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A new magnetless axial-flux doubly-salient DC-field (AF-DSDC) machine is proposed and implemented into the application of a range-extended electric vehicle (RE-EV). By employing the radial active part for the torque production, the proposed machine can produce satisfactory torque density to fulfill the requirements of the RE-EV system. With the support of the 3D finite element method (3D-FEM), the performances of the proposed machine are calculated and compared with the requirements of the typical passenger RE-EV applications. To offer a more comprehensive illustration, the common radial-flux (RF) machines are included for comparison.

With the ever increasing concerns on energy efficiency and hence environmental protection, the development of electric vehicles (EVs) has been speeding up [

As the key element of the RE-EV system, electric machines have to offer high efficiency, high power density, high controllability, wide-speed range, and maintenance-free operation [

Compared with PM machines, the magnetless machines have the definite merit of low material costs, but suffer from lower torque density [

The purpose of this paper is to newly incorporate the DC-field winding into the AF-DS machine, hence creating a new AF-DSDC machine, purposely for RE-EV applications. The design criteria and operating principles of the proposed machine will be discussed. Its performance will be analyzed by using the 3D finite element method (3D-FEM), and then quantitatively compared with the requirements of typical passenger RE-EV applications. For better illustrative purposes, the profound RF machines, namely the RF-switched reluctance (RF-SR), RF-DSDC and RF-doubly-salient permanent-magnet (RF-DSPM) machines are included for comprehensive comparison.

When the RE-EV needs to accelerate or run uphill, the motor has to operate with the torque boosting mode, namely, the motor is requested to output the peak torque under a short period of time. Under this mode, the current density of the DC-field excitation is purposely boosted up to strengthen the flux densities, resulting in the maximized output torque.

After the RE-EV has started up with the torque boosting mode, the generated electromotive force (EMF) among the armature windings starts to increase with the increased operating speed. In order to maintain a certain power level within the constant power operation region, the flux density should be weakened accordingly. By regulating the independent DC-field excitation, the proposed RE-EV system can effectively achieve this flux-weakening operation. Theoretically, RE-EVs can provide infinite speed ranges by regulating the DC-field excitations [

Whilst the DC-field winding is under open-circuit fault or short-circuit fault conditions, the DC-field current should be cut off and the proposed motor can still be operated by the remaining sets of healthy armature winding [

Once the battery is almost exhausted, the ICE can be applied to operate the RE-EV and to charge the battery for range extension. As the engine speed varies along with the wide range of vehicle speeds, the generated voltage is varying accordingly. Meanwhile, when the RE-EV runs downhill or in braking occasions, the motor undergoes a regenerative braking stage and starts to charge the battery [

Based on this structure and accompanied by the proper dimensions, the proposed machine is able to mount its rotor to the tire directly, such that in-wheel motor drive can be achieved instinctively as shown in

The proposed machine is designed based on the four-phase topology due to considerations of stability and cost-effectiveness. The corresponding key design data are tabulated in

Since the AF-DSDC machine is derived from the conventional RF-DSDC machine, its design equations such as the pole arrangement [_{s} is the number of stator poles; _{r} is the number of rotor poles; _{s} = 8 and _{r} = 10, resulting with the proposed structure for the AF-DSDC machine.

The proposed AF-DSDC machine uses two types of winding, namely the DC-field winding and armature winding, in each of its sided-stators, and both sets of winding adopt the concentrated winding arrangement. With this configuration, each of the two sets of winding is installed with their magnetic axes parallel to each other, albeit in opposing directions. These lead to the fact that the flux in the rotor yoke is divided into two equal paths as shown in

Since the machine has a symmetrical structure, the torques produced by the two sided-stators are complementary with each other, leading to a balanced resultant torque. In addition, to simplify the operation complexity, each of the corresponding sets of windings in the two sided-stators can be purposely connected in series.

