A novel split-teeth Vernier pseudo-direct-drive permanent-magnet machine with concentrated winding

Abstract

1. IntroductionThe demand for low-speed and high-torque electrical machines is increasing in different industrial applications. Vernier permanent-magnet machines (VPM) and Pseudo-direct-drive machines (PDD) are especially suitable for this situation due to their magnetic gearing effect [1,2]. Generally, the power factor of the conventional VPM can be as low as 0.66 [3]. PDDs have the capability of high power factor as well as high torque density [2].To output higher torque density compared with PDDs, a novel Vernier Pseudo-direct-drive machine (VPDD) is proposed in [4] as Fig. 1 (a) shows. In this paper, the subdomain analytical model is used to analyze and preoptimize a VPDD. After the preoptimization, a finite element model (FEM) is also used to optimize the slot opening and check the demagnetization. However, in this machine, the winding has not been optimized to reduce the size of end-winding, and as a result, the real torque density taking the end-winding into account is much lower.Our research focuses on further improving the torque density of the VPDD considering a structure that reduces the size of end-winding. A concentrated winding split-teeth VPDD topology (ST-VPDD), shown in Fig. 1 (b), is hence proposed.2. Modeling and optimizationThe ST-VPDD can be regarded as a combination of a magnetic gear and a VPM. For the part that functions as a magnetic gear [4], the speed ratio can be expressed as Gtr=Ωt/Ωr=pr/pt=-Tr/Tt, where Ωt, Ωr are the rotational angular speed of the transmitter and of the low-speed (LS) rotor; pt, pr are the pole-pairs of permanent-magnets (PM) of the transmitter and of the LS rotor; Tt, Tr are the torque of the transmitter and of the LS rotor. The rotating magnetic field created by the armature winding drives the transmitter. The LS rotor will be driven by both the transmitter and by the field created by the armature winding.If the transmitter is removed, the ST-VPDD can be regarded as a VPM. According to [5], if Pf=nZ=pr±ps is satisfied, the rotating field of armature winding can drive the LS rotor. n is the number of split teeth per tooth, Z is the number of slots and ps is the pole-pairs of armature winding. The gear ratio can be expressed as Gsr=Ωs/Ωr=pr/(nZ-pr)=pr/ps, where Ωs is the rotational angular speed of the field created by the winding.We keep ps=pt so that Gsr=Gtr. Therefore, the part of the magnetic gear and the VPM will drive the low-speed rotor together at the same speed.The ST-VPDD is analyzed through a FEM. Its ferromagnetic material is made of M-45 steel.Fig. 2 (b) shows the proposed concentrated winding model. For comparison, the distributed winding model of the prototype VPDD is put in Fig. 2 (a). In the figure, lw is the length of the half-width of a slot, and ls depends on the winding material. According to the winding model, a mathematical model is built to calculate the torque density taking end-winding into account.The torque density of the ST-VPDD is optimized. The variable parameters are:1. r1 is the radius of the transmitter iron;2. hmt is the thickness of the transmitter PM;3. hmr is the thickness of the rotor PM;4. ls is the length of the split teeth;5. c1=α1/α2;6. c2=lt/ls;7. c3=β1/β2.Once the optimization is done, we do the demagnetization check on the optimized ST-VPDD.3. ResultsThe following parameters of the ST-VPDD are assumed to be similar to the parameters of the VPDD [4]: Z=9, n=5, pt=ps=3 and pr=42, and its out radius ro is 90 mm and active length la is 75 mm. The optimized ST-VPDD can output 172.1 Nm of torque in theory. Without taking the end-winding into account, it provides 90.2 Nm/L of torque density, which is slightly lower than the torque density of 91.8 Nm/L of the VPDD in [4].From the winding model of the VPDD and of the ST-VPDD, we get that the lengths of end-winding of VPDD and ST-VPDD are 96 mm and 49.6 mm, respectively. So, taking the end-winding into account, the torque density of VPDD and ST-VPDD are 40.3 Nm/L and 54.3 Nm/L. There is a 34.7% improvement in the torque density.The PM used for this design is VACODYM AP 776 [6]. When the temperature is 60 Celsius degree, its remanent flux density is 1.25 T and the demagnetization field is 1300 kA/m. No demagnetization occurs.4. ConclusionsWith concentrated winding, the optimized ST-VPDD topology improves the torque density capability compared with the VPDD. And the concentrated winding is less complicated, which simplifies manufacture and hence decrease its cost. The split tooth allows more combinations of poles and slots, so the structure can be further optimized.5. AcknowledgmentThis work was supported by the National Natural Science Foundation of China under Grant 51851110761. Corresponding author: P.-D. Pfister (e-mail: pierredaniel.pfister.public@gmail.com). **