Novel Self-Excited Brush-Less Wound Field Vernier Machine Topology

To realize the brush-less operation for wound field vernier machines (WFVMs), a novel self-excited topology is proposed in this paper which involves a four-pole main armature winding and a two-pole excitation winding connected in series using an uncontrolled rectifier. Upon supplying current from a single current-controlled voltage source inverter (VSI), this procedure results in four-pole and two-pole magnetomotive force (MMF) components in the airgap. The four-pole MMF produces the main stator field while the two-pole MMF component develops a sub-harmonic field in the airgap. The rotor is altered to house a forty-four-pole rotor field and two-pole harmonic windings. The two-pole harmonic field is employed to induce a current in the harmonic winding, which is rectified by means of a full-bridge diode rectifier to energize the rotor field winding and realize brush-less operation for WFVM. Two-dimensional finite element analysis (2-D FEA) is implemented in JMAG-Designer to confirm the operation of the proposed WFVM topology and attain the electromagnetic performance for a four-pole, twenty-four-slot outer-rotor vernier machine.

INDEX TERMS Wound field vernier machine, harmonic field excitation, self-excited machines, wound rotor. 15 In regular electric drive systems, the mechanical gearing sys- 16 tems are generally used to achieve the speed/torque differ- 17 ence between the high-speed motor and the low-speed prime-18 mover shafts. Mechanical gearing systems are noisy and 19 lead to high maintenance costs. Furthermore, these gearing 20 systems encounter lifetime challenges as their life span is 21 usually shorter than electromechanical systems whereas the 22 The associate editor coordinating the review of this manuscript and approving it for publication was Atif Iqbal . direct-drive systems operating at low-speed and high-torque 23 conditions result in substantially large machines because of a 24 large number of rotor poles [1]. 25 In the 1960s, a new machine that uses the magnetic gear- 26 ing phenomenon caused by flux modulation was developed. 27 This machine is known as the vernier machine (VM) and 28 has received much attention over the last couple of decades 29 because of the progress in permanent magnets (PMs) devel-30 opment technology. It is because the vernier PM machines 31 can generate two-to-three times the output torque in com-32 parison with the conventional PM machines of the same 33 FIGURE 1. Conventional brush-less WFVM topology proposed in [6]. [5], [6], [7], [8], [9], [10], [11].

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The reluctance vernier machines (RVMs) are the least 55 expensive in comparison with PMVMs as they do not require 56 any PM for the rotor field excitation; however, these machines 57 suffer from low torque density [2].

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On the other hand, wound field vernier machines 59 (WFVMs) address the high cost and low torque den-60 sity concerns associated with PMVMs and RVMs, respec-61 tively. However, these machines require brushes and slip-62 rings for the energization of the rotor field winding, which 63 raises the maintenance cost of the machine system. This 64 encouraged researchers to explore different brush-less 65 topologies of WFVMs.

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The conventional brush-less WFVM topologies require 67 exciters and pilot-exciters for their rotor field excitation. 68 Exciters and pilot-exciters are small-sized electrical machines 69 fitted on the shaft of the WFVM machine to realize 70 brush-less operation through the electromagnetic induction 71 phenomenon; however, these small-sized machines enhance 72 the overall cost and size of the machine system. Therefore, 73 these brush-less WFVM topologies could not get consider-74 able attention in the market [12], [13], [14]. 75 Recently, researchers have started investigating differ-76 ent brush-less topologies for the wound rotor synchronous 77 machines whose rotor field is energized through the har-78 monic field excitation method. In principle, the harmonic 79 field excitation method involves developing a supplementary 80 harmonic MMF component in the airgap besides the funda-81 mental component [15], [17], [18]. In the case of WFVMs, 82 the only successful attempt in this regard is recently made 83 in which a brush-less WFVM topology for electric vehi-84 cle and washing machine applications while considering the 85 sub-harmonic field excitation method is developed [19]. This 86 topology is based on a dual-inverter-outline. The armature 87 winding is split up into two halves (ABC and XYZ), each 88 of which is having a different star connection, and is supplied 89 current (I ABC and I XYZ ) from a designated inverter, namely 90 current-controlled VSI-1, and VSI-2. The frequency of the 91 output inverter currents is the same; however, the magnitude 92 of currents is different. This procedure generates the funda-93 mental and sub-harmonic MMF components in the airgap. 94 The fundamental MMF develops the main stator field, while 95 VOLUME 10, 2022   dual-inverter-outline makes it expensive and increases its 105 overall size. In addition, it exhibits some performance issues 106 such as low average torque, high losses, and low efficiency 107 because of the different magnitude of current flowing in two 108 halves of the armature winding [20], [21], [22], [23].
109 This paper proposes a new self-excited brush-less WFVM 110 topology, which employs a four-pole main armature wind-111 ing and a two-pole excitation winding connected in series 112 by means of an uncontrolled rectifier. As the machine is 113     injected current from current-controlled VSI, a four-pole 114 main stator field and a two-pole pulsating sub-harmonic field 115 are established in the airgap. The rotor is housed with har-116 monic and field windings connected in series by means of 117 a full-bridge diode rectifier. The harmonic field will induce 118 a current in the rotor harmonic winding, which will be rec-119 tified to energize the forty-four-pole rotor field winding to 120 realize brush-less operation. The proposed brush-less WFVM 121 topology is based on a single-inverter-outline, making it cost-122 effective and compact in comparison with the conventional 123 sub-harmonic-based brush-less WFVM topology.

