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Four-Quadrant Energy Transducer Test Bench Sylvain Chtelet, Chandur Sadarangani Abstract An innovating system called four-quadrant energy transducer (4QT) has been developed, at the division of Electrical Machines and Power Electronics, Royal Institute of Technology (KTH), Stockholm. It consists of two permanent magnet machines: a double rotor machine (DRM) and a conventional machine (STATOR). The outer rotor of the DRM is connected to the shaft of the STATOR-machine. This system presents a trade-off between series and parallel hybrid architectures. As its name suggests, the transducer can operate in all four quadrants defined from the working point of the Internal Combustion Engine (ICE) in the torque-speed diagram. In a hybrid vehicle, the 4QT allows independent speed and torque of both the ICE and the transmission load. The DRM and STATOR provide respectively the speed and torque differences. 4QT machine prototypes, which integrate the DRM and the STATOR into one unit, have been built. In order to test the performances of the prototypes and measure the system efficiency a laboratory set- up was built. Two more electrical machines are used in the test bench to replace the ICE driving the 4QT and to simulate the transmission load. Each of the four machines is supplied from a common DC link through an inverter. The setup is controlled by a digital signal processor (DSP). Different operation conditions are programmed, resulting in speed and torque set points, which are sent to the bench controller. Mechanical and electrical power meters are installed to measure the efficiency of each machine of the 4QT. Efficiency measurement can be performed at various operating points as well as during the simulation of a drive cycle. This paper presents the equipment and the solutions used to build the test bench. Measuring equipment, and methods will be described. The state of completion of the test bench is illustrated by experimental results from operation tests. Copyright 2002 EVS19. Keywords: Control system, Efficiency, Hybrid Electric Vehicle (HEV), Inverter, Permanent Magnet Motor. 1. Introduction Hybrid electric vehicles are raising considerable interest as environmentally friendly vehicles. They are expected to fulfill criteria as low emissions, low consumption and possibility of zero emission vehicle (ZEV) operation. A solution for hybrid vehicles is to combine an internal combustion engine with an electric system to reach a high efficiency. Different architectures of such a hybrid vehicle exist. The most common are the series hybrid and the parallel hybrid 7. The present paper introduces a new topology of hybrid vehicle: the Four-Quadrant Energy Transducer (4QT) 1. The Four-Quadrant Energy Transducer topology consists of a combination of two electric machines, inserted on the transmission line of a vehicle, between the combustion engine and the wheel transmission, as shown in Figure 1. One of the electric machines is a double rotor machine (DRM) 2. The second machine has a shaft going through, it is referred to as STATOR machine. Both machines are permanent magnet (PM) machines for an optimum efficiency. The two machines are integrated in one unit, which is referred to as 4QT 3, 4. Figure 1 presents the two machines separated, to ease the understanding. Each machine of the 4QT is supplied via an inverter from a battery. Changchun University (92) - 2015/4/24 Download = = Battery InverterInverter DRM ICE “STATOR“ 4QT ?1 T1 ?2 T2 Figure 1: Schematic of the 4QT architecture. 1 and T1 are the input speed and torque, 2, T2 are the output speed and torque. The 4QT system allows the internal combustion engine (ICE) to operate at a speed and a torque independent from the speed and torque of the wheel transmission. The DRM compensates the speed difference; while the STATOR machine compensates the torque difference, see Figure 2. The ICE can thereby operate on its optimum operation line (OOL). The DRM and STATOR can be operated as motor or generator, depending on the 4QT operation mode 3. 100020003000400060005000 Torque Nm Speed rpm 45 ICE OOL 90 ?T ? ICE wheels T1 T2 ?2?1 Figure 2: An example of operation of the 4QT: the DRM works as a motor while the STATOR works as a generator. Prototype machines of the 4QT have been designed and build. To test the prototype machine performances in operation it was decided to build a laboratory test bench. The test bench should permit to run the prototypes at their nominal ratings, in different operation modes, which they would be submitted to in a vehicle, and emulate standard drive cycles. The test bench will also be equipped to measure the efficiency of the 4QT. The present paper describes the test bench. The control of the set-up is presented, along with the implementation of the solutions. Then the achievements will be presented. Changchun University (92) - 2015/4/24 Download 2. Experimental Set-up 2.1. Functionality/requirements The test bench characteristics follow from a series of requirements and constraints. Different prototypes are to be tested. They have different ratings and dimensions and each of them should fit on the test bench. The control of the bench should be flexible to adapt to the different machines, and test different control strategies. The user interface should be user friendly, and pedagogic for demonstrations. The test bench should be used for efficiency measurement. Efficiency of both machines should be measured at different operation point, as well as during standard drive cycles. The set-up should be housed in the laboratory of the division of Electrical Machines. The laboratory is not equipped with gas exhaust system. Therefore it was decided to replace the combustion engine by an electric machine. For practical reasons it was decided to limit the size of the test object to the size of personal vehicle motor. Other constraints as equipment availability decided for large parts of the hard ware used to build the test bench. 