Design and Control of Electrically Excited Synchronous Machines for Vehicle Applications
Doktorsavhandling, 2021

Electrically excited synchronous machines (EESMs) are becoming an alternative to permanent magnet synchronous machines (PMSMs) in electric vehicles (EVs). This mainly attributes to the zero usage of rare-earth materials, as well as the ability to achieve high starting torque, the effectiveness to do field weakening and the flexibility to adjust power factor provided by EESMs. Furthermore, in case of converter failure at high speed, safety can be improved by shutting down the field current in EESMs.

The purpose of this study is to investigate the potential application of EESMs in EVs. To achieve this aim, several topics are covered in this study. These topics are studied to confront the challenges before EESMs could become prevalent and to maximumly use the advantages of EESMs for EV applications. In control strategies, the challenge is to properly adjust the combination of stator and field currents so that high power factor and minimum copper losses can be achieved. To tackle this, control strategies are proposed so that reactive power consumption and total copper losses are minimized. With the proposed strategies, the output power is maximized along the torque-speed envelope and high efficiency in field-weakening is achieved. In dynamic current control, due to the magnetic couplings between field winding and stator winding, a current rise in one winding would induce an electromagnetic force (EMF) in the other. This introduces disturbances in dynamic current control. In this study, a current control algorithm is proposed to cancel the induced EMF and the disturbances are mitigated. In machine design, high starting torque and effective field weakening are expected to be achieved in the same EESM design. To realize this, some criteria need to be satisfied. These criteria are derived and integrated into the design procedure including multi-objective optimizations. A 48 V EESM is prototyped during the study. In experimental verification, a torque density of 10 N·m/L is achieved including cooling jacket. In field excitation, a contactless excitation technology is adopted, which leads to inaccessibility of the field winding. To realize precise control of field current in a closed loop, an estimation method of field current is proposed. Based on the estimation, closed-loop field current control is established. The field current reference is tracked within an error of 2% in experimental verifications. The cost of an EESM drive increases because of the additional converter used for field excitation. A technique is proposed in which the switching harmonics are extracted for field excitation. With this technique, both stator and field windings can be powered using only one inverter.

From all the challenges tackled in this study, it can be concluded that the application of EESMs in EVs is feasible.

Electric Machine Modeling

Brushless Excitation

Electrically Excited Synchronous Machines

Electric Machine Control

Electric Drives

Electric Machine Design

ZOOM
Opponent: Mats Alaküla

Författare

Junfei Tang

Chalmers, Elektroteknik, Elkraftteknik, Elmaskiner och kraftelektronik

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Electric vehicles (EVs) have become a fast-growing area in recent years due to an increase of environmental concerns from society. Permanent magnet synchronous machines (PMSMs) are the most prevalent choice in EV propulsions. In a PMSM, the magnetic field is generated by rare-earth magnets. Due to this, high power density and high efficiency can be achieved. However, the usage of rare-earth magnets introduces an imbalance between demand and supply, challenges to environment and safety risks at high speed operation. This leads to a trend to search for alternatives to PMSMs. Electrically excited synchronous machines (EESMs) are considered as a promising alternative, in which the magnetic field is generated by electric current in a field winding.

The aim of this study is to investigate the potential of EESM applications in EVs. To tackle the challenges in machine design, machine control and magnetic field excitation is the focus in this study. In machine design, one problem is how the machine performance can be maximized. In this study, a few design criteria are concluded to maximize the torque and power density of the machine. Following these criteria, a multi-objective design procedure is formulated. In machine control, due to the magnetic mutual coupling between armature and field windings, a current rise in one winding would induce an electromotive force (EMF) in the other. This causes disturbance in dynamic control. In this study, a current control algorithm is proposed to cancel the induced EMF and the disturbances are mitigated. In field excitation, to transfer power to the rotating field winding, the traditional solution is to use brushes and sliprings. This leads to friction losses and maintenance efforts. In this study, a contactless technology is employed. Power is transferred to the field winding at 100 kHz through a 1 mm airgap in a rotating transformer. Furthermore, a control method and a circuit topology are proposed to utilize switching harmonics to do the field excitation. With this technique, the armature and field windings can be powered from only one converter. 

From all the challenges tackled in this study, it can be concluded that the application of EESMs in EVs is feasible. The focus of ongoing and future studies is mainly in the mechanical and thermal topics of the EESMs.

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Ämneskategorier

Elektroteknik och elektronik

ISBN

978-91-7905-466-3

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4933

Utgivare

Chalmers tekniska högskola

ZOOM

Online

Opponent: Mats Alaküla

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Senast uppdaterat

2021-03-16