Electromagnetic and Thermal Analysis of Interior Permanent Magnet Motors Using Filled Slots and Hairpin Windings

This paper analyzes the electromagnetic and thermal design of interior permanent magnet motors using filled slots and hairpin windings for electric vehicle applications. Two models of ∇ shape of the interior permanent magnet motors have been proposed to evaluate the temperature distribution and cogging torque performance. A narrow opening slot of the interior permanent magnet of 48 slots/8 poles with the filled winding has been designed to investigate the electromagnetic torque because the cogging torque depends on opening stator slots. A parallel– rectangle slot of the interior permanent magnet with the hairpin winding has been also implemented with finite element analysis to evaluate their performances. Normally, the slot opening of the interior permanent magnet stator equals the slot width, it is greater than the size of hairpin windings, and the cogging torque is increased significantly with a bigger slot opening. The main advantage of the hairpin winding design is the high slot fill factors. Hence, the lower the current density, the higher torque, and efficiency are, than the normal design with the same geometry parameters. To improve the cogging torque due to the wide slot opening, the step–skew rotor slices have been arranged to minimize the torque ripple with different skewing angles. Keywords-interior permanent magnet; finite element analysis; Ansys Maxwell; SPEED software; hairpin windings

INTRODUCTION Hairpin-type windings are gaining increasing popularity [1,2], especially in automotive traction motors due to advantages which include reduced manufacturing time, high fill factor, shorter end-winding overhang, and better high voltage protection. Moreover, with respect to the random nature and the packing style of traditional distributed windings, hairpin windings consist of a plurality of accurately placed stator bars [1,3]. However, the cogging torque is very high with a wider slot opening, and it is also very complicated to manufacture.
Thus, in order to improve the torque ripple, a novel design of a step-skew V shape magnet has been presented for Interior Permanent Magnet (IPM) motors of 150kW (48 slots/8 poles). The stator stepped slot-openings with optimal shift angles have been implemented for the IPM motors in [4][5][6]. Cogging torque reduction has become an increasingly important issue in IPM motors. Effective methods for reducing the cogging torque, such as skewing slots, step-skewing rotor, and magnets are possible for the slot-filled windings. For the hairpin windings, only step-skewing rotor can be applied to improve the torque ripple. In this paper, a stepped skewing shift magnet rotor method for reducing the cogging torque of IPM motors, which can avoid the drawbacks of the wider slot opening of stator slots, is proposed. The electromagnetic torque and efficiency performance of the IPM motor are significantly affected by winding topology. The electromagnetic performance of multilayered IPM motors keeping in mind their potential use in Electric Vehicle (EV) applications. The torque and back Electromagnetic Force (EMF) waveforms are verified with different step skewing models. The torque harmonics have been compared for different topologies. Finally, an IPM motor with a step-skewing magnet rotor is manufactured to verify the results of Finite Element Analysis (FEA) [9][10][11][12].

II. HAIRPIN AND SLOT FILL WINDING DESIGNS
The designed IPM motor with ∇ shape of permanent magnet rotor with slot filled and hairpin windings is presented in Figure 1. The stator slot parameters are pre-determined in and are shown in Table I. The slot-filled winding has an opening width of 1.2mm, which is greater than the wire diameter of 1mm (Figure1(a)) and the stator slot opening of the hairpin winding is 5mm, bigger than the rectangle bar sizes of 4.3×3.3mm (Figure 1  The two IPM motors have 48 slot/8poles, the stack length is 51mm, the diameter of stator and rotor is 250mm and 175mm respectively, the air-gap length is 1mm, the thickness of the electrical steel sheet is 0.2mm, the continuous phase current amplitude is 400A, the continuous rated power is 150kW, and the maximum speed of the machines is 12000rpm. The main differences are winding overhang, slot opening, and the copper sizes of 15 (Φ 0.85) and 4.3×3.3mm. The slot opening is 1.2mm in slot filled and 5mm in hairpin designs. In both models, the slot openings are arranged along the motor axis and the central position of the slots remains the same. The cogging torque of IPM motors results from the interaction between the stator teeth and rotor magnetic poles, which can be expressed in Fourier series [6][7][8]: where i is the order of the cogging torque harmonics, T ci is the amplitude of i order cogging torque component, and N is the least common multiple between the number stator slots and rotor poles. The mechanical angle of the stator opening θ is: where t S is slot opening width, D S is the stator diameter.
The cogging torque of slot filled winding with 1.2mm (0.55 0 ) opening is 2.8N.m, while the cogging torque of Hairpin winding with 5mm opening width (2.22 0 ) is 8N.m, which is three times bigger than the slot filled winding designs.

