Synchronous Machines with High-Frequency Brushless Excitation for Vehicle Applications
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