Electrical Variable Transmission Research Articles

Wind Energy Conversion System with Integrated Power Smoothing Capability based on an EVT-coupled Flywheel

This paper presents a wind energy conversion system that integrates, in a compact form, both generation and power smoothing functions. The system is based on a vertical axis wind turbine (VAWT) and a low-speed flywheel that are coupled by an electric variable transmission (EVT). The EVT performs a dual function by simultaneously injecting power to the grid and regulating the power flow to the flywheel to smooth the power fluctuation caused by wind turbulences. The main advantages of the presented generation/smoothing topology are integrating two machines (generator and motor) in the same physical structure and the reduced power demanded by the converter that feeds the flywheel’s rotor. A theoretical analysis is carried out to determine an appropriate control law and flywheel sizing to satisfy a desired attenuation of the power fluctuation. Simulation results show an effective smoothing action, as the power fluctuation is reduced to an adequate level. In addition, the validity of the flywheel sizing procedure is verified.

Fuel saving potential of a long haul heavy duty vehicle equipped with an electrical variable transmission

The series–parallel architecture is the most interesting for hybrid electric vehicles, allowing the lowest fuel consumption. Unlike passenger cars, this architecture is not commercially available on the heavy-duty vehicles market. This is due to technical limitations associated with unsufficient load capability of the geartrain. To address this issue, new transmissions, such as the electrical variable transmission, have been developed. The novelty of this paper relies on the hybridization of a long-haul truck using the electrical variable transmission. This study aims to investigate the potential of using this new transmission for trucks. For that aim, fuel consumption benchmarking of three powertrain topologies is performed, considering: (a) a gearless topology; (b) a geared topology that uses one gearbox inserted between the engine and the mechanical input port of the electrical variable transmission; (c) a geared topology similar to the second one, but, with an additional multi-stage gearbox inserted to the mechanical output port of the electrical variable transmission. For a fair comparison between the different topologies, a bi-level optimization process has been used, incorporating the optimization of both components sizing and control. Results show that the fuel consumption of the gearless powertrain is higher than the engine-powered truck due to higher losses in the electrical variable transmission. While maximum fuel reduction of 14.2% was obtained by a geared topology that uses two gearboxes. Furthermore, emphasis is given to understand the effect of the powertrain component sizing on fuel consumption. Depending on the defined sizing, a possible fuel reduction is achieved from 3.3% to 14.2% for the two geared topologies. The reduction of CO2 emissions is found to be proportional to the fuel savings. Considering a long-haul mission, the last findings prove that the electrical variable transmission exhibits potential to reduce fuel consumption, if an adequate powertrain topology and its sizing are well defined.

Comparison of an optimized electrical variable transmission with the Toyota Hybrid System

This paper discusses the comparison of two series–parallel hybrid electrical vehicles. The first one is based on the Toyota hybrid system, while the second one is equipped with an electrical variable transmission. The problem with previous comparisons between these two transmissions is the lack of validated data used to support the comparison as well as a comprehensive study on the sizing of the electrical variable transmission for a given vehicle and load cycle. To tackle these issues, a validated model of an electrical variable transmission is used in combination with validated scaling laws to assess design changes. This scalable model is used to determine the optimal design and the impact of sizing on the fuel consumption of the vehicle. To exclude the impact of the chosen control methodology, dynamic programming has been used. This technique is not only used to optimize the operating points of the internal combustion engine, but also to find the optimal DC-bus voltage in order to optimize the system level efficiency. The comparison is performed for multiple driving cycles that all show the added value of the electrical variable transmission based hybrid electrical vehicle. On average, over the different driving cycles, a reduction in fuel consumption of 4.75% is achieved while using an electrical variable transmission.

