Efficiency improvement of a high-speed permanent magnet excited synchronous machine by the use of spot-welding as lamination packaging technology

Abstract

In this paper, an efficiency improvement of a high-speed permanent magnet excited synchronous machine (PMSM) by the use of spot-welding as lamination packaging technology is realized. Welding, interlocking, clinching and gluing are typical technologies to manufacture electrical steel stacks of electrical machines. However, these processes deteriorate the magnetic properties of the steel sheets, thereby decreasing the overall efficiency of the machine. Variations of line-welding (six, eight, twelve, thirty two and forty) line welds and spot-welding procedure is performed and analyzed. A semi-physical iron loss separation approach is used to extract the IEM iron loss model (Eggers et al., IEEE Transactions on Magnetics48(11) (2012), 3021–3024). These parameters are utilized during the finite element simulation to calculate the effective iron loss (efficiency) of the machine while using each joining procedure. A low speed, high-speed and WLTC-c3 driving cycle is exemplarily used to analyze the effects of each procedure on the machine efficiency at the different operating conditions.

Keywords

Welding process ferro-magnetic lamination numerical simulation electrical steel

1. Introduction

For the construction of a mechanically stable magnetic core of an electrical ac machine, the electrical steel laminations must be firmly connected. Welding, interlocking, clinching and gluing represent the current state of the art of the mechanical connection of such thin steel sheets [14,15]. Each of the connecting processes leads directly to the deterioration of the material’s magnetic properties. Increased iron losses as well as a decreasing permeability of the electrical steel can be observed. The connection of the single lamination sheets by welding is a commonly used technology. In this process, the sheets are joined mechanically along a seam accompanied by an apparent dissipation of heat along the welding area. This local concentration of dissipated heat yields a thermal degradation and increase of the residual stress distribution inside the material[18,19]. The residual stress reduces the magnetizability of the electrical steel used due to the Villari-effect. In this paper, a method for improving the overall efficiency of a high-speed permanent magnet synchronous machine is discussed. The decreasing effects of the aforementioned weld process (spot-welding) on the eddy-current loss component of iron loss can be utilized for a high-speed operation, where most of the operations are at higher frequencies in the range 400 Hz–1000 Hz. A finite element simulation of the high-speed permanent magnet synchronous machine is performed and the analysis of the joining process with driving cycles has been done. The effects of welding have been reported [1–3] to lead to a significant magnetic deterioration of the sample of a ring core. These deterioration are in part, due to increase of the average grain size [13,17] in and around the welded area, and also due to the increase in residue stresses in and around the weld-point [5,8–12]. However, a comprehensive and systematic analysis of the welding process is still lacking. This paper studies the effects of different welding scheme such as spot-welding and line-welding on the iron losses and efficiencies of electrical machines. The simulated results will then be evaluated with vehicle data and driving cycles with view of understanding the exact extent of the influence of welding on actual application.

2. Experimental approach

2.1. Examined samples

Ring core samples welded with spot-welding and line-welding procedure were manufactured for this study. The ring core laminations have inner D_i and outer D_o diameters of 60 mm and 48 mm respectively (Fig. 1).

Fig. 1.

Geometry of the samples.

The height h of the examined ring samples is 9 mm, i.e. 30 ring laminations are joined together. The reference ring core is joined together with a conventional glue, so as to exclude the effects of the material degradation. The specific iron loss and magnetization emanating from this sample represents the electromagnetic properties of the laminated stack. The individual rings are laser cut to the aforementioned dimensions.

Fig. 2.

(a) Line-weld sample and (b) spot-weld sample.

The reference ring core is compared to other ring cores of different welding concepts (spot- and line-welding). The spot welded ring cores connects only two to three rings per spot at each instance Fig. 2b and 3b ). There is a minimum of 4 welds on each rings to ensure the mechanical stability of the motor. Less spot-welds can be used depending the mechanical requirements of the application.

Fig. 3.

(a) Line-weld sample and (b) spot-weld sample.

For the welding process, a laser beam power of 320 W, focal position of 0 mm, resulting in a focal diameter of 400 μm and working pressure of 1000 mbar. Whereas the line-welds are performed at a weld speed of 0.5 m/min, the spot welds are instantaneous at a pulse duration of 70 ms. To ensure the comparability of the ring cores, all other weld process parameters are held constant. A multi-mode disc-laser with a focal length of 400 mm is used for this study (Table 1). Both spot- and line welding are performed with the laser.

