Design of a High-Efficiency 12V/1kW 3-Phase BLDC Motor Drive System for Diesel Engine Emissions

Reductions

Allan Taylor, Chenguang Jiang, Kevin (Hua) Bai Department of Electrical and Computer Engineering,

Kettering University Flint MI 48504 USA hbai@kettering.edu

Adam Kotrba, Argun Yetkin, Arda Gundogan Tenneco, Inc.

3901 Willis Road Grass Lake, MI 49240 USA

akotrba@tenneco.com

Abstract- For the conventional vehicle, an efficient and effective heat source that provides autonomous exhaust temperature control is of interest, and one solution is a diesel burner which needs to adjust its air delivery based on transient operating conditions through a three-phase motor drive system powered by a 12V lead-acid battery. The system proposed in this paper consists of a series-resonant LLC MOSFET full-bridge converter, which provides high-efficiency energy transfer through implementing Zero Voltage Switching, and an IGBT inverter which provides the high-side phase currents to a 1kW brushless DC motor. Experimental results on this prototype system demonstrate the LLC DC/DC part efficiency is 97.6% and the inverter efficiency is 92%.

I. INTRODUCTION Diesel Particulate Filters (DPFs) have been

successfully applied for several years to reduce Particulate Matter (PM) emissions on-highway applications, and similar products are now also applied in off-highway markets and retrofit solutions [1]. For the traditional diesel engine, when the exhaust goes through a DPF, soot particles accumulate, and unless managed over time it results in high exhaust backpressure and eventual soot overload that damages the DPF. If exhaust temperatures are not controlled, low-temperature duty cycles could lead to the ineffectiveness even of passive regeneration, NO to NO2 conversion with a catalyst, which then reacts with the soot to slowly yet continuously burn [2]. Thus, autonomous heat sources have been developed to eliminate system performance risks of such cold duty cycles, applying an ignition-based combustor that leverages the current diesel fuel supply and safely provides necessary energy whenever needed, which is shown as Figure 1 [3].

This system applies air and fuel within the burner simultaneously to increase the exhaust temperature to

control DPF soot load or other catalyst activities [4]. Typically, the air compressor system is powered mechanically through the engine crankshaft mounted within the front end accessory drive (FEAD) system. However, this power source has its own disadvantages, i.e., the compressor's speed cannot easily be controlled and unless a clutch is integrated, its pulley cannot be disengaged, constantly applying engine power and consuming fuel. As the only available DC electric power source in conventional vehicle, a 12V lead-acid battery is preferred to power the air compressor, offering autonomous control across all conditions regardless of engine speed.

Figure 1. Thermal Regeneration Unit for Diesel Exhaust System

This paper proposes a topology using the on-board 12V battery as the source to drive a 1kW BLDC motor coupled to an air compressor. But, in order to avoid large current demand, a more convenient method is applied, boosting 12V to a high DC voltage, feeding it to a DC/AC inverter and driving a high voltage (~300V)

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motor, reducing its packaging demands and mass. Therefore the whole system contains a DC/DC and a DC/AC portion, with an overall topology shown in Figure 2. Section II describes the analysis of the DC/DC converter using an LLC resonant, as well as the control theory for the DC/AC inverter to drive a BLDC motor. Section III describes the experimental results, and Section IV is the conclusion and discussions on future work.

II. OPERATION ANALYSIS OF THE LLC RESONANT DC/DC CONVERTER AND 3-PHASE INVERTER

Figure 2. Topology for the 12V/1kW Motor Drive System

The DC/DC converter used in this topology is a full-bridge LLC series-resonant converter, of which its primary goal is to transform the input voltage while maintaining high power density and low switching losses using zero-voltage switching (ZVS) of the MOSFETs [5, 6]. Two pairs of complimentary MOSFET sets, supplied by a 12V battery, are used to create low-voltage AC output across a series resonant tank and coupling transformer. The transformer features a large turn ratio (1:28) to yield a high voltage gain, and a full-bridge rectifier is used to convert the transformer secondary AC to a high-voltage DC (~300V). Having a high-voltage DC bus will yield lower current stress in the inverter, however because the output of the motor was selected as 1kW, pulling the necessary energy from the 12V source will require significant amounts of current. Therefore the current stress and power loss of the DC/DC converter need careful attention. A full bridge topology is necessary to decrease the current stress as compared to other topologies [7]. More importantly, ZVS should be realized to eliminate the reverse recovery loss of MOSFET body diodes.