Experimental Implementation of Power-Split Control Strategies in a Versatile Hardware-in-the-Loop Laboratory Test Bench for Hybrid Electric Vehicles Equipped with Electrical Variable Transmission

The energy management strategy (EMS) or power management strategy (PMS) unit is the core of power sharing control in the hybridization of automotive drivetrains in hybrid electric vehicles (HEVs). Once a new topology and its corresponding EMS are virtually designed, they require undertaking different stages of experimental verifications toward guaranteeing their real-world applicability. The present paper focuses on a new and less-extensively studied topology of such vehicles, HEVs equipped with an electrical variable transmission (EVT) and assessed the controllability validation through hardware-in-the-loop (HiL) implementations versus model-in-the-loop (MiL) simulations. To this end, first, the corresponding modeling of the vehicle components in the presence of optimized control strategies were performed to obtain the MiL simulation results. Subsequently, an innovative versatile HiL test bench including real prototyped components of the topology was introduced and the corresponding experimental implementations were performed. The results obtained from the MiL and HiL examinations were analyzed and statistically compared for a full input driving cycle. The verification results indicate robust and accurate actuation of the components using the applied EMSs under real-time test conditions.

Comprehensive Analysis and Optimal Configurations of the EVT Powertrain

This paper investigates the design of a series-parallel hybrid electric powertrain that includes an electric variable transmission (EVT) and two mechanical gears that connect the EVT to the rest of the powertrain. The EVT is implemented as a dual rotor electric machine (EM), which from a design perspective can be studied as two separate EMs. The goal is to optimize the size of the two EMs and the two mechanical gears, while minimizing fuel consumption, subject to performance requirements on top vehicle speed and acceleration time. A practical optimization methodology is proposed that decouples the multi-objective problem into three different control subproblems: a slow dynamic subproblem for fuel economy, a fast dynamic subproblem for acceleration performance and a static optimization subproblem for the top speed performance. Efficient dynamic programming formulations are presented to solve the two dynamic subproblems. Two different scenarios are discussed and analyzed, where the first one presents the influence of the EMs’ sizes on the vehicle performance and the second one delivers near optimal configurations of the EVT powertrain for different gear ratios. A case study is also carried out to compare the sizing results between the proposed method and an existing benchmark method, showing that the two EMs could be reduced by 45% and 11.87% respectively, while powertrain performance could still be maintained at the same level.

Sizing Methodology Based on Scaling Laws for a Permanent Magnet Electrical Variable Transmission

This paper proposes a method to design an electrical variable transmission (EVT) of which the stator and inner rotor contain a distributed three-phase winding, and the outer rotor is equipped with permanent magnets and a dc-field winding. The main problem with the design of EVTs is that the modeling relies on time-consuming finite-element (FE) calculations. Studying the effect of design changes is, therefore, a cumbersome task. To minimize the need for FE calculations, scaling laws are applied, which reduce the computational effort to a matter of milliseconds per design. Therefore, the losses of multiple scaled designs can be analyzed for a given load cycle while taking constraints into account such as maximum torque. By presenting this data in a performance map, the optimal design regarding the losses is easily deduced. To validate the presented methodology, FE calculations, measurements, and available literature are used.

Optimal sizing of the EVT for a hybrid urban delivery truck

Abstract The objective of this paper is to find the trade-off solutions among fuel economy and machines sizes for a hybrid urban delivery truck with an electric variable transmission (EVT). To solve the optimization problem, exhaustive search is conducted over the feasible EVT set obtained by different drivability performances. Proper design parameters for each machine of the EVT are selected and defined. Dynamic programming (DP) is used to benchmark the fuel economy. To overcome the drawback of huge computation burden of the optimization process caused by DP searching, a bi-level DP method is formulated. Simulation results show that EVT truck could lead to 21% reduction of fuel consumption compared to a conventional truck. Finally, the Pareto front of the optimal sizing problem is presented.

