2007 IEEE International Future Energy Challenge

Transcription

1 007 IEEE International Future Energy Challenge Topic B: Integrated Starter/Alternator-Motor Drive for Automotive Applications Final Report Presented by: Consortia of the University of Colorado at Boulder Electrical and Computer Engineering Department, USA, and the Indian Institute of Technology at Delhi Electrical Engineering (Power), India August 0, 007

2 . Acknowledgements.. Undergraduate Team Richard Tan, student, CU-Boulder, Electrical and Computer Engineering Maung Myat, student, CU-Boulder, Electrical and Computer Engineering Geoff Sanders, student, CU-Boulder, Electrical and Computer Engineering Ankit Tripathi, student, CU-Boulder, Electrical and Computer Engineering Marc Hesse, student, CU-Boulder, Electrical and Computer Engineering Megha Gupta, student, IIT-Delhi, Electrical Engineering Aditya Bhatla, student, IIT-Delhi, Electrical Engineering Dhruv Vatsal, student, IIT-Delhi, Electrical Engineering Ekansh Aggarwal, student, IIT-Delhi, Electrical Engineering Ishita Mukhopadhyay, student, IIT-Delhi, Electrical Engineering Parag Arora, student, IIT-Delhi, Electrical Engineering Vineesh Kumar, student, IIT-Delhi, Electrical Engineering.. Faculty Support Dr. Frank Barnes, CU-Boulder, Electrical and Computer Engineering Dr. Ewald Fuchs, CU-Boulder, Electrical and Computer Engineering Dr. Dragan Maksimovic, CU-Boulder, Electrical and Computer Engineering Dr. Regan Zane, CU-Boulder, Electrical and Computer Engineering Dr. Robert Erickson, CU-Boulder, Electrical and Computer Engineering Dr. Tom Brown, CU-Boulder, Electrical and Computer Engineering Dr. Bhim Singh, IIT-Delhi, Electrical Engineering Dr. G. Bhuvneshwari, IIT-Delhi, Electrical Engineering

3 .3. Graduate Student Support Gregory Martin, CU-Boulder, Electrical and Computer Engineering Richard Moutoux, CU-Boulder, Electrical and Computer Engineering.4. Sponsors National Academy of Engineering Bernard M. Gordon Prize CU-Boulder Engineering Excellence Fund CU-Boulder Undergraduate Research Opportunities Program. Introduction The University of Colorado at Boulder and the Indian Institute of Technology jointly entered the 007 IFEC in September of 006. This move was sparked by collaborative work happening among IIT and CU students during the summer of 006, on the CU campus. The team has grown to include several undergraduate students at IIT and CU, faculty advisors, graduate assistants, and sponsors. Continued collaboration among students and faculty has created an excellent learning experience for students in designing, constructing, and testing the motorgenerator components and system. This effort has resulted in a working deliverable unit for the final IFEC Topic B competition. 3. Status A pole-changing induction machine with winding switching has been developed, constructed, and tested for the IFEC Topic B competition finals. The system designed functions as a starter, developing high torque at zero speed and accelerates up to 500 rpm. It also functions as a generator, generating kw at 3000 rpm. 3

4 While the team strived to meet all of the design requirements of the IFEC, not all of the goals have been met. The machine accelerates to 3000 rpm in under 5 seconds, and in generating mode tests show that it generates. kw. However, the target of 30 Nm of starting torque is not met. Tests show that the machine will deliver about Nm of starting torque. Also, the machine is over the weight limit, weighting a little over 30 pounds. The off-the-shelf inverter cannot handle the current transient required to zero speed starting. However, a slightly larger inverter of exactly the same design would suffice, and the concept used in this team s design has been proven. Due to time and financial constraints, the inverter was purchased off-the-shelf. Although the inverter is rated for 5 amps (at 80 deg C) (which is about the current needed for the machine to reach 30 Nm starting torque), the inverter in test cannot handle this amount of current, and is destroyed. Thus, the team has not been able to achieve more than 5 Nm of starting torque consistently. The team is confident that with the same inverter with a slightly higher rating, 30 Nm could be achieved. On the positive side, the team is quite pleased with the operation of the polechanging and winding switching induction machine. Tests show that this is an effective way to develop as simple electrical machine that works as both a motor and a generator over a very wide speed range. 4. Learning Experience The students at both CU-Boulder and IIT-Delhi have expressed a very positive learning experience from their participation in the 007 IFEC. These students recognize the value in learning to function on an international design team across time, space, and 4

