Efficiency Evaluation of A 55kW Soft-Switching Module Based Inverter for High Temperature Hybrid Electric Vehicle Drives Application

Transcription

1 Efficiency Evaluation of A 55kW Soft-Switching Module Based Inverter for High Temperature Hybrid Electric Vehicle Drives Application Pengwei Sun, Jih-Sheng Lai, Hao Qian, and Wensong Yu Virginia Tech, Blacksburg, VA 24060, USA Chris Smith, John Bates, and Beat Arnet Azure Dynamics Inc., Woburn, MA 01801, USA Alexander Litvinov, and Scott Leslie Powerex Inc., Youngwood, PA 15697, USA Abstract This paper presents a 55kW three-phase softswitching inverter for hybrid electric vehicle drives at high temperature conditions. Highly integrated softswitching modules have been employed to achieve switching loss as well as conduction loss reduction. Detailed experimental evaluations of inverter efficiency have been conducted through both inductive load and motor-dynamometer load at coolant temperatures ranging from 25 C to 90 C. Efficiency measurement using power meter showed that the peak efficiency is around 99%, and it drops slightly at lower speed and higher temperature conditions. To ensure measurement fidelity, a double chamber differential calorimeter system was designed and calibrated for the inverter testing. Through long-hour testing, the measured efficiencies consistently showed 99% and higher. The soft-switching inverter has been operated reliably and demonstrated high efficiency at different temperature and test conditions. I. INTRODUCTION Optimized device and system structure for high efficiency power conversion can prove to be beneficial to many industries, especially for high-temperature operation requirement. One of the driving forces of high-temperature operation is the elimination of bulky and expensive cooling systems, which are necessary to protect power electronics system from extreme working conditions [1-4]. The state-ofthe-art hybrid electric vehicles have a separate cooling loop for electronics at a maximum coolant temperature of 70 C. It would be desirable to eliminate such an extra cooling loop and use the engine coolant system, which would have a maximum temperature of 105 C. Nevertheless, the performance of the semiconductor devices degrades rapidly with the increase of temperature. Despite challenges, a coolant temperature requirement of 105 C has been established for 2015 with an intermediate step of 90 C for 2010 by FreedomCAR and Fuel Partnership Program under U.S. Department of Energy [5]. To meet the design challenges, there are several alternative ways [6], including using more silicon, by means of wideband-gap semiconductors [7-9], improvement of thermal management techniques. The main barrier is the rising cost imposed by adopting those measures. In [6], it points out that with the help of soft-switching technique, the conventional silicon power devices have the chance of meeting hightemperature operation requirements with much more reasonable cost than other approaches. However, it only explored the use of IGBT under soft-switching for reduction of switching loss. For IGBT devices, there is a fixed voltage drop even at lower current region. In order to reduce the conduction loss, a hybrid switch in the form of MOSFET and IGBT parallel operation is proposed. The advantage of this switch combination is to have MOSFET conducting the current at low current and IGBT conducting the high current. The hybrid switch voltage drop at low currents is proportional to current, and at high currents is dominated by the IGBT and is increased as the current increases. Using the hybrid switch in the soft-switching inverter circuit described in [10], a liquid-cooled soft-switch module has been developed. The module integrates main IGBTs, MOSFETs, auxiliary IGBTs, and diodes with capability of 400-A continuous current operation. The integration of these chips allows significant parasitic inductance and thermal resistance reduction. Using such a highly integrated liquidcooled soft-switch module, a 55-kW three-phase softswitching inverter was designed and assembled. The inverter efficiency was evaluated under both inductive load and motordynamometer load tests with coolant temperatures ranging from 25 C to 90 C. A double chamber differential calorimeter was introduced for precision inverter efficiency measurement. The soft-switching inverter was successfully operated at various temperature and test conditions. The power meter measurement from 20% to 100% output consistently shows This material is based upon work supported by the U.S. Department of Energy (DOE) under Award Number DE-FC26-07NT /10/$ IEEE 474

