High Efficiency Three-Phase Soft-Switching Inverter for Electric Vehicle Drives

Pengwei Sun, Jih-Sheng Lai, Hao Qian,Wensong Yu Future Energy Electronics Center

Virginia Polytechnic Institute and State University Blacksburg, U.S.A.

Chris Smith, John Bates Azure Dynamics Inc.

Woburn, U.S.A.

Abstract— This paper presents a high efficiency three-phase soft-switching inverter for electric vehicle drives application. A 12kW three-phase soft-switching inverter using CoolMOS as the main device and IGBT as the auxiliary device has been designed and fabricated. The electric vehicle drive test system has been assembled with soft-switching inverter, induction motor and dynamometer. The soft-switching inverter has been tested with closed-loop torque direction control under speed reversal conditions. The measured efficiency of the inverter in a wide load range is above 98.5%. The calorimeter test at 12kW shows 98.8% efficiency.

Keywords-soft-switching inverter; high efficiency;electric vehicle drives;

I. INTRODUCTION For the emerging hybrid electric vehicles (HEV) and

electric vehicles (EV) industry, there are many key technologies [1-3] involved in the field of power electronics. Optimization of the power inverters for electric motor drives is one of the indispensable technologies. As such, different types of inverters [4-8] have been studied and tested to address different issues. However, few of them discuss the efficiency improvement of the inverter, which is definitely an important facing challenge, especially for a pure EV. In [9], it presents a three-phase inverter for automotive traction with SiC Schottky diodes and it showed that the Si-SiC hybrid inverter losses are up to 33.6% less than the all Si inverter with the peak efficiency at around 97%. The EV utilizes electric motor for propulsion and battery as the only source of energy. With limited source of energy, efficiency is a major concern, which leads directly to the range of the vehicle [10]. In order to improve the system efficiency, several types of soft-switching inverters have been studied in the EV application. The soft-switching inverters have demonstrated efficiency improvement over hard-switching inverters [11-13]. With the increasing frequency to lower the ripple current into the motor and thus boost the system efficiency, soft switching has much superiority over hard switching when switching frequency is higher than 10kHz [13]. At high temperature applications, soft-switching technique can be a good candidate to maintain high efficiency to employ low-cost conventional silicon power devices [14].

However, a comparison research [15] has indicated soft-switching inverters were not necessarily having higher

efficiency in this application than their hard-switching counterparts. One of the reasons is that in EV drives, the inverter has to operate in a wide range of loads and power factors. Many existing soft-switching inverters cannot achieve full current range soft-switching conditions. Another unfavorable factor to soft-switching inverters is that they mostly rely on additional auxiliary switches and passive components for completing soft-switching process, which imposes extra loss.

This paper presents a high efficiency three-phase soft-switching inverter for electric vehicle drives. The proposed inverter utilizes the combination of variable timing control and independent coupled-magnatics techniques [16-17] to address the above mentioned issues and thus to improve the efficiency further more. The variable timing controlled soft-switching inverter [14] achieves zero-voltage switching (ZVS) of main switches over the entire load current range by simply sensing the device voltage. Significant efficiency improvement was found at light-load conditions compared to fixed timing control. The soft-switching inverter using two coupled-magnetics for each phase leg [17] avoids the magnetizing current circulating loop and thus eliminates the need for a saturable inductor with additional core loss. Additional efficiency benefit of this topology is the zero-current switching (ZCS) of the auxiliary switches over the entire load range with the fully reset magnetizing current.

The design of proposed three-phase soft-switching inverter was presented first along with the motor test system setup. The motor was tested at speed reversal conditions with motoring and regenerating modes. Finally, the efficiency test of the inverter was conducted under different load and power factor conditions. The measured efficiency of the inverter in a wide load range is above 98.5% and peaks at 99% at room temperature. In addition, for more accurate measurement, the calorimeter test was performed and at 12kW the inverter efficiency is 98.8%.