Research on the Efficiency Optimization Control of the Regenerative Braking System of Hybrid Electrical Vehicle Based on Electrical Variable Transmission

Electrical variable transmission (EVT) is a novel electromechanical energy converter that can be applied in hybrid electrical vehicle (HEV). In this paper, the hybrid electrical vehicle based on EVT (EVT-HEV) and the efficiency optimization strategy of regenerative braking system (RBS) is studied. Firstly, this paper analyzes the dynamic coupling mode of EVT-HEV and establishes the expression of the dynamic coupling relationship between EVT and the engine. Then, four different braking modes including two novel ones are divided. Next, a hierarchical controller is proposed in which the rule-based control strategy is used for braking mode switching and neural network algorithm is used for the optimal efficiency control for the system. The simulation of the control strategy is carried out under New European Driving Cycle (NEDC) based on the joint platform of CRUISE and MATLAB software. And the results indicate that the proposed strategy can improve the performance of RBS in EVT-HEV. Thus the research in this paper can bring a new theoretical foundation for the application of EVT in HEV and is of great significance to enrich the energy management strategies of EVT-HEV.

Predictive Energy Management for Electric Variable Transmission HEV

This paper introduces predictive energy management for a hybrid electric powertrain with an electric variable transmission. The optimization of the energy management in hybrid mode is formulated as a bi-level program, in which one static level optimizes the engine speed of a compound unit consists of the engine and electric variable transmission (EVT). The other layer optimizes a dynamic program by splitting power between the compound unit and the battery using equivalent consumption minimization strategy (ECMS). By combining ECMS with dynamic programming (ECMS-DP), the proposed strategy allows the engine on/off control to be also taken into account by iteratively updating the costate. Finally, the efficient ECMS-DP optimization is incorporated into the framework of model predictive control (MPC) and solved in a moving horizon fashion. Computation efficiency and optimization results are presented by simulation. (C) 2019, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.

Convex modeling for optimal battery sizing and control of an electric variable transmission powertrain

This paper provides convex modeling steps for the problem of optimal battery sizing and energy management of a plug-in hybrid electric vehicle with an electric variable transmission. Optimal energy management is achieved by a switched model control, with driving modes identified by the engine on/off state. In pure electric mode, convex optimization is used to find the optimal torque split between two electric machines, in order to maximize powertrain efficiency. In hybrid mode, optimization is performed in a bilevel program. One level optimizes speed of a compound unit that includes the engine and electric machines. Another level optimizes the power split between the compound unit and the battery. The proposed method is used to minimize the total cost of ownership of a passenger vehicle for a daily commuter, including costs for battery, fossil fuel and electricity.

Design Optimization of an Electric Variable Transmission for Hybrid Electric Vehicles

An electric variable transmission (EVT) for hybrid electric vehicles (HEVs) is investigated in this paper. With a special double rotor structure, the EVT splits and reintegrates the output power of the internal combustion engine (ICE) to run at its optimum working efficiency. However, the high electromagnetic coupling degree causes torque ripple and affects the dynamic performance of the EVT. After introducing the configuration and working principle, the torque mathematical model of the EVT in an ABC three-phase coordinate system is proposed to analyze the cause of this torque ripple. Besides, a finite element method-based (FEM) structural optimization design for reducing the torque ripple and improving the working stability is presented. The magnetic field distribution, induced voltage and torque property validate the rationality of optimization.

Efficiency of a CVT-Operated EVT Experimentally Evaluated Against Half-Toroidal and Push-Belt CVTs

This paper describes how an electrical variable transmission (EVT) can be used as an alternative for mechanical continuously variable transmission (CVT) systems. An EVT is an electromagnetic power split device having two mechanical and two electrical ports. The speed ratio between both mechanical ports (rotors) can be varied in a continuously variable way, so that this machine can be used as an electromagnetic alternative for the mechanical push-belt and half-toroidal CVT. Its operating principle is based on splitting the power into an electromagnetic direct path and an electrical path involving two inverters connected back to back. Its potential use as a CVT is investigated by comparing experimental data against toroidal and belt CVT data from literature having comparable power ratings. To this end, measurements are performed on a prototype $\text{120-kW}$ permanent magnet EVT.