5 cultural barriers. Both of the groups have been frustrated at times over the difficulty in working with a team on the other side of the world, however, in the end these students realize that this is great preparation for the challenges that a global economy will present to them in their future careers. According to one team member, the project has exorbitantly improved our global perspective and working across distance, time and cultural differences. Also, the importance of such an experience is noted by a student, This kind of exposure at the stage when the zeal to learn is at a maximum is of great importance and should be made a part of curriculum. In addition to the challenges presented by working in an international design team, the students have been complimentary of the experience of working on a year-long design project. To quote a student team member, the project was great learning experience as far as understanding how to set short term goals, long term goals, and milestones in order to reach the end goal. Another students summary of their experience is as follows: Overall, we applied the techniques that the university wants us to use as an engineer, and we even learned more about planning the project and the teamwork experience. 5. Technical Design 5.. Overall System The goal of this project is to design, construct, and verify an induction machine system to serve as both the starter and alternator in a hybrid automobile. Operating off of a 00 V DC supply from the battery, the drive starts the engine, accelerates to 3000 rpm, and then generates at least kw of electricity at 3000 rpm and above. In order to serve as 5

6 a starter, the machine must provide 30 Nm of torque at stand-still, and then provide constant shaft power up to 3000 rpm. At 3000 rpm the machine will switch to generating mode where it will act as an alternator to charge the battery. The starter-alternator system is comprised of four major hardware components; an induction machine, switching board, control board, and power inverter. Software algorithms loaded in the control board implement PWM vector control of the motor and rectifier action during generating. For motoring operation, the induction machine requires a three-phase AC input that is provided from the power inverter. The gating signals for the power inverter are provided by the control board using the vector PWM control method. The control board is also responsible for controlling the winding switching of the machine. The control board provides system status output to an LCD display, and also takes user input commands to control the system. Safety functions are implemented in the control board. Figure shows a conceptual diagram of the system. Figure : Conceptual system diagram 6

7 5.. Electric Machine The basic design of the machine consists of a pole-changing squirrel cage induction machine. In order to provide 30 Nm of torque, the machine will start as an 8- pole machine operating at 7 Hz and a line-to-line voltage of V RMS provided by the power inverter. In order to increase the operating speed of the machine the frequency is increased from 7 Hz to 50 Hz. The shaft speed then increases to approximately 750 rpm. At this point the machine is changed from 8-pole operation to 4-pole operation. This switch is controlled by an ST micro-controller and implemented through switching boards. The change from 8-pole to 4-pole operation causes the machine to speed up to 500 rpm. In order to increase the speed from this point the operating frequency is once again increased, this time from 50 Hz to 75 Hz. This causes the speed of the machine to increase to 50 rpm. At this high of a speed, the back EMF generated by the machine reduces the torque output. To remedy this, the number of effective stator turns per phase (N) in the machine is reduced by half. This increases the flux generated by the stator windings, and give a torque boost to the shaft output. After the configuration is changed the operating frequency is increased to 00 Hz, resulting in a final speed of 3000 rpm. Figures and 3 show the timing diagram for frequency changes and the resultant change in shaft speed, respectively. The 8-pole to 4-pole change occurs at.5 seconds and the N-reduction occurs between 3 and 3.5 seconds. 7

8 Figure : Frequency-Time Diagram Figure 3: Speed-Time Diagram For this project a -pole, 3 hp, Dayton motor was purchased from Boulder Electric Motor Company. The primary specifications when choosing the motor are the number of stator slots and the power output. The motor design chosen for this project has 4 stator slots and at 3 hp the stator should have enough iron to supply 30 Nm of torque. Figure 4 shows the cross section of a typical squirrel cage induction machine. The design and actual parameters of the machine are given in Table. 8