2.2. Solution overview The above requirements and constraints led to the present set-up. Instead of the 4QT prototype machine, which will be tested later, two separate DRM and STATOR have been used. An overview of the system is presented in Figure 3. The tested 4QT machines are mounted on a floor plate and connected to a drive and load machine respectively referred to as ICE and LOAD. Each machine is supplied via an inverter from a common DC link. The power flows therefore in a loop; only the power dissipated in the set-up is supplied. The DC link voltage is supplied by a generator, which is part of the laboratory infrastructure. No battery is used. Additional passive components can be added later on to account for a load dependent battery voltage. LOADStator InverterInverterInverterInverter DSP PC DC RsRs Laboratory power Supply Sampling command Data acquisition Power meter Instruments Torque meter Temperatur e Measure OutputsInputs Water cooling DRMICEStatorLOADDRMTMTM Figure 3: Test bench organization scheme. The control of the system is computed by a DSP. Currents and rotor positions are used for feed back control. Two resolvers are needed, one for the position of each shaft. The DSP is programmed using a personal computer (PC), which also provides the user interface under operation of the test bench. The measurement system is synchronized with the control and data acquisition is made in the user interface PC. Changchun University (92) - 2015/4/24 Download The machines are water-cooled. It is planned to use a dedicated cooling system for the test prototype object while the rest of the set-up is cooled with tap water. As it is build now all machines and inverters are water-cooled except for the air-cooled DRM. 2.3. Machines Before the first 4QT prototype is ready, two other machines are used. The ratings of the machines are given in Table 1. An inverter supplies each machine. The drive and load machine inverters have built- in torque control. Torque set-points are send to them via a CAN1 bus. The machine shaft ends are coupled with steel lamina couplings. Table 1: Rating of the machines mounted on the test bench. ICE DRM STATOR LOAD Nominal torque (Nm) 70 95 58 135 Nominal Speed (rpm) 5000 2000 5000 4800 Max Torque (2 min) (Nm) 170 - 90 450 Max speed (rpm) 10000 - 10000 8000 Current (A) 68 84 135 100 Voltage (V) 340 140 130 400 Inverter (kW) 89 89 89 150 2.4. Control Hardware The control of the test bench is computed in is a DSP system from dSPACE2. It is mounted in the PC rack. The DSP system consists of a processor board and several input and output boards, as depicted in Figure 4. The processor board is based on a DSP from Texas Instrument TMS320C40 running at 60MHz. This DSP was purchased in 1996. Since then, considerable progress have been made in terms of computation power and level of development of such DSP solution. The feed back sensors are the resolvers and the current sensors. The resolvers are mounted one on each shaft, and the ends of the DRM. They are conditioned by an electronic circuit board and the position of each shaft is converted into a 12bit signal sent to the DSP. The current sensors are electronic current transformers, mounted in the inverter box. There are two current sensors per inverter. The machines are fed with pulse width modulation (PWM). The PWM is generated on an electronic circuit board, because of the DSP computation time limitation. There is one oscillator for each machine of the 4QT. 1CAN: Control Area Network. 2dSPACE GmbH Technologiepark 25 33100 Paderborn GERMANY, www.dspace.de. Changchun University (92) - 2015/4/24 Download ICE DRM STATOR LOAD Inverter Inverter Inverter Inverter DC rotating generator CAN D/APWM PWMD/A A/D 4x8bits 4x8bits Resolver board Resolver board DSP 6 pulse signals 6 pulse signals 4 current measures 3 3 12 bits 12 bits IRQ PC COM CAN Dongle RS232 Figure 4: Control hardware. A CAN bus ensure the communication between the DSP and the torque controller of the ICE and the LOAD. The torque control parameters of the ICE and the LOAD can be accessed and modified via a series communication bus RS232. 2.5. Control Software The DSP is programmed via the PC. The DSP can be programmed from Simulink using block diagram representation. The Real Time Workshop Toolbox of MATLAB transforms it into C code that is then compiled to DSP code. It is also possible to program directly in C, using the function libraries provided by dSPACE. A user interface software is used to monitor in real time the program variables and modify program parameters. This software is called ControlDesk. It offers the possibility to build advanced control panels to access to the program variables and parameters. 