ICE EM2 = TEM1?EM2 (8) The shaft output torque T2m of the outer rotor is a combined torque of the inner motor and the outer motor .as shown in equation (9): T2m = TEM1 + = PEM1 ?EM2 = TEM1 + TEM1 (?ICE ? ?EM2 ) ?EM2 TEM1?ICE ?EM2 (9) According to equation (9), the EVT realizes the CVT function of the traditional vehicle?and the transmission ratio iEVT of the EVT can be defined? iEVT ? T = ICE = 2m ?EM2 TEM1 (10) The transmission ratio iEVT of the EVT can be continuously. As a result, the powers of EM1 and EM2 can be gotten? 1 ? ? PEM1 = TEM1 (?EM2 ? ?ICE ) = TEM1?ICE ( i ? 1) ? EVT ? 1 ?P = T ? ) EM2 EM2 = (T2m ? TEM1 )? EM2 = TEM1? ICE (1 ? ?? EM2 iEVT (11) As equation (5) implied, in CVT mode, the output electromagnetic power of inner motor is equal to that of outer motor when transmission ratio iEVT=2; compared with outer motor, the inner motor will output higher torque and lower speed when transmission ratio iEVT between 1~2; it is in contrary when transmission ratio iEVT>2. (2) Starter mode It is free of starter that traditional vehicle installs when hybrid vehicle has been installed EVT, the inner motor start engine via the power that battery supplied. It is time to fuel supply and ignition so that starting engine when the engine has been dragged at relatively high speed by inner motor. In auxiliary assist mode, vehicle is drive by the energy that engine and battery supply together, so that the dynamic performance is improved. According to the above analysis about main working mode, the summary of working states under each mode of vehicle powertrain was shown in table 1. Tab.1 Status of power-trains in different operation modes Work mode generator Inner motor Outer motor Mechanical braker CVT mode work Electricity generation electromotion stop Starter mode work electromotion stop stop Generation mode work Electricity generation work stop Pure electric mode stop stop electromotion stop Regenerative braking mode stop stop generating stop Auxiliary assist mode work Electricity generation electromotion stop Reverse mode stop stop electromotion stop Mechanical braking mode stop stop stop work Hybrid braking mode stop stop Electricity generation work V. ANALYSIS OF POWER FOLLOWING CONTROL STRATEGY The power follower management strategy is to adjust the output of the engine according to the different requirements of the load. According to the vehicle load demand and SOC of battery, it is determined the engine operating points, the output power of the engine for the difference between the load power requirements and battery power output. (3) Generation mode In generation mode, the battery will be charged by inner motor which is in generation mode. Under such condition, the mechanical energy which output by engine will transform into electric energy and store in battery, it can supply auxiliary energy to the vehicle. (4) Braking mode If we use drum or dish type brakes during the braking, the energy of braking will be wasted by fiction. Because of electrical powertrain, the hybrid vehicle based on EVT can work in feedback brake mode. (5) Pure electric mode In pure electric mode, the engine will be stopped and EVT that battery supply will drive the vehicle solely. Under such condition, the inner motor will stop and the outer motor work in generation state. (6) Auxiliary assist mode Fig.7 The optimal operation curve of ICE As is shown in the Fig.7, the engine is operating in optimal working curve in the road, which is independent of cycle. That is, the output of the engine can be different from the wheels? speed and torque output. At this time, by adjusting the EM2 output torque, it is compensated torque deviation ?T, while by adjusting the EM1 output speed, it is compensated speed deviation ?n. When the engine operating point coincides with the wheel working point, the battery output 2017 IEEE Transportation Electrification Conference and Expo, Asia-Pacific (ITEC Asia-Pacific) power is zero. At the time, the transmission in the work process is equivalent as a gear of ratio of 1. According to the current load power demand and battery SOC, by looking up in table the engine work in the current state of the optimal operating speed is got. The optimal transmission ratio iEVT of an electrical transmission can be calculated according to follow equation. iEVT = 0.377 rd ? ?ICE_opt i0 ? VVHE (12) The power follower strategy is used to The HEV with the EVT. In the 10.15 urban driving cycles the simulation results shown in Fig.8. Because of the demand of low power, the vehicle pure always work with electric driving mode, thus EM2 providing the main power output. The engine mode stays the zone of the low torque or closed. The torque of EM1 is depended on the torque output of the engine, while the EM1 speed changes according to the change of the vehicle speed. So the EM1 is constantly switching between the electric state and the power generation state. Fig.8 The simulation waveforms of 10.15 urban driving cycles based on power following strategy VI. EXPERIMENTAL STUDY In the experiment, two induction motors simulate the engine and traffic load, and simulation experiment platform is shown in Fig.9. From left to right three motors respectively simulate the engine motor EM0, EVT and traffic load motor EM3. The 10.15 city conditions is applied in HEV with EVT for experiment. Because the motor rated value is relatively small, therefore, the actual rotational speed value is 0.5 times than the calculated value of the torque control strategy and the actual value of 0.25 times than the calculated value for the control strategy. Fig.9 The experiment bench (a) The waveforms of speed In this paper, the power follower strategy used in this paper is used to configure the working points of the motor simulated the engine and EVT. The experimental waveforms are shown in Fig.10. From Fig.10, the control system is designed in this paper can successfully complete the various operation modes of HEV and torque and speed errors are smaller compared with its target value given, thus verifying the correctness that the EVT is applied in HEV. (b) The waveforms of torque (a) The waveforms of EM1 speed (c) The waveforms of power (d) The waveforms of SOC and battery power (b) The waveforms of EM2 speed 2017 IEEE Transportation Electrification Conference and Expo, Asia-Pacific (ITEC Asia-Pacific) vehicle, thus exploring a new way for the study of hybrid electric vehicles. EVT, as a continuously variable transmission, realize that change of transmission ratio the can be continuous. The vehicle control strategy can ensure that the engine is always working in high efficient area, which is of important significance to improve the vehicle?s power, fuel economy, and emission reduction. REFERENCES (c) The waveforms of EM1 torque  (d) The waveforms of EM2 torque Fig.10 The experiment waveforms of 10.15 urban driving cycles VII. CONCLUSION This paper analyzes the application of EVT in HEV, the equivalent magnetic circuit and Ansoft finite element simulation can find the magnetic coupling law. 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