Coordinated Control Algorithm of a Dual Motor for an Electric Variable Transmission Hybrid System

With the successful application of the electric variable transmission hybrid system, the dynamic quality of electric variable transmission (EVT) has received significant attention. The dual motor integrated by the EVT plays a vital role in riding comfort during mode shifting. Therefore, it is of great significance to study the coordinated control algorithm of the dual motor for improving driving comfort. However, current research on single motor torque compensation indicates that the poor accuracy of the compensation torque affects the coordinated control performance. In this paper, a dual motor coordinated control algorithm for the EVT is presented to solve the problem of shock in the mode transition, which is from the electric starting mode to the engine starting mode. First, the lever analogy is used to analyze the cause of the shock during this mode shifting. Then, the coordinated control algorithm of the dual motor is designed, in which the first motor MG1 adopts compounded feedforward and feedback control and the second motor MG2 adopts the torque compensation based on the engine torque estimation algorithm. Finally, the EVT system model and its dynamic coordination control algorithm model are built in the Simulink environment. The simulation results demonstrate that the presented control algorithm markedly reduces the shock of the mode shifting process. When applied in an actual driving cycle, the coordinated control algorithm proposed in this paper performs well, thus improving the dynamic quality of the EVT system.

An up to date review of continuously variable speed wind turbines with mechatronic variable transmissions

As a promising and potential alternative to conventional fixed or variable speed wind turbines, continuously variable speed wind turbines (CVSWTs) with variable transmissions offer improved power efficiency and enhanced power control capabilities. The CVSWTs can be generally achieved by adapting mechatronic variable transmissions in the turbine drive train for continuously variable speed operations for wind turbines. Therefore, this paper serves to provide an up to date and exhaustive review of the CVSWTs with mechatronic variable transmissions such as mechanical variable transmission, electrical variable transmission, and power splitting transmission. In this paper, the analysis of CVSWTs with different mechatronic transmission topologies is performed regarding basic configurations, dynamic characteristics, control principles, and experimental or simulation results. Review results indicate the feasibility of applying CVSWTs with such mechatronic transmissions and highlight superiorities of the CVSWTs with power splitting transmission. The CVSWT with power splitting transmission will be particularly suitable for megawatt-scale turbine systems and will hence increase the economic competitiveness of these turbines due to its large power capacity and high reliability. The directions or challenges for future investigations of CVSWTs with such mechatronic transmissions are also presented to foster in-depth understanding of such CVSWTs and their control strategies.

Grid-connected Control Strategies for a Dual Power Flow Wind Energy Conversion System

A novel dual power flow (DPF) wind energy conversion system (WECS) based on an electrical variable transmission (EVT) machine is proposed recently. The structure of the novel EVT-based WECS, especially the grid-connected configuration, is different from any other existing WECSs. In this paper, the grid-connected control strategies of the DPF- WECS are proposed by controlling the DC-link and the inner rotor side converter, where the DC-link with and without the energy storage system (ESS) is considered separately. The relevant control strategies are presented to enhance the grid fault ride through capability of the DPF-WECS. Moreover, the maximum power point tracking control strategy and the pitch control strategy are also studied to implement the complete functionality of the DPF-WECS. The simulation model of the DPF-WECS is developed based on the EVT mathematical models and proposed control strategies. Finally, the simulation results verify the efficiency of the proposed control strategies for the DPF-WECS.

Analytical behavior of brushless doubly fed electric variable transmission for wind power generation

Summary Mechanical gearboxes in wind power plant have low efficiency, reliability, and high maintenance cost. To overcome these problems, it is substituted by an electric gearbox, ie, electrical variable transmission (EVT). This paper deals with the study of several kinds of EVT and 4 quadrant (4QT) energy transducer which helps in the analysis of the dynamic equations of brushless doubly fed EVT system. Brushless doubly fed electric variable transmission (BL-DF-EVT) can be used as a replacement of mechanical gear in the wind power generation. Proficiency and performance of the electrical variable system can be increased by the use of BL-DF-EVT. As no power-driven part is not present, then it results in negligible mechanical losses which decreases the maintenance cost of the system, and also output speed is greater in contrast with power-driven gear. The dynamic model and operational features of this machine like flux and speeds of inner and outer rotors are illustrated in this paper. MATLAB/Simulink software has been used to simulate the dynamic equations.