II. SOFT-SWITCHING INVERTER DESIGN AND SYSTEM SETUP

A. Three-Phase Soft-Switching Inverter Topology Fig. 1 shows the power circuit of the proposed three-

phase soft-switching inverter. There are three identical phase legs in the topology. Each phase leg consists of its main switches, auxiliary switches as well as resonant capacitors,

This material is based upon work supported by the U.S. Department of Energy (DOE) under Award Number DE-FC26-07NT43214.

978-1-4244-2601-0/09/$25.00 ©2009 IEEE 761

coupled-magnetics, and diodes. Power MOSFET switches S1-S6 serve as the main inverter switches. Each main switch is paralleled with a lossless snubber capacitor C1-C6 to reduce the turn-off loss as well as function as resonant capacitors. IGBT switches Sx1-Sx6 are the auxiliary switches that assist the main switches to achieve ZVS. The auxiliary switch only turns on in a very short period but requires large peak current capability. Coupled-magnetics Lr1-Lr6 serve as the resonant components to establish zero-voltage condition for the main switches and as the resetting component to reset the resonant current so that the auxiliary switches can turn off at ZCS condition.

B. Soft-Switching Invertr Design The soft-switching inverter is mainly composed of three

parts: power board, gate driver board and interface board. The power board consists of all the power devices and resonant capacitors shown in Fig. 1. Fig. 2 shows the designed three-phase soft-switching inverter. The gate drive board sits on top of the power board, which connects power devices with thick traces. On top of the gate drive board there is an interface board that allows the inverter directly talking to the DSP board. All the control functions are performed with Azure Dynamics DSP motor control board.

The gate driver board incorporates the variable timing control by a simple device voltage detecting and comparison circuit. All the auxiliary power supplies for gate driver board and DSP board come from the interface board. The interface board also performs the functions of sensing DC bus voltage and AC output phase currents as well as communication with DSP board.

Major circuit components are listed as follows:

* Main devices S1–S6: CoolMOS, IPW60R045CP, rated 650V, 45mΩ; each S consists of two paralleled devices. * Auxiliary devices Sx1–Sx6: IGBT, IXGH30N60BD1, rated 600V, 60A. * Resonant capacitors C1–C6: polypropylene, rated 630V, 6.8nF. * Coupled-magnetics Lr1–Lr6: ETD49 core; 0.6mH magnetizing inductance and 5μH leakage inductance measured from primary; turns ratio n = 1.4.

Vdc

Lr1

Sx1

Cdc

Lr3

Sx3Lr5

Sx5

Sx4 Sx6 Sx2

Lr4 Lr6 Lr2

S1 C1

S4 C4

S3

S6 C6

S5

S2 C2

C3 C5

ac motor

Figure 1. Circuit diagram of three-phase soft-switching inverter.

Devices

Heat sink

Gate drive

Interface

Coupled magnetics

Figure 2. Picture of three-phase soft-switching inverter.

C. System Setup The test system setup is shown in Fig. 3. All the control

functions were performed with the digital signal processor (DSP) board. The inverter is controlled with 10-kHz discontinuous SVPWM using TI TMS320F2407 DSP. The battery bank provides the DC bus voltage and power to the soft-switching inverter. An AC55 motor was connected with the dynamometer by proper couplers. Fig. 4 shows the complete dynamometer setup with a hysteresis brake and the AC55 motor assembly.

Figure 3. Test system block diagram.

Figure 4. Dynamometer setup with a hysteresis brake and the AC55

motor assembly.

III. SPEED REVERSAL TEST Repetitive tests with forward and backward rotating have been conducted to ensure the closed-loop controller

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works with the soft-switching inverter. The test can also verify if the inverter can operate in motoring and regenerating modes. Fig. 5 shows three-phase motor current waveforms under speed reversal test. Initially the motor was accelerated under constant torque condition. A negative torque is suddenly commanded in the middle of motoring mode operation. The motor starts with a slow speed turning in the reverse direction and gradually accelerates with the new constant torque.