Table 1

Specification of the used laser beam source

Producer	TRUMPF
Model	TruDisc 16002
Medium	Yb:YAG
Type	Disc-laser multi-mode
Power range (W)	320–16 000
Wavelenght 𝜆 (nm)	1030
Fibre diameter (μm)	200

2.2. Measurement method

In the measurement setup, during the sample preparations, the ring core dimensions (magnetic length, effective active area and effective active volume) are calculated according to DIN-IEC-60205 standard. During measurement the current on the primary winding induces a voltage response on the secondary winding. With the voltage response the magnetic parameters are derived according to DIN-IEC-60404-6 standard see Fig 4. $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle H=\frac{N_{1}\cdot I}{l_{m}}.\end{eqnarray}$ (1) The magnetic field strength H is calculated from the current I on the primary winding and magnetic length l_m see Eq. ((1)). $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle \overline{U_{2}}=4\cdot f\cdot A\cdot B\cdot N_{2}\end{eqnarray}$ (2) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle P_{s}=\frac{N_{1}\cdot P_{m}}{m\cdot N_{2}}-\frac{{\pi}\cdot (\overline{U_{2}})^{2}}{2\cdot \sqrt{2}\cdot m\cdot R_{i}}.\end{eqnarray}$ (3) The specific iron loss P_s is then derived from the induced voltage on the secondary winding and the ring core geometry (area) see Eq. ((3)). Whereas P_m and m are the measured iron loss and ring core weight respectively. The ring cores are wound with 80 turns on the primary N₁ and secondary side N₂.

Fig. 4.

Measurement setup.

3. Measurement results

In this section, the extent to which each welding scheme contributes on the magnetic deterioration of the electrical sheets is quantified and analyzed. In Figs 5 and 6 the measured results of the specific iron loss and magnetization curves at 50 Hz is shown.

Fig. 5.

Specific iron loss at 50 Hz.

Fig. 6.

Magnetization curve at 50 Hz.

It can be observed, that in general welding leads to the degradation of the electromagnetic properties of the material. This is due to an apparent increase in the thermal energy that penetrates into the sheets. This energy leads to the change of the effective grain size and the deposition of residual stress at the affected area [4], thereby leading to the magnetic deterioration of the sheets. The level of deterioration increases with increasing number of weld lines. The spot welded sample is compared to the 8-line welded sample because of a similar effective welded area of 12.44 mm². It is observed that the decrease of the magnetization of the welded samples, when compared to the reference sample is more pronounced at flux densities between 0.6 T and 1.5 T. At high frequencies, Figs 7 and 8 depicts a more significant deterioration of the material magnetic properties compared to the reference sample. This is due to the effects of the increasing grain sizes, which increases the excess and non-linear loss component.

Fig. 7.

Specific iron loss at 1000 Hz.

Fig. 8.

Magnetization curve at 1000 Hz.

The non-dependency of the eddy-current loss from the effects of welding is observed, at the relatively slight percentage of deterioration of the material for both low and high frequencies. The difference in the deterioration of iron losses at higher frequencies compared to lower frequencies can be attributed to the grain size dependency of the excess loss component of iron loss. Furthermore, there was a slight change of the magnetization curves with increasing frequencies Figs 6 and 8 observed. Therefore, the effects of welding on the magnetization of the ring core is also frequency dependent. In Figs 9 and 10 is the depiction of deterioration of the iron loss of the investigated ring samples at a flux density of 1 T and frequencies of 50 Hz and 1000 Hz respectively. It can be observed, that line-welding leads to more deterioration compared to spot-welding. It can also be observed, that spot-welding and line-welding with 4 lines performed better than the reference sample. This effect will be evaluated in details, because it could be from the margin of error or due to changes in grain texture around the weld-point. The Figs 11 and 12 depicts the magnetization value of the investigated ring samples at a flux density of 1 T and frequencies of 50 Hz and 1000 Hz respectively. It can be seen, that the deterioration tendency is independent on the frequency. A higher level of deterioration of magnetization is observed compared to degradation of the iron loss. The total iron loss is defined as the sum of the hysteresis loss, classic eddy-current loss, local eddy-current loss component (excess) and non-linear loss component [6,7,16]. $\begin{eqnarray}\displaystyle P_{\text{total}}=a_{1}\cdot B^{{\alpha}}\cdot f+F_{\text{skin}}\cdot a_{2}\cdot B^{2}\cdot f^{2}\cdot (1+a_{3}\cdot B^{a_{4}})+a_{5}\cdot B^{1.5}\cdot f^{1.5}. & & \displaystyle\end{eqnarray}$ (4) Whereby a₁, 𝛼 are the hysteresis loss parameter and the exponential coefficient of the hysteresis loss, a₂, a₅ are classical eddy-current loss constant and excess loss constant respectively and a₃, a₄ are the non-linear loss constants [6]. As can be seen in Eq. ((4)), the excess loss component is proportional to the frequency to power 1.5. This meant that an increase in excess loss component gets more pronounce at higher frequencies leading to more specific iron loss generation.

Fig. 9.

Specific iron loss changes at 1 T and 50 Hz.

Fig. 10.

Specific iron loss changes at 1 T and 1000 Hz.

Fig. 11.

Magnetization changes at 1 T and 50 Hz.

Fig. 12.

Magnetization changes at 1 T and 1000 Hz.