9 Figure 4: Cross-section diagram of induction machine 9

10 Table : Comparison between original machine and custom design Original machine design parameters Stator: One sided air gap (g) = 0.35mm Number of stator slots (N ) = 4 slots Outer diameter of stator core (D os ) = 53.mm Number of poles (p) = poles Inner diameter of stator core (D is ) = 74mm Axial length of stator core (l) = 00mm Cross-section of one stator slot (A N ) = 9mm Height of stator back iron (h j ) = 0.75mm Height of one stator slot (h N = h z ) = 4.05mm Opening of stator slot (S n ) = mm b z = 4.95mm Rotor: Number of rotor slots (N) = 30 slots Outer diameter of rotor core (D or ) = 73.3mm Cross section of one rotor slot (A N ) = 3.3mm Height of rotor back iron (h j ) =.8 mm Height of rotor slot (h N = h z ) =.3 mm Opening of rotor slot (S N ) =.0mm Inner diameter of rotor core (d i ) = mm b z = 3.6mm Our custom machine parameters Stator: One sided air gap (g) = mm Number of stator slots (N ) = 4 slots Outer diameter of stator core (D os ) = 53.mm Number of poles (p) = 8 poles or 4 poles Inner diameter of stator core (D is ) = mm Axial length of stator core (l) = 88mm Cross-section of one stator slot (A N ) = 78mm Height of stator back iron (h j ) = mm Height of one stator slot (h N = h z ) = 4.9mm Opening of stator slot (S N ) =.85mm b z =.85mm Rotor: Number of rotor slots (N ) = 34 slots Outer diameter of rotor core (D or ) = 78.5mm Cross section of one rotor slot (A N ) = N/A Height of rotor back iron (h j ) = 4.5 mm Height of rotor slot (h N = h z ) = 4.5 mm Opening of rotor slot (S N ) = N/A Inner diameter of rotor core (d i ) = mm Total winding turns = 400 turns per phase with AWG # Winding Switching Once the off-the-shelf motor is obtained the stator windings are customized. The Boulder Electric Motor Company assisted in removing the existing windings and replacing them with the required custom windings. Figures 5 and 6 give the winding 0

11 diagrams for 8-pole and 4-pole configurations respectively. The 8-pole configuration is implemented using a delta (Δ) connection and the 4-pole uses a double wye (YY) connection. Leads from all 4 slots both for 8-pole and 4-pole operation are brought out of the motor for external switching connections. These leads are connected to the switching board so that they can be configured as required. Figure 7 shows the 3-phase diagrams for the 8-pole delta connection and the 4-pole double-wye connection. Figure 5: Winding configuration for 8-pole operation

12 Figure 6: Winding configuration for full N 4-pole operation Figure 7: Delta and double-wye equivalent 3-phase diagram The schematic for the switching board is shown in Figure 8. In addition to the Tyco relays, two 5 V SPST switches are required for each board. Due to size restrictions on the PCBs, two boards were required as only 5 relays could fit on each PCB. The purpose of the 5 V relays is to provide a means of triggering the switches using the 5 V

13 logic output of the micro-controller. The switching board also requires a 4 V source to power the Tyco relays. The 5 V relays short the 4 V source to the relays when the micro-controller signals them to do so. Each wire connector represents either a normally closed (NC) or normally open (NO) switch. In order to have enough switches one 5 V relay controls 3 of the Tyco relays on one of the boards and is dedicated to switching the 8-pole to 4-pole operation. The remainder of the 5 V relays can be controlled at the same time, as they are all responsible for the N reduction switching. There are now three main power lines coming out of the switching board. These lines connect to the three-phase output of the power board. Before powering up the motor a good test to make sure everything is connected properly is to check the line-to-line resistance and inductance between all phases for each operating winding configuration. For each configuration these resistances and inductances should be approximately equal. K P0 5V Relay-SPST K Header Wire conn9 Wire conn Wire conn3 Wire conn34 Wire conn64 Wire conn70 gate in Header 5V Relay-SPST Header Wire conn0 Header Wire conn P V relay 6PDT Header Wire conn3 Header Wire conn4 Header Wire conn3 Header Wire conn33 P V relay 6PDT Header Wire conn35 Header Wire conn36 Header Wire conn65 Header Wire conn66 P V relay 6PDT Header Wire conn7 Header Wire conn7 GND Header Header Header Header Header Header Wire conn5 Wire conn8 Wire conn6 Wire conn67 Header Wire conn6 Header Wire conn7 P V relay 6PDT Header Wire conn9 Header Wire conn30 Header Wire conn6 Header Wire conn63 P V relay 6PDT Header Wire conn68 Header Wire conn69 Header Header Header Header Figure 8: Schematic for switching board 5.4. Power Inverter Rectifier There are several options for driving the induction motor. Among them are V/f speed control, vector control, and direct torque control (DTC). A two level inverter has 3