Since the negative torque command can be applied to any point of operation, the speed reversal may not be as smooth as it would be. Fig. 6 shows the current waveform under speed reversal test with a lower acceleration frequency immediately following the negative torque command. The case can also happen with a fast initial acceleration frequency, as shown in Fig. 7, which indicates a faster acceleration frequency immediately following the negative torque command. This initially accelerating frequency is automatically adjusted by the controller depending on the state of motor speed and inverter operation.

Fig. 8 shows the current waveform under speed reversal test with varying frequency and phase following the negative torque command. The initial frequency after the command was not low enough to turn in reverse direction, but the controller automatically adjusted the frequency to reaccelerate again. The repetitive test indicated that the initial frequency after negative torque command is as low as 1 Hz.

200ms/div

speed reversal with negative torque commanded

Figure 5. Three-phase motor current waveforms under speed reversal test.

200ms/div

speed reversal with negative torque commanded

Figure 6. Motor current waveforms under speed reversal test with lower initial acceleration frequency.

100ms/div

speed reversal with negative torque commanded

Figure 7. Speed reversal test with fast initial acceleration frequency.

200ms/div

speed reversal with negative torque commanded

Figure 8. Speed reversal with automatic adjustment of frequency and phase.

IV. EFFICIENCY TEST

A. Inductive Load Test Fig. 9 shows the steady-state three-phase current test

waveforms at 11kVA inductive load condition. Three 13.9mH inductors are connected in Y-configuration to serve as the load. The inverter operates at 300-V dc bus voltage. The fundamental frequency in this case is 83.3 Hz, which corresponds to the nominal motor speed of 2500 rpm. Output currents are smooth and balanced.

Fig. 10 shows test waveforms of three-phase output currents and phase-c resonant inductor current. It can be seen that the PWM is discontinuous with 60° peak current region non-conducting. This discontinuous SVPWM equivalently reduces the switching frequency and is preferred for high efficiency motor drive design.

2ms/div

ia icib

ia, ib, ic: (20A/div)

Figure 9. Three-phase current waveforms at 11-kVA test condition.

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2ms/div

ia icib iLr

ia, ib, ic: (20A/div)

iLr: (50A/div)

Figure 10. Three-phase current waveforms and resonant inductor current.

B. Efficiency Test with Passive Loads Fig. 11 shows the measured efficiency at 300-V dc bus,

83.3 Hz output condition. The load is a resistor bank in series with 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 power can be controlled. At lower modulation index condition, the power factor is low, and the inverter efficiency is also low because it needs to circulate current. Additional factor that lowers the light-load efficiency is the fixed gate driver board and interface board power consumption. Note the total efficiency here takes the gate drive and other auxiliary power supply of 26.5 W into account. The loaded efficiency, however, gradually increases as the load increases.

Fig. 12 shows the measured efficiency at 325-V, 83.3-Hz condition. Three resistance values were used in the test: 18 Ω, 9 Ω, and 4.5 Ω. In each case, a fixed three-phase inductor is connected with the resistor bank. At a lower modulation index condition, the output voltage and power factor are also lower. The efficiency increases as the modulation index or output power increase. The peak efficiency of 99.1% was found with 9-Ω resistor in series with 4.5mH inductor per phase.

Fig. 13 shows the efficiency plot under constant power factor condition. At power factor of 0.83, the peak efficiency is 98.8%. At power factor of 0.87, the peak efficiency is 99%. The AC55 induction motor has a power factor of 0.83 in the most operating range. Therefore, the peak inverter efficiency can be expected as 98.8%. For other types of motors, especially like permanent magnet motors where the power factor is typically higher than 0.9, the soft switching inverter efficiency should exceed 99% in a wide load range.

Note that all the above efficiency measurements were done by using digital power meters with accuracy of ±0.1%, which could lead up to 0.4% efficiency difference at full power range. For a high efficient 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 [18-20]. Therefore, the calorimeter measurement was conducted with AC55 motor.