With the additional DC-field excitation, the proposed AF-DSDC machine can be operated at two different conduction schemes, namely the bipolar conduction scheme and unipolar conduction scheme. To be specific, the bipolar conduction scheme mainly serves for normal operations, whereas the unipolar conduction scheme is devoted to fault-tolerant operation [

When the DC-field excitation is available, the proposed AF-DSDC machine can be operated by using the bipolar conduction scheme, which is similar to the conventional RF-DSPM machine operation [_{DC} is increasing and the no-load EMF is positive, a positive armature current _{BLDC}_{DC.} Similarly, a negative armature current −_{BLDC} is applied when the Ψ_{DC} is decreasing and the no-load EMF is negative, hence also producing a positive torque. The operating waveforms under the bipolar conduction scheme are shown in

Each phase performs 90° conduction with θ_{2} − θ_{1} = θ_{4} − θ_{3} = 90°. The resulting electromagnetic torque _{DC} can be expressed as:
_{D} is the self-inductance. Under this conduction scheme, the torque is majorly constituted by the DC-field torque component, whereas the reluctance torque component is small and pulsates with a zero average value [_{DC} is the slope of Ψ_{DC} with respect to θ.

Whilst the DC-field winding is under any fault condition, the DC-field current should be cut off and the proposed machine can then be operated by using the unipolar conduction scheme, which is similar as the RF-SR machine [_{SR} is fed to the armature winding during the increasing period of the self-inductance _{SR}, resulting as the positive corresponding reluctance torque _{SR} within θ_{2} − θ_{1} = 90° as shown in _{SR} is the slope of _{SR} with respect to θ. In order to maintain the same torque level at both modes, the

Electromagnetic field analysis has been widely applied for the design of electric machines, and it can be basically categorized as the analytical field calculation [

^{2}. These show that both air-gaps achieve the same flux pattern. Hence, it can be confirmed that under the same electric supply, the output torques produced by the two sided-stators should be the same.

^{2}) and the self-inductance waveforms for the unipolar conduction scheme (without any DC-field excitation), respectively.

In order to maintain the same torque level between both conduction schemes, the armature current at the unipolar one is given by _{DC}, and _{SR} can be deduced from

By using 3D-FEM, the performance of the proposed machine can be thoroughly analyzed. Firstly, the no-load EMF waveforms at the base speed 300 rpm with the rated DC-field excitation of 5 A/mm^{2} are simulated as shown in

Secondly, ^{2}) and at strengthened DC-field excitation (10 A/mm^{2}), respectively. Since the independent DC-field winding can be controlled effectively, it can be utilized to regulate the air-gap flux densities to fit different situations. It can be observed that the average steady torques of the machine at rated and at strengthened DC-field excitation are 154.2 Nm and 203.2 Nm, respectively. These confirm that the proposed machine can fulfill the torque requirements of normal operation, including the

Thirdly, the cogging torque waveforms of the proposed machine at rated and strengthened DC-field excitations are simulated as shown in

Fourthly, the torque-speed characteristics of the proposed machine are simulated and shown in ^{2}, the operating speed of the proposed machine is approximately 907 rpm. The result can cover the whole targeted speed range and fulfill the requirements of the

Fifthly, when the DC-field excitation is cut off, the proposed machine can be operated with the unipolar conduction scheme and the corresponding torque performance is shown in

Finally,

The performances of the proposed AF-DSDC machine are thoroughly analyzed by 3D-FEM, and summarized in

Meanwhile, to further illustrate the merits of the proposed AF-DSDC machine, the profound RF machines, namely the RF-SR, RF-DSDC and RF-DSPM are also included [

Based on the FEM, the performances of the RF machines are calculated and summarized in

Even though the RF-DSPM machine provides the highest torque densities and outperforms its magnetless counterparts, the supply of the PM materials is limited and irregular, resulting to the surged materials cost [

In this paper, a new magnetless AF-DSDC machine has been proposed and implemented for RE-EV applications. By adopting the radial active part for torque production, the proposed AF-DSDC machine offers better performance than its conventional RF counterparts do, especially from a cost-effectiveness perspective. The AF-DSDC machine is under prototyping and the experimental data will be the substance of our future papers. With the support of the external DC-field winding, the proposed machine can accomplish different operation modes, namely the torque boosting, flux-weakening, fault-tolerant and battery charging modes. Hence, it is anticipated that the proposed machine will have great potential for implementation into RE-EV systems.