This system is sensitive to the parameters, e.g., the resonant leakage inductance Lr, capacitance Cr, reflected excitation inductance Lm and the load. In order to shrink the system size, high-frequency operation is necessary. Thus, the semiconductor switching losses are the primary concern, especially the reverse recovery loss of MOSFET body diodes. Softly turning on a MOSFET (or equivalently, softly turning off its body diode) requires ZVS of the device. In order to realize ZVS with the four MOSFETs on the primary H-bridge, the whole system needs be operated at a switching frequency between ω1 and ω2, where

rmrrr CLLand

CL )(11

21 +== ωω

(1) The overall impedance reflected to the H-bridge

through the transformer and resonant tank is shown as (2) where Reo is the equivalent load. For a given DC-bus output voltage equal to Vo at a load current equal to Io, the equivalent resistance can be defined as

o

oeo I

VnR 22 8

π= , where n is the turn ratio of the

transformer.

2 2

1 / /

(1 ( ) ) (1 )( )

r r eor

eo m r r m r r

r eo m

Z j L j L Rj C

R L L C j L L Cj C R j L

ω ωωω ω ω

ω ω

= + +

− + + −=+

(2) From (2), through changing the switching

frequency fs, the overall impedance of the resonant network will change. Within the limits above, a lower fs will lead to higher power transfer through the transformer. Thus, through controlling the LLC switching frequency, the inverter's DC input voltage can be varied, allowing more flexible inverter operation at different power levels. Table 1 shows the DC/DC converter parameters.

TABLE I. PARAMETERS OF LLC RESONANT DC/DC CONVERTER

Maximum Output Power 1kW

Low Side Voltage 12V High Side Voltage 70V to 350V

Switching Frequency 50kHz to 100kHz Battery-Side DC-bus Capacitor 188µF

Resonant Capacitor 20µF Resonant Inductor 0.3µH

Transformer Winding Ratio 1:28 High Side DC-bus Capacitor 100µF

Based on the above parameters, a MATLAB/Simulink model was constructed. Simulation waveform of the MOSFETs H-bridge voltage and current output operating near full power are displayed in Figure 3 to illustrate the ZVS condition. At this operating point, the simulated peak current is ~170A while the turn-off current is around 120A. For each of the four semiconducting switches, two 40V/190A MOSFETs are applied in parallel, decreasing the equivalent internal resistance of the MOSFETs (0.5mΩ

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for two paralleled) and increasing the system efficiency, to be validated experimentally.

Figure 3. Simulated LLC Bridge Voltage (top) and Current (bottom) operating at ZVS

From the inverter side, a three-phase two-level topology is the most widely used, as shown in Figure 2, which could drive either PMSM or BLDC motors, but BLDC (340V/1kW) is chosen to simplify its control strategy. Hall-effect sensors are used to achieve closed-loop speed control of the motor through sampling the shaft position. Various control algorithms for the BLDC motor [8, 9] are available, and Figure 4 is the simulated voltage and current of the BLDC motor, with the top channel being the line-line voltage and the bottom being the motor phase current.

Figure 4. Line-line Voltage and Phase Current of the BLDC at 6000rpm, 1kW

III. EXPERIMENTAL RESULTS The experimental setup is shown in Figure 5, with

the entire system designed onto a single PCB. Both the DC/DC converter and high-voltage DC/AC inverter are controlled via an onboard Texas Instruments digital signal processor (DSP). The low-voltage resonant DC/DC converter is supplied by a 12-V 140Ah battery bank and two 55A battery chargers acting as an on-board alternator, all parallel connected to supply up to 110A while regulating the input voltage to ~13.7V. To realize the mechanical loading up to 1kW, two identical 3-phase BLDC motors were placed end-to-end with one operating as a motor and the other as a generator. The re-generated AC electrical power was then rectified and

dissipated in a DC load bank. A torque transducer placed inline between the two motor shafts measures the applied output torque, and the speed information was computed by measuring the time between encoder signal transitions.

(a) 1kW Drive System with LLC and Inverter Circuits

(b) Test Bench with 12V PbA Batteries, BLDC Motors, and Load

Bank Figure 5. Experimental Setup for the 1kW Motor Drive System

Below in Figure 6 are experimental waveforms for the DC/DC converter operating at 500W and 1000W, with operating frequencies of 55kHz and 50kHz, respectively. For 500W operation, the peak current flowing through the H-bridge and MOSFETs is 85A, and the actual turn-off current of the MOSFETs is 60A. For 1kW operation, the peak current flowing through the H-bridge and MOSFETs is 160A, while the MOSFET turn-off current is only ~20A. Difference between the simulation and the experiments is caused by the dead band. Since the MOSFET's turn-off current is close to zero, most of the current goes through MOSFET channel instead of body diodes. Therefore the system efficiency is expected to be very high considering only 0.5mΩ internal drain-to-source resistance, which is close to being synchronous rectifying control (SRC). At the operating point shown in Figure 6(b), the average DC input voltage and

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current were 10.41V and 105.94A, respectively, yielding an input power of 1102.8W with the average DC output power of 1074.7W for the DC/DC part. Thus, the LLC operating efficiency at this condition was 97.5%, losing only 25W of power.