14 been selected for the project, using six MOSFET switches. This configuration can produce a total of 8 switching voltage vectors. Two of these are zero output vectors. Hence we have a total of 6 controlling vectors. These are shown below in Figure 9: Figure 9: Two-Level Inverter Switching States The method for determining the switch state is as follows: Using the sensors, the current and voltage input to the machine are found. The torque and flux produced by the machine are determined. Now the reference speed and load torque are known. We regulate the motor speed using a PI controller, which also gives the value of the reference torque. Based on the reference and the actual torque values, the required voltage vector is read from the look-up table (Table ). This table is also known as the optimal switching table. Table : Switching table Sector Δλ ΔT ve V V3 V4 V5 V6 V +ve 0 V0 V0 V0 V0 V0 V0 -ve V6 V V V3 V4 V5 +ve V3 V4 V5 V6 V V -ve 0 V0 V0 V0 V0 V0 V0 -ve V5 V6 V V V3 V4 4

15 For generating control of the induction machine, there are two options: ) Use capacitors to provide the quadrature exciting current to the machine, while using the MOSFET bridge as a simple inverter. ) Use the MOSFET bridge to supply excitation current to the machine while simultaneously rectifying the output. This method works only when the DC load is a battery. Since no battery is available for the competition, this method cannot be used. The power board was designed to be used in conjunction with the ST control board and few modifications were required to make it operational for the initial tests. The power board takes in a DC voltage between 70 V and 340 V and outputs a 3-phase AC output. The value used for the four bypass capacitors is expected to be 470 uf at 350 V. The Transil diode used is the P6KE400A and it was sampled from Littelfuse. Figure 0 shows the power board schematic. 5

16 Figure 0: Power Board Schematic 6

17 5.5. Controller The control board is designed to operate in conjunction with the SEMITOP 3 power board from ST and an InDART-STX debugger board. The debugger board is used to load code onto the micro-controller and allows for in-circuit debugging. In order to operate the control board, one must first download the necessary program to the control board using the debugger board and then attach the control board onto the power board. The power board provides +5 V supply to the control board. The control board developed by ST uses the 44-pin package of the ST7FMC family of micro-controllers specifically designed for motor control. For the purposes of this project more I/O pins were needed so the ST board was adapted to fit the 64 pin ST7FMCS4T6-TQFP64 ST micro-controller. This provided an additional 0 pins of I/O that is used for implementing the winding switching and the LCD display. To implement closed-loop speed control, a position encoder is attached to the shaft of the machine. The output of the encoder goes to an additional op-amp on the control board in order to amplify the signal so that the micro-controller can use the signal to control the switching and speed regulation of the motor. The encoder used for this project was the TRD-S000BD. This encoder provides 000 voltage pulses per revolution. In order to use this encoder with the ST micro-controller this needed to be reduced to 8 pulses per revolution. This was accomplished by adding counters to the control board that made the conversion. This signal can now be used to implement closed-loop speed control of the motor. 7

18 6. Test Results and Performance Analysis Several tests were performed on the motor/generator during the testing phase. These tests included: Locked rotor test of starting torque No-load motor operation Motoring acceleration test Generating test Full system test Only the latest, most informative test results are included here. These tests were performed in the final phase of the system development, and time was very limited. The motor/generator system met many of the specifications required for the 007 IFEC. This section gives a brief synopsis of how the machine performs in meeting the specifications of the challenge. The requirement of 30 Nm starting torque was not met due to the limitation on current in the inverter. The inverter failed at about 0 amps, which allowed the machine to achieve Nm of starting torque without compromising the inverter. The group is confident that by using the same inverter, rated for slightly higher current, 30 Nm starting torque would be achieved. Testing showed that the motor was able to accelerate from 0 to 3000 rpm in 4.5 seconds. Figure shows the motor current during startup and acceleration for two different tests. The controller determines the points at which the number of poles is halved and the number of windings is halved. The timing has been set to achieve the 8

19 required acceleration while minimizing the transients created at the pole and winding changes. Current Time Current Time Figure : Phase current input to machine during start and acceleration to 3000 rpm for two different tests. 9