This work was supported by a grant (Project No. HKU710612E) from the Hong Kong Research Grants Council, Hong Kong Special Administrative Region, China.

The authors declare no conflict of interest.

Range-extended electric vehicle (RE-EV) system. ICE: internal combustion engine; DC: direct current; and AC: alternating current.

Proposed axial-flux doubly-salient DC-field (AF-DSDC) machine.

In-wheel direct drive internal structure.

DC flux-linkage paths of the proposed machine: (

Operation principle schemes: (

No-load magnetic field distributions: (

Air-gap flux density waveforms: (

DC flux-linkage waveforms: (

Self-inductance waveforms: (

No-load electromotive force (EMF) waveforms at rated conditions: (

Torque waveforms at bipolar conduction scheme: (

Cogging torque waveforms at bipolar conduction scheme: (

Torque-speed characteristics of the proposed machine.

Torque waveforms under the unipolar conduction scheme.

No-load EMF waveforms at various speeds: (

Radial-flux (RF) machines: (

Targeted motor specifications for the RE-EV system.

Peak DC voltage | 360 | V |

Rated power | 4.7 | kW |

Rated torque | 150 | Nm |

Constant-torque operation | 0–300 | rpm |

Constant-power operation | 300–900 | rpm |

Peak torque | 130% for 10 s | - |

Wheel dimension | 195/65 R15 | - |

Key data of proposed machine.

Radial outside diameter | 381 | mm |

Radial inside diameter | 100 | mm |

Axial stack length | 195 | mm |

Air-gap length of both segments | 0.5 | mm |

Number of stator poles of both segments | 8 | - |

Number of rotor poles of both segments | 10 | - |

Number of armature phases | 4 | - |

Number of turns per armature coil | 50 | - |

Rotor and stator material | Steel sheet: 50JN700 (JFE Steel Corporation, Tokyo, Japan) | - |

Armature and DC-field winding material | Copper | - |

Key parameters at different conduction schemes.

Rated current at bipolar conduction | 10 A | 10 A |

Slope of DC flux-linkage _{DC} |
1.04 | 1.04 |

Slope of self-inductance _{SR} |
0.079 | 0.079 |

Rated current at unipolar conduction | 22.9 A | 22.9 A |

Proposed AF-DSDC machine performances.

Rated Power | 4.8 kW | 4.6 kW |

Power density | 35.4 W/kg | 33.8 W/kg |

Operating frequency at base speed | 50 Hz | 50 Hz |

No-load EMF of Winding 1 | 153 V | N/A |

No-load EMF of Winding 2 | 153 V | N/A |

Rated DC-field excitation | 5 A/mm^{2} |
N/A |

Rated torque | 154.2 Nm | 148.2 Nm |

Torque ripple at rated torque | 27.4% | 40.1% |

Cogging torque at rated torque | 18.6 Nm | N/A |

% Cogging torque at rated torque | 12.1% | N/A |

Boosted DC-field excitation | 10 A/mm^{2} |
N/A |

Boosted torque | 203.2 Nm | N/A |

Torque ripple at boosted torque | 43.2% | N/A |

Cogging torque at boosted torque | 42.4 Nm | N/A |

% Cogging torque at boosted torque | 20.9% | N/A |

Axial-flux (AF) and RF machine performance comparisons.

Rated torque/mass | 0.51 Nm/kg | 0.61 Nm/kg | 1.21 Nm/kg | 1.13 Nm/kg |

Rated torque/volume | 4.12 kNm/m^{3} |
4.85 kNm/m^{3} |
9.51 kNm/m^{3} |
8.97 kNm/m^{3} |

Material cost | 208.4 USD | 209.8 USD | 411.9 USD | 239.5 USD |

Rated torque/cost | 0.34 Nm/USD | 0.41 Nm/USD | 0.39 Nm/USD | 0.65 Nm/USD |