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The proposed self-excited brush-less WFVM topology, 127 as presented in Fig. 4, involves a four-pole main arma-128 ture winding (ABC) and a two-pole excitation winding (X). 129 Both windings are connected in series by means of an un-130 controlled rectifier and are supplied current (I ABC ) from a 131 single current-controlled VSI. This current is given by: where I p denotes the peak value of the current and ω is used 134 to represent the rotor angular speed.

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The balanced three-phase currents injected to the ABC 136 winding get rectified and simultaneously energize the two-137 pole excitation winding. This procedure results in a four-pole 138 main stator MMF and a two-pole sub-harmonic MMF in 139 the airgap. The four-pole MMF develops the main stator 140 field; however, the two-pole sub-harmonic MMF develops the 141 where F is the airgap flux, N a is used to denote the ABC 146 winding turns per phase, N e represents the excitation winding 147 (X) turns, and I X is the excitation winding current [24], [25].

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The rotor is altered to house the harmonic and field wind- the machine must satisfy the following equation (3) to work 158 in the vernier mode: where P r denotes the rotor field winding pole pairs, N s is used 161 to represent the stator slots number, and P s denotes the stator 162 pole pairs.

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For the proposed self-excited brush-less WFSM topology, 164 a four-pole, twenty-four-slot outer-rotor machine is used. 165 To satisfy equation (3), the number of rotor field winding 166 poles is kept at forty-four.

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The main reason behind using an outer-rotor vernier 168 machine to implement the proposed brush-less topology is 169 its suitability for various applications such as in-wheel motors 170 and washing machines. In addition, it offers a greater slot area 171 for the rotor field winding poles, which helps avoid manu-172 facturing complications associated with many rotor poles for 173 the inner-rotor vernier machines. The speed of the vernier 174 machine can be calculated using equation (4) where ω r denotes the rotor speed, and ω MMF represents the 177 main stator field speed [26], [27].

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For the operational validation of the conventional and pro-194 posed brush-less WFVM topologies and to realize their elec-195 tromagnetic performance for the comparative performance 196 analysis, two-dimensional machine models, as shown in 197 Fig. 2 and 6, are developed in JMAG-Designer. The designing 198 parameters of these machines are presented in Table. 1.

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To study the behavior of the conventional and proposed 201 brush-less WFVM topologies under no-load conditions, 202 no-load analysis is implemented in JMAG-Designer. Both 203 machines are operated at 300 rpm, and their field wind-204 ing is energized with a direct current of 1 A. The gener-205 ated back-EMF of the conventional and proposed WFVMs 206 are shown in Fig. 8(a) and (b), respectively. These figures 207 illustrate that the magnitude of back-EMF for the ABC 208 and XYZ windings of the conventional topology is around 209 10.003 V rms ; however, this magnitude for the proposed topol-210 ogy is 52.443 V rms . The lower magnitude of the back-EMF 211 for the conventional topology is due to the split of armature 212 winding in two-halves. The harmonic contents of the gen-213 erated back-EMF are presented in Fig. 9. From this result, 214 the total harmonic distortion (THD) of the back-EMF for 215 the conventional brush-less WFVM topology is calculated 216 as 10.92%, whereas in the proposed self-excited brush-less 217 WFVM topology, the THD for the back-EMF is around 218 11.33% which is 0.412% higher than the conventional topol-219 ogy. As the THD of the back-EMF under no-load condi-220 tions affects the torque ripple of the machine, the proposed 221 brush-less WFVM topology will exhibit a higher magnitude 222  of the torque ripple than the conventional WFVM topology.

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This will further be confirmed from the loaded analysis pre-  For the loaded analysis of the conventional and proposed 229 brush-less WFVM topologies, the shaft of the machines is 230 rotated at a speed of 300 rpm, whereas the simulations of 231 these machines are performed for 0.6 s. For the conventional 232 VOLUME 10, 2022 brush-less topology, the input armature currents for ABC and 233 XYZ windings are 1 A (peak) and 0.7 A (peak), respectively.   To calculate the efficiency of the conventional and 273 proposed brush-less WFVM topologies, loss studies for the 274 stator and rotor cores of the employed machines are imple-275 mented in JMAG-Designer. Fig. 16 Table. 3.

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A self-excited brush-less WFVM topology based on a single- ing. The sub-harmonic field was used to induce a current in 296 the harmonic winding, which was rectified to energize forty-297 four-pole rotor field winding to realize brush-less operation.

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The proposed topology was implemented in FEA using a 299 four-pole, twenty-four-slot machine and compared with the 300 conventional sub-harmonic-based brush-less WFVM topol-301 ogy under the same loading and operating conditions. The 302 results showed that the proposed topology generates 34.71% 303 more average output torque than the conventional topology.

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The total losses of the proposed topology are calculated as 305 232.191 W, which is 3.2% lower than the total calculated 306 losses of the conventional topology. The efficiency of the 307 proposed topology was found to be 7.19% higher than the 308 conventional one.

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On the other hand, a slightly higher magnitude of torque