20 Current Time Current Time Figure : Start from standstill machine phase current for two different tests. 0

21 In generation mode, testing has shown that the machine will generate at least. kw of electricity. This satisfies the IFEC requirement of kw in generation mode. Preliminary results from the IFEC testing indicated an efficiency of around 60%. Generating is achieved by connecting exciting capacitors to the machine leads. These capacitors are adjusted to allow the system to generate greater than 00Vdc at 3000 rpm. An unregulated rectifier is used to provide the DC power, so that variance in the load or shaft speed will shift the voltage generated. 7. System Cost and Weight This system was designed with cost as a major design consideration, and we believe that this machine has the lowest possible cost for this application. Table 3 shows the cost breakdown for the starter/alternator system, including what we paid for the parts and the estimated mass production cost. The total mass production cost is estimated to be approximately $35 dollars. This is assuming mass production rates, use of standard electronics, iron and aluminum parts. Table 3: Cost and Weight Data and Production Estimates. Component Prototype Cost Estimated Production Cost Prototype Weight Estimated Production Weight Induction $300 $60 4 kg 9 kg Machine Controller: Control Board $45 $5 0. kg 0. kg Inverter $30 $30 0. kg 0. kg Switch Board $30 $30 0. kg 0. kg Rectifier $0 N/A 0. kg N/A Capacitors $5 N/A 0.4 kg N/A Total $740 $35 5 kg 9.5 kg Note: Capacitors will not be required in mass production, and the rectifier and inverter will be one item The machine used for this system is a simple, standard induction machine, which has been rewound as described in this report. As a mass production part, the machine cost

22 could be reduced significantly because of the simple construction using common and inexpensive materials. The controller houses several relays, a programmed inverter and protection and safety provisions. In a mass production unit solid state switches would be used, reducing cost and weight and increasing reliability. This unit uses standard electronic parts whose cost could be reduced significantly in a mass production scenario. The packaging requirements for the IFEC are to meet the NEMA 56 standards and to have a total system weight under 0 kg. The NEMA 56 standards were met in terms of bolt patterns and shaft diameter, in order to interface with the test setup. The weight requirement was not met, as the machine weights just over 4 kg (3 lbs). Several strategies are envisioned for lightening the machine in a second design iteration. These options include using a lighter aluminum housing and a lighter rotor (the housing and rotor were obtained free of charge), and changing the dimensions of the machine to optimize the length-to-diameter ratio. The off-the-shelf induction motor used for the prototype had more back iron in the stator laminations than is required. Optimization of the utilization of metal in the machine could reduce its weight by an estimated third. The expected lowest weight of this machine would be about 9 kg (0 lbs). 8. Conclusions and Future Work This project has been an excellent experience for all of the students involved. Designing, building, and testing this machine has taught skills that cannot be taught in a classroom setting and will prove invaluable as these students enter the workforce. Participation in the final competition should prove to be a very positive experience as well.

23 The group feels strongly that a second iteration of this design process would yield even more positive results. There are several new ideas to try, including a double squirrel-cage induction machine, a customized housing and rotor, and developing our own (more robust) inverter. In short, the group feels that we now know what we are doing much more so now than at the beginning of this project, and we believe that the pole switching, winding switching induction machine is a good idea to pursue for the design of this type of motor/generator. Our team will pursue funding options for continued development of the concept. 9. References [] E. F. Fuchs and J. Schraud, "Analysis of Critical-Speed Increase of Induction Machines via Winding Reconfiguration with Solid-State Switches," IEEE Trans. on Energy Conversion, Paper No. TEC [] J. Schraud and E. F. Fuchs, "Experimental Verification of Critical-Speed Increase of Induction Machines via Winding Reconfiguration with Solid-State Switches," IEEE Trans. on Energy Conversion, Paper No. TEC [3] E. F. Fuchs and M. A. S. Masoum, Power Quality in Power Systems and Electric Machines, Elsevier, Inc., 007. [4] G. Bhuvaneshwari, Direct Torque Control of Induction Machine applied to Automobile System, Senior Member, IEEE. [5] J. C. Trounce, S. D. Round, R. M. Duke, Evaluation of Direct Torque Control Using Space Vector Modulation for Electric Vehicle Applications, University of Canterbury, New Zealand. 3