2.6. Measurement system The efficiency measurements are taken using the following method: the mechanical and electrical powers at the inputs/outputs of the 4QT are measured. The efficiencies are then computed as given in Table 2. Changchun University (92) - 2015/4/24 Download Table 2: Efficiency computation. PDRM and PSTATOR are the input electrical power of the DRM and STATOR respectively. DRM STATOR Motor ? DRM P T1 12 ? ? STATOR P TT 122 ? Generator ? 112 T PDRM ? ? 122 TT PSTATOR ? The measuring equipment consists of two power-analyzers from Yokogawa 3. They are remote controlled from the PC via a GPIB4 cable. External shunts are used to measure the currents. Torque meters from HBM5 measure the torques and speeds. The two power analyzers are synchronized. 3. Control and Implementation 3.1. Overview The goal of the test bench control is to regulate the input and output speeds and torques, see Figure 1. The control of the test bench is separated in cascade controllers. The main controller controls the speeds of both shafts and the input and output torques of the 4QT. Secondary controllers in cascade control the current of the 4QT machines. The current control of the drive and load machines are built in the respective inverters. 3.2. Current Control Each 4QT machine is vector controlled as a permanent magnet synchronous machine (PMSM) 5, see Figure 5. The feed back signals are the rotor position and the current in two of the three phases. For the DRM the rotor position is obtained by subtracting the positions of the two shafts. The transformation blocks 3/2 and /dq are used to transform the measured currents from stator coordinates to synchronous coordinates. M InverterPWM 2 3 dq ? Controller 3 2 ? dq Isref Is + - A DSP Figure 5: Vector control of one of the PMSM. The current controllers are proportional integral (PI) controllers. They have been tuned using Ziegler- Nichols method 8. 3.3. Speed Control The torque set-points are sent directly to the ICE and LOAD machines, while the speed of the input and output shaft are regulated. The speed controller regulates the speed of the two shafts. In a first approach the controller is designed using inverse-based loop shaping 6. The speed controller is shown in Figure 6. This controller gives an acceptable result for steady state operations. 3Yokogawa Electric Corporation, Yokogawa Europe B.V. Radiumweg 30, 3812 RA Amersfoort, The NETHERLANDS. 4GPIB: General Purpose Interface Bus. 5HBM Mess- und Systemtechnik GmbH, Postfach 10 01 51, D-64201 Darmstadt GERMANY. www.hbm.de. Changchun University (92) - 2015/4/24 Download DRM speed controller (PI) STATOR speed controller (PI) -1 speed1 ref speed1 speed2 ref speed2 Torque1 Torque2 Tdrm Tst Tice Tload + - - + - + + + - + + Figure 6: Speed controller of the test bench. 3.4. Implementation The control was implemented into a program to run on the DSP. At first, the program was written using Simulink. This solution turned out to be too slow for our application. The same program was rewritten directly in C, using Dspace RTI library. The program is divided into two tasks running at different time rates. The current control is computed in the fast task running at 4500Hz, while the speed control is computed in the slow task, at 50Hz. These computing frequencies are limited by the computation time required by the DSP. 4. Achievements The test bench has been assembled and programmed. It is already possible to operate it at steady state, in different operation modes. 4.1. State of completion of the Test bench The four machines have been coupled. The torque meters are to be inserted when the set-up proved to be operated without torque jerk. It appeared some preoccupying mechanical vibrations when attempting to run the second shaft over 2000rpm. This limited until now the tested operation range. Figure 7 shows a view of the test bench at its actual stage of completion. Changchun University (92) - 2015/4/24 Download Figure 7: Test bench 4.2. Example of operation Tests have been run to verify the possibility of operating the set-up in different conditions. The tests run until now have been performed to validate the set-up. Different examples of speeds and loading performed are presented in Table 3. Table 3: Examples of operation. Here, PICE, PDRM, PST and PLOAD are the mechanical output power of the corresponding machines. Nb n1 (rpm) n2 (rpm) T1 (Nm) T2 (Nm) PICE (kW) PDRM (kW) PST (kW) PLOAD (kW) Mode 1 0 1700 0 12 0 0 2.1 -2.1 Electric 2 200 1000 20 15 0.82 1.7 -0.52 -1.6 Hybrid+ 3 200 1200 40 50 0.84 4.2 1.3 -6.3 Hybrid+ 4 1600 200 50 40 8.4 -7.3 -0.21 -0.84 Hybrid- The examples of operation given in Table 3 have low ratings, due to the speed and torque limitation during validation test. Nevertheless they show the principle of operation of the test bench. The output power given in Table 3 are computed from the torque and speed measured. Example 1 shows that in electric mode, the first shaft holds stand still, and all the traction power is delivered by the battery. Examples 2 and 3 illustrate hybrid operation mode, where the battery power is used to add to the ICE power. Example 4 illustrates hybrid operation mode where the battery is being charged. 5. Conclusion An experimental set-up for testing performances of designed 4QT prototypes has been built. The technical solutions used have been presented. It shows that the set-up built is in pass to fulfill our expectations. However some points have to be improved like mechanical aspect: Balancing and alignment of the second shaft should be improved to overcome the vibrations of the bench and thereby reach higher speeds. The cont