Loss Identification in a Double Rotor Electrical Variable Transmission

An electrical variable transmission (EVT) is an electromagnetic power-split device with two mechanical and two electrical ports. It can be used in hybrid electric vehicles to split the power to the wheels in a part coming from the combustion engine and a part exchanged with the battery. Although crucial for the EVT design and evaluation, no papers are found to give a detailed overview of the different loss components in such a machine and how they can be calculated and measured. In contrast to conventional electrical machines, this machine has more degrees of freedom, which can be exploited to measure the different loss contributions separately. This paper proposes a methodology to identify and measure the different loss components in this kind of machines. The proposed method is able to identify the iron losses in stator and inner rotor, the copper losses, bearing losses, and slip ring friction losses separately. To this end, measurements of both torque and speed sensors in different operating points are combined. The methods are applied to identify the different loss contributions in a prototype permanent magnet assisted EVT, both in no-load operation as under load where its functionality as power-split device is evaluated.

Sensorless Control for the EVT-Based New Dual Power Flow Wind Energy Conversion System

The dual power flow wind energy conversion system (DPF-WECS) is a novel system which is based on the electrical variable transmission (EVT) machine. The proposed sensorless control for the DPF-WECS is based on the model reference adaptive system (MRAS) observer by combining the sliding mode (SM) theory. The SM-MRAS observer is on account of the calculations without the requirement of the proportional-integral (PI) loop which exists in the classical MRAS observer. Firstly, the sensorless algorithm is applied in the maximum power point tracking (MPPT) control considering the torque loss for the outer rotor of the EVT. Secondly, the sensorless control is adopted for the inner rotor control of the EVT machine. The proposed sensorless control method based on the SM-MRAS for the DPF-WECS is verified by the simulation and experimental results.

Optimal Control for a Hybrid Excited Dual Mechanical Port Electric Machine

An electrical variable transmission (EVT) is an electrical machine consisting of a stator and two concentric rotors. It is a competitive alternative for power split devices used in current series–parallel hybrid electric vehicles (HEVs). Although tending to have high power densities, permanent magnet (PM) machines suffer from no-load iron losses. Since the torque interaction between stator and outer rotor is low during large time intervals, the EVT in this paper possesses a flux bridge with dc-field winding to modulate the stator field, while maintaining the inner rotor flux linkage. In this paper, a torque controller is proposed that optimally combines the stator, dc-field, and inner rotor currents in order to minimize the copper and iron losses in every operating point of torque and speed on both rotors. Also the electromagnetic coupling between both rotors is considered. It turns out that there are two optimal load angles for the stator currents, of which one of the two is dominant depending on speed and torque. The control is tested on a 120 kW prototype EVT, and measurement results are given to illustrate the optimal performance obtained with the proposed torque control. In addition, efficiencies are measured in order to support the statements made concerning the proposed method.

Benchmarking the permanent magnet electrical variable transmission against the half toroidal continuously variable transmission

In most applications where a variable transmission is required, either an EVT or a CVT would be viable options. In order to obtain a justified decision between both variable transmissions, the operating range and efficiency should be known better. To be able to pinpoint the pros and cons based on efficiency and operating range, a detailed comparison is necessary. However, to the knowledge of the authors there is no such study reported in literature so far. In this paper a permanent magnet Electrical Variable Transmission (EVT) is benchmarked against a half toroidal Continuously Variable Transmission (CVT) and this during steady state conditions. This paper provides the constraints for the comparison and results based on detailed models of both technologies. Two comparative studies are carried out: on one hand a study based on equal maximum output torque and on the other hand a study based on equal rated power. The outcome of the study shows that the CVT is in favor for high input torques and that the EVT is dominant for high input speeds.