2 efficiency higher than 98% under different temperature conditions, and the peak efficiency with calorimeter measurement exceeds 99%. II. INVERTR SYSTEM ASSEMBLY AND SETUP The three-phase soft-switching inverter consisted of three identical soft-switching modules, which are shown in shaded area in Fig.1. S 1 to S 6 are main switches composed of paralleled IGBT and MOSFET devices. S x1 to S x6 are auxiliary IGBT switches. In each module, there are also four auxiliary diodes. All the devices have the voltage rating of 600V. L r1 to L r6 are coupled magnetics with turns-ratio 1:1.35. C 1 to C 6 are resonant capacitors with value of 100nF. Fig. 2 shows the photograph of the complete integrated liquid-cooled softswitching module based inverter. The coolant can be pumped to each individual switch through manifold, and the inlet temperature is regulated by a chiller/heater. The manifold is designed to make sure each module has the same watercooling flow rate and same length cooling path. Figure 3. Module-based soft-switching inverter. S x1 S 1 C 1 S x3 S 3 C 3 S x5 S 5 C 5 L r1 L r3 L r5 V dc L r4 L r6 L r2 S x4 S 4 C 4 S x6 S 6 C 6 S x2 S 2 C 2 Figure 4. Soft-switching inverter motor test system. Figure 1. Circuit diagram of three-phase soft-switching module based inverter. Figure 2. Liquid-cooled soft-switching modules. A complete three-phase inverter has been designed and assembled using the integrated liquid-cooled soft-switching modules. Fig. 3 shows the assembled inverter. The gate drivers sit on top of each soft-switching module. They incorporate variable timing control circuit to ensure the entire load range zero voltage switching of main devices [11]. The inverter is controlled with a 10-kHz discontinuous space vector modulation using TI TMS320F2407A DSP. The DC power supply provides the DC bus voltage (325V) and power to the soft-switching inverter. An AC55 TM induction motor with 2500rpm nominal speed, 30kW continuous shaft-power, and 55kW peak power was connected to an eddy-current braked dynamometer through torsional coupling. Fig. 4 shows the picture of the complete motor-dynamometer system setup in the lab. III. EFFICIENCY TEST UNDER INDUCTIVE LOAD The inductive load test was performed in order to push to higher output voltage and current conditions. The reactive power kva and the line frequency represent the output power and speed of the motor load, and their losses are in the similar scale. Therefore, it is reasonable to project the efficiency with reactive power test. The load is a Δ-connected three-phase inductor. Equivalent inductance is about 4.5mH per phase. By controlling the modulation index, the output voltage, and thus the output reactive power can be controlled. The dc bus voltage was fixed at 325V, and the output line frequencies varied at 45Hz, 60Hz and 83.3Hz. Temperature was regulated at four different conditions: 25 C, 50 C, 75 C and 90 C. Fig. 5 shows the projected efficiency based on the inductive load measured loss results at different output line frequency and different temperatures. The power factor in these cases is assumed 0.83, which is the same as what has been tested on the motor drive cases. It is noted that at the light load condition, the efficiency difference is more obvious than that at the heavy load condition. The reason is that at light loads, MOSFET shares more current, and with positive temperature coefficient, the efficiency suffers. However, at heavy loads, the LPT IGBT shares more current, and with the negative temperature coefficient [6], its efficiency hit by temperature is not as severe. The peak efficiency approaches close to 99%. Efficiency drops slightly with higher temperatures, typically 0.1% per 50 C. 475

3 (c) (c) Figure 5. Efficiency comparison under different temperatures and line frequencies: 83.3Hz, 60Hz, (c) 45Hz. To compare the efficiency under different frequency or motor speed conditions, the above results are rearranged to have the same temperature condition but under different frequencies. Fig. 6 shows the projected efficiencies between different output line frequencies at 25 C, 50 C, 75 C and 90 C, respectively. As can be seen, at the same output power point, the efficiency is higher at a higher output line frequency. That can be translated into higher speed with higher efficiency, which is proven by later motor tests. (d) Figure 6. Efficiency comparison under different line frequencies and temperatures: 25 C, 50 C, (c) 75 C, and (d) 90 C. IV. EFFICIENCY TEST UNDER MOTOR LOAD The motor test setup is shown in Fig. 4. We tested motor at different speed conditions, 1000rpm, 1500rpm and 2000rpm with different output current values, 30A, 40A and 50A at different temperatures, 25 C, 50 C and 75 C. Due to limited DC power supply capacity in the lab, the test was conducted for up to 30% load. The high power test with calorimeter measurement was then conducted with regeneration type dynamometer, and the results will be discussed in the next section. Table 1 shows the tested inverter efficiency at different speeds and different output currents at different temperatures with power factor At lower output power, the lowtemperature efficiency is slightly higher than the hightemperature one. At higher output power, the high temperature efficiency catches up and may surpass low temperature one. At higher motor speed and thus higher output frequency, the inverter efficiency is higher than lower speed conditions. 476