III. COGGING TORQUE AND THERMAL ANALYSIS
To eliminate the cogging torque by the stator slot-openings, the step-skewing rotor has a shift angle of β mechanical degrees. The cogging torque can be transformed as [6]: If the rotor is divided into segments along the motor axial direction, and the relative shift angle between the adjacent two rotor poles is δ n mechanical degrees, the resultant cogging torque can be expressed as [6]: The above equation can be simplified to: Hence, for eliminating the i-th order cogging torque component, the theoretical shift angle β n must fulfill the following requirements: sin ேఉ ଶ =0 (6) By applying the cogging torque for straight skewing and Vshape skewing magnet model, the conventional skewing model with straight skewing has a smaller shift angle than the V-shape skewing model with the same total skewing angle. The V shape magnet model has more advantages than the straight skewing because two harmonic orders can be eliminated together. For example, the harmonic orders with mechanical degrees of 1 0 or 3 0 are reduced to zero if they fulfill (6). The cogging torque of the 3 models is shown in Figure 3. The cogging torque of the V shape skewing design is the lowest value because the second order harmonics are eliminated in this model. The back EMF harmonic of the 3 models has been analyzed by Fourier transform from the electromagnetic force waveform with the harmonic order from the first to the 15 th order. The back EMF amplitude versus harmonic order was implemented by the   The temperature distribution of slot fill and hairpin winding designs is depicted in Figure 6. The hot spot of Hairpin winding is 105.7 0 C, which is lower than 111.5 0 C. Based on these results, the IPM V skewing with the hairpin winding design is preferable for EV applications. The efficiency map of hairpin winding design is shown in Figure 7, with a speed range of 12000rpm and a current density of 8.9A/m 2 .

IV. SKEWING ROTOR DESING
The prototype with the 6-segmented magnets has been manufactured and is shown in Figure 8. Each segment with a thickness of 20mm is skewed by 1.5 mechanical degrees. To insert the six segments simply, every segment block is designed with one guide pin to fix the correct position when all segments assembly together.  The whole hardware of the IPM motor has been assembled as in Figure 9. The high accurate torque transducer is used to measure the torque and speed values under different load conditions. The measured no-load phase EMF at 1500rpm of IPM is shown in Figure 10. To verify the electromagnetic performance, the 48 slot/8 pole proposed PM machine is built. Distributed windings were adopted. It should be noted that the PM motor is a prototype. The no-load back-EMF is carried out by using a voltage sensor. The simulated and measured results are exhibited in Figure 10. One circle of the back EMF has been measured by the oscilloscope and the data were recorded for plotting those curves. Fig. 9.
Back EMF measurement of the IPM motor system.

V. CONCLUSION
The obtained performance of IPM with slot filled and hairpin windings in the wide speed range has been presented in this paper and the effect of slot opening width on the cogging torque has been discussed. The thermal simulation of the two designs has been investigated to find out the hot spot or maximum temperature of hairpin and slot filled windings. A significant contribution of this study is the analytical calculation of the V skewing angle for the elimination of the second order harmonics of the cogging torque. A prototype of the V skewing magnet shape of the IPM hairpin winding has been manufactured and assembled. The simulation results achieved peak torque of 300N. m at the base speed of 5500rpm. The back EMF waveforms obtained from the FEA are close to the measured ones.