(a) At 500W

(b) At 1000W

(c) Starting Process

Figure 6. DC/DC Low-voltage H-bridge Experimental Waveforms

During the starting process or at very low power levels, the MOSFET H-bridge operates at a maximum frequency of 100kHz. This helps limit the switching losses and reduce the power transfer. However, to further reduce the power transferred to the high-voltage

secondary side, a phase-shift between the two complementary legs of the H-bridge is needed to shorten the conduction time during which the resonant LLC circuit is connected to the low-voltage power source. Both, the phase shift and switching frequency are modulated by a PI controller to vary the power transfer. To prevent large in-rush currents during startup, the reference of this PI controller is fed by a ramp input. This reference, compared to the measured secondary side DC bus voltage provides the error signal which is fed to the PI controller.

Fig.6 (c) shows the starting process of the system to build the DC-bus voltage, where a heavy phase-shifting at low power levels is required. The high-voltage DC bus was ramped up at 25 Volts per second. It is worthwhile to point out that this heavy phase shift will make the MOSFETs in the H-bridge no longer operate under ZVS. However, since both the voltage and current are very low at this operating condition, this working condition does not lead to significant power losses, allowing the DC/DC converter to operate without damaging the MOSFETs.

The high-voltage three-phase inverter's DC input is supplied by the DC/DC converter. At the end this input voltage is controlled to 300VDC. Figure 7 shows a phase current of the motor, the corresponding encoder signal, and the torque transducer output while operating the system at 1kW. The large variations of the torque transducer are believed to be from mechanical chattering and from the inherent torque ripples resultant from BLDC motor control. The averaged values at the bottom of this graph are computed over many revolutions of the motor shaft.

Figure .7 Inverter / Motor Phase Current, Corresponding Hall-encoder

Signal, and Torque Transducer Output @ 6000rpm, 1kW

IV. CONCLUSIONS AND FUTURE WORKS This paper presents a 12V/1kW BLDC motor drive

system concept applying ZVS operations to achieve a highly efficient DC voltage increase from 12 to 300V. Future work will focus on the system’s dynamic response, particularly when coupled to an air

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compressor and applied within a DPF exhaust system. Integration will continue within dynamometer development and graduate to vehicle fleet applications. Throughout the application development the system behaviors on board will be tested and tuned to optimize. Package size, mass, costs, noise, risks, and other potential uses will be considered as well.

REFERENCES [1] Adam Kotrba, Ling Bai, Argun Yetkin, Robert Shotwell and

Timothy Gardner, “DPF Regeneration Response: Coupling Various DPFs with a Thermal Regeneration Unit to Assess System Behaviors”, SAE 2011-01-2200

[2] Adam Kotrba, Tim Gardner, Ling Bai, Argun Yetkin, “Passive Regeneration Response Characteristics of a DPF System”, SAE 2013-01-0520

[3] Adam Kotrba, Argun Yetkin, Bradley Gough, Arda Gundogan, Dan Mastbergen and Clark Paterson, “Performance Characterization of a Thermal Regeneration Unit for Exhaust”, SAE 2011-01-2208

[4] Z Zhao, F Zheng, X Shan, A Kotrba, “CFD Modeling of Mini and Full Flow Burner Systems for Diesel Engine Aftertreatment Under Low Temperature Conditions”, 2012-01-1949

[5] Hua Bai, Allan Taylor, Wei Guo, et al, “Design of An 11kW Power Factor Correction and 10kW ZVS DCDC Converter For A High-efficiency Battery Charger in Electric Vehicles ”, IET Power Electronics, 2012, pp.1-9.

[6] Ray-Lee Lin and Chiao-Wen Lin, “Design criteria for resonant tank of LLC DC-DC resonant converter”, IECON, 2010, pp. 427-432.

[7] Hua Bai, Chris Mi, “Comparison and Evaluation of Different Charger Topologies for Plug-in Hybrid Electric Vehicles”, International Journal of Power Electronics, vol.4, no.2, 2012, pp.119-133.

[8] U. Ansari, S. Alam and S. Minhaj un Nabi Jafri, “Modeling and Control of Three Phase BLDC Motor Using PID with Genetic Algorithm”, International Conference on Computer Modelling and Simulation (UKSim), 2011, pp. 189 – 194.

[9] Kai Sheng Kan and Ying-Yu Tzou, “Adaptive wide angle PWM control strategy of BLDC motor drive for efficiency optimization and wide speed control range”, Energy Conversion Congress and Exposition (ECCE), 2011, pp.1721 – 1727.

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