4 Table 1. Efficiency measurement with motor test at different temperatures. Figure 7. Diagram of the calorimeter measurement setup. Table 2 shows the efficiency comparison between inductive load test and motor test when the efficiency is reflected to 0.83 power factor. At the same output power, the motor test efficiency is higher than the pure inductive load test efficiency. The reason is during motor dynamometer test, the current is mainly conducting through MOSFET and IGBT channels, while in inductive load test, the duty cycle of the anti-paralleled diodes increases, and the efficiency is suffered slightly. Previous inductive load test shows that at 83.3Hz (30.6kVA), the efficiency is 98.8%; and at 60Hz (46.8kVA), the efficiency is 98.6%. Therefore, the peak efficiency at higher motor load can be expected to exceed 99%. Table 2. Efficiency comparison between motor test and inductive load test at different temperatures. Figure 8. Calorimeter with reference chamber in foreground and inverter chamber in back. Figure 9. The high temperature heat exchanger and pump hooked into the back of the calorimeter. V. EFFICIENCY TEST UNDER CALIROMETER All the above efficiency measurements were done by using digital power meters with accuracy of ±0.1%. For a high efficiency inverter, the calorimeter method to measure the total power loss of the high-frequency switched inverter is considered the more accurate way to determine the efficiency [12-14]. Therefore, the calorimeter measurement was conducted for precision efficiency determination. Fig. 7 shows the diagram of the calorimeter measurement setup. We used a double chamber differential calorimeter method [15], which removes the need for measuring fluid properties and associated measurement errors. Fig. 8 shows photograph of the calorimeter used to test the inverter efficiency. Fig. 9 shows the high temperature heat exchanger and pump hooked into the back of the calorimeter. Assuming that the properties of the cooling fluid in the setup remain constant across the system, a simple energy balance condition can be used to find the power losses in the inverter using the temperature rise across the first and second chambers, ΔT 1 and ΔT 2 respectively, and the power input by an adjustable heater. This balance is ΔT Tout T P = P = P T T T 1 inv loss heater heater Δ 2 Precision temperature sensors were placed in the system to measure points T out, T mid, and T in. The power of the electric heater was obtained by measuring its current and voltage. The calorimeter test was performed for more than five hours to wait until the thermal condition reached its steady state. Fig. 10 to Fig. 12 shows the coolant temperatures and the measured inverter efficiency at 12kW, 18kW and 27kW, respectively. Different speeds and power factors were tested, which were indicated in the figures. As can be seen, at the initial stage, the efficiency fluctuates; after thermal balanced mid mid in (1) 477

5 is well established, the efficiency flattens. From the test results, it is clear that at higher speed and higher power factor, the efficiency is higher, which proves the motor test in previous section. The calorimeter-tested inverter efficiency is 98.8% at 12kW, 99.1% at 18kW, 98.8% at 27kW. Figure 12. Calorimeter measurement at 27kW: coolant temperatures, inverter efficiency. Figure 10. Calorimeter measurement at 12kW: coolant temperatures, inverter efficiency. VI. CONCULSION A high efficiency three-phase soft-switching inverter has been developed and evaluated for high temperature hybrid electric vehicle application. The inverter is designed and assembled by three liquid-cooled soft-switching modules, which reduce both switching and conduction losses. Complete efficiency tests have been performed at different coolant temperatures ranging from 25 C to 90 C under inductive and motor dynamometer loads. Test results indicate that peak efficiency at the rated speed is around 99%. For the same torque, efficiency drops as speed lowers, typically 0.1% per 500 rpm. For the same output, efficiency drops slightly as temperature increases, typically 0.1% per 50 C. The calorimeter test has been conducted to verify the test results with power meters. Peak efficiency of 99.1% at 30% load was observed. Experimental results prove the feasibility of soft-switching module based inverter operating at high temperature for hybrid electric vehicle application. Figure 11. Calorimeter measurement at 18kW: coolant temperatures, inverter efficiency. ACKNOWLEDGMENT The authors would like to thank their DOE FreedomCAR Program partners for their valuable contributions. The softswitching modules were designed and fabricated by colleagues at Powerex. The calorimeter test setup was provided by colleagues at Azure Dynamics. Special thanks go to the lab mechanical engineer, Gary Kerr, for his effort on mechanical design and assembly of the system. 478

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