Synchronous Machines with High-Frequency Brushless Excitation for Vehicle Applications
Licentiate thesis, 2019

Electrically excited synchronous machines (EESM) are becoming an alternative to permanent magnet synchronous machines (PMSM) in electric vehicle (EV) applications. This mainly attributes to the zero usage of rare-earth material as well as the capabilities of high starting torque and good field weakening provided by EESM. EESM also improves safety in case of converter failure at high speed. The prevalence of wireless power transfer (WPT) technologies enables the employment of high frequency brushless excitation in EESM. This reduces the friction loss and maintenance effort compared with traditional excitation through brushes and sliprings.

Hence this study aims at investigating the potential of EESM with high frequency brushless excitation in EV applications. Modeling, design and control are the main aspects of interest in this study. Due to the varieties of different vehicle applications, this study covers the developments of three EESM drive systems, one for mild hybrid vehicles, one for electric passenger cars and one for heavy duty vehicles.

To achieve a comprehensive understanding of the system, modeling is firstly studied. This includes the modeling of the machine as well as the modeling of the high frequency brushless excitation system. Nonlinear properties of magnetic material are taken into considerations. Based on the machine modeling, the vector loci of current, voltage, torque and power factor in dq-frame as well as the envelop in torque-speed map are derived analytically. One step further, algorithms to achieve unity power factor along with minimizations of copper loss or field current are studied. To achieve unity power factor at high speed, the field excitation needs to be stronger than the armature reaction. 

The design of the system starts with profiling of the specifications for the three applications. The varieties in specifications lead to the differences in design strategies. This study adopts a general design procedure with interactions of FEM analysis and operation point iterations. Then the design strategies are established based on each set of design specifications to tune the parameters of the machine geometry accordingly. The design for mild hybrid vehicles emphasizes on widening the flux path. In terms of the design for electric passenger cars, a good balance is required between copper area and flux path. Comparisons between open-slot and closed-slot designs bring a trade-off of torque ripples and average torque. Adding ferrite to the top of rotor slots introduces a study of influence from the ferrite pieces to the field excitation. This includes a possible ease of local saturation in rotor and a reduction of copper losses etc. As for the machine design for heavy-duty vehicles, investigations show that, the adjustable field in EESM brings a significant benefit in field weakening operation.

A 48 V EESM with high frequency brushless excitation for mild hybrid vehicles is prototyped. The experimental results of both machine and exciter are consistent with the FEM calculation results. This verifies the modeling and the methods that are applied in the design and analysis.

One challenge for the prevalence of EESM is the difficulty to access the field winding after assembly. As a solution, an algorithm is developed to estimate the field winding current and temperature. The dc-link current is utilized as a feedback in the algorithm to correct the estimations. The current and temperature variations are tracked quite well. As one step further, a closed-loop field current control is established. The ability to track field current reference is experimental verified as well. This closed-loop field current control enables a complete dynamic closed-loop control of the EESM.

Electrically Excited Synchronous Machines

Machine Modeling

Brushless Excitation

Machine Control

Machine Design

Electric Vehicles

EA, Hörsalsvägen 11
Opponent: Pär Ingelström


Junfei Tang

Chalmers, Electrical Engineering, Electric Power Engineering

Modeling and Experimental Verification of High-Frequency Inductive Brushless Exciter for Electrically Excited Synchronous Machines

IEEE Transactions on Industry Applications,; Vol. 55(2019)p. 4613-4623

Journal article

Design and Experimental Verification of a 48 V 20 kW Electrically Excited Synchronous Machine for Mild Hybrid Vehicles

International Conference on Electrical Machines,; (2018)

Paper in proceeding

Study of Voltage Spikes and Temperature Rise in Power Module Based Integrated Converter for 48 V 20 kW Electrically Excited Synchronous Machines

2018 IEEE Applied Power Electronics Conference and Exposition (APEC) ,; Vol. 2018-March(2018)p. 210-217

Paper in proceeding

Comparison of copper loss minimization and field current minimization for Electrically Excited Synchronous Motor in mild hybrid drives

2017 19th European Conference on Power Electronics and Applications (EPE'17 ECCE Europe),; (2017)

Paper in proceeding

Sensorless control of a PMSM with a transmission system including shunt branches

Proceedings - 2016 22nd International Conference on Electrical Machines, ICEM 2016. Lausanne; Switzerland; 4-7 September 2016,; (2016)p. 1160-1166

Paper in proceeding

High frequency exciter of electrically excited synchronous motors for vehicle applications

Proceedings - 2016 22nd International Conference on Electrical Machines, ICEM 2016. Lausanne; Switzerland; 4-7 September 2016,; (2016)p. 378-383

Paper in proceeding

Variable Flux Machine for electric vehicles

Swedish Energy Agency (41396-1), 2016-04-01 -- 2019-03-31.

Subject Categories

Other Electrical Engineering, Electronic Engineering, Information Engineering



EA, Hörsalsvägen 11

Opponent: Pär Ingelström

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