95.0%95.5%96.0%96.5%97.0%97.5%98.0%98.5%99.0%

0 1 2 3 4 5 6 7 8 9 10 11 12

Power (kW)

Inve

rter E

ffici

ency

(%)

Fixed RL load 9ohm+4.5mH Fixed RL load 2 of 9Ω+4.5mH in parallel

Fixed power factor at 0.83

Vdc = 300Vf1 = 83.3 Hzfsw = 10 kHz

Figure 11. Efficiency measurement with RL load at 300V dc bus condition.

95.0%95.5%96.0%96.5%97.0%97.5%98.0%98.5%99.0%99.5%

100.0%

0 1 2 3 4 5 6 7 8 9 10

Power (kW)

Inve

rter E

ffici

ency

Fixed RL load 9Ω+4.5mH

Fixed RL load: 2 of 9Ω+4.5mH in parallel

Fixed RL load 18Ω+4.5mH

Vdc = 325Vf1 = 83.3 Hzfsw = 10 kHz

Figure 12. Efficiency measurement with RL load at 325V dc bus condition.

96.0%

96.5%97.0%

97.5%98.0%

98.5%

99.0%99.5%

100.0%

0 1 2 3 4 5 6 7 8 9 10Power (kW)

Inve

rter E

ffici

ency

Fixed power factor at 0.83

Fixed power factor at 0.87

Vdc = 325Vf1 = 83.3 Hzfsw = 10 kHz

Figure 13. Efficiency measurement with RL load at 325V dc bus

condition with constant power factors.

C. Calorimeter Test with AC55 Motor The soft-switching inverter was tested with AC55

motor at 12kW output power. The motor operates at speed 2800rpm, torque 35Nm and power factor 0.83 conditions. Fig. 14 shows the diagram of the calorimeter measurement setup. We used a double chamber calorimeter method [21], which removes the need for measuring fluid properties and associated measurement errors. Fig. 15 shows photograph of the calorimeter used to test the inverter efficiency. Assuming that the properties of the cooling fluid in the setup

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remain constant across the system, a simple energy balance can be used to find the power losses in the inverter using the temperature rise across the first and second chambers, ΔT1 and ΔT2 respectively, and the power input by an adjustable heater. This balance is

1

2

out midinv loss heater heater

mid in

T TTP P PT T T−

−Δ= ⋅ = ⋅Δ −

(1)

Figure 14. Diagram of the calorimeter measurement setup.

Figure 15. Calorimeter with reference chamber in foreground and

inverter chamber partially open in back.

RTD sensors were placed in the system to measure points Tout, Tmid, and Tin. The power of the electric heater was obtained by measuring its current and voltage. The calorimeter test was performed for five hours to wait until the thermal measurement reached its steady state. Fig. 16 shows the coolant temperatures of Tout, Tmid, and Tin. Fig. 17 shows the measured inverter efficiency. As can be seen, at the initial stage, the efficiency fluctuates; after thermal balanced is well established, the efficiency flattens. The calorimeter-tested inverter efficiency at 12kW is 98.8%.

Figure 16. Coolant temperatures of calorimeter measurement points.

Figure 17. Calorimeter measurement of inverter efficiency.

V. CONCLUSION A high efficiency three-phase soft-switching inverter is

proposed for electric vehicle application. The high efficiency inverter adopts the variable timing control technique and uses two coupled-magnetics structure for each phase leg, which both result in efficiency improvement. The three-phase soft-switching was designed and assembled with DSP, motor and dynamometer in the system test. It has been successfully tested with closed-loop torque controller for severe speed reversal conditions under constant torque acceleration and deceleration. Efficiency measurement with passive loads demonstrated peak efficiency at 99% along with above 98.5% efficiency at motor most operating power factors. The calorimeter measurement of the soft-switching inverter driving AC55 at 12kW output power shows efficiency of 98.8%.

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