4. FEM simulation

For the proposed FEM-simulation comes the motor topology. A high speed permanent magnet synchronous motor PMSM with four poles, an axial length of 98 mm and three-phased winding. The Fig. 13 depicts the torque-speed map of the attainable efficiency, when the material is welded with the line-welding scheme. In Table 2 is the data of the simulated machine shown.

Fig. 13.

Torque-speed map of the efficiency.

Table 2

Data of the simulated machine

Machine parameter	Units	Values
Maximum torque	Nm	100
Maximum speed	r/min	18000
Rated voltage	V	300
No of slots	—	36
Outer diameter	mm	200
Axial length	mm	98

Fig. 14.

Torque-speed map of the WLTC-c3 driving cycle.

Fig. 15.

Torque-speed map of the city driving cycle.

Fig. 16.

Torque-speed map of the highway driving cycle.

5. Evaluation of the result

After modeling of the vehicle (here: Volkswagen golf), the operating points of the different driving cycles are calculated from the velocity-time characteristics of each driving cycle. The motor is incorporated so as to evaluate the motor characteristics with respect to the efficiencies and losses at the different operating points. An algorithm is used to calculate the operating points (torque and rotational speed) values equivalent of the driving cycles at an allowed maximum vehicle velocity of 180 km/h. Figures 14, 15 and 16 show the depiction of the operating points emanating from WLTC-c3, city and highway driving cycles respectively. These points show exactly the torque-speed characteristics of the respective driving cycle. After the calculation of the torque and rotational speed equivalent of the driving cycles, i.e. operational points and its distributions (Prob(M_cycle, n_cycle, 𝛼)) are then incorporated into the loss map of the motor. Whereas 𝛼 represents here the utilized driving cycle. The mathematical description of the aforementioned process can be seen in Eq. (5). $\begin{eqnarray}\displaystyle P(M_{\text{cycle}},n_{\text{cycle}},{\alpha},{\beta})=P(M_{\text{motor}},n_{\text{motor}},{\beta})\cdot \text{Prob}(M_{\text{cycle}},n_{\text{cycle}},{\alpha}). & & \displaystyle\end{eqnarray}$ (5) Where P (M_motor, n_motor, 𝛽) are the motor loss power depending on the used welding scheme 𝛽. Afterwards the corresponding losses at various operating points were then calculated by the integration of equation. $\begin{eqnarray}\displaystyle P({\alpha},{\beta})=\iint P(M_{\text{cycle}},n_{\text{cycle}},{\alpha},{\beta})∼dM_{\text{cycle}}∼dn_{\text{cycle}}. & & \displaystyle\end{eqnarray}$ (6) Due to the fact that loss energy and not loss itself is the most appropriate value for the evaluation of a drive’s efficiency during a traction’s operation, the total loss energies of the corresponding driving cycles were calculated. The calculation were done according to Eq. (7). $\begin{eqnarray}\displaystyle E({\alpha},{\beta})∼[\text{kWh}]=P({\alpha},{\beta})∼[\text{kW}]\cdot \frac{\text{total time}}{3600∼\text{s}}. & & \displaystyle\end{eqnarray}$ (7) The calculated loss energy in Eq. (7) of each driving cycle is then depicted against the different packaging technology. The Figs 17, 18 and 19 depict these losses depending on the WLTC-c3, city and highway driving cycles. It can be seen in all driving cycle, that there is no observable influence of welding on the eddy current loss component of iron loss. High observable change in the excess loss component of iron loss, due to the its high negative dependency on the average grain size. Welding processes leads to an increase of the average grain size at the welded area, because of the influx of thermal energy into the material. The welding parameters such as laser power or welding speed influence the rate of diffusion of the thermal energy into the steel sheet. The rate of diffusion during spot-welding is slower, due to the higher welding speed (spontaneous) associated with the welding scheme. This meant, that the increase in grain sizes is relatively smaller during spot-welding as with line-welding. Figure 17 shows the iron loss energy dissipated during the WLTC-c3 driving cycle. It can be seen, that whereas 4.5% more loss energy is dissipated with spot-welding compared to the reference, 7.8% more loss energy can be attributed to the line-welding scheme. A total reduction of 3.3% loss energy can be attained by the use of spot-welding as lamination packaging method instead of the conventional line-welding scheme. Figure 18 depicts the iron loss energy dissipated during the city driving scheme. It can be observed, that the value of excess loss energy is higher than the eddy current loss energy during the city driving scheme with line-welding, this is due to the low or average speeds attributed to this driving cycle, at which the influence of excess loss component of iron loss becomes more pronounced. This is the reason for an increased reduction of the total dissipated loss energy of around 3.6%. A less rate of diffusion leads to the performance improvement of the drive train during the city driving scheme. Figure 19 shows in average more deterioration of the electromagnetic properties of the electrical steel sheet. This is in part due to the probability of higher rotational speeds and also to the magnetization of the utilized electrical steel sheet. The higher value of the hysteresis loss energy attributed to line-welding as spot-welding is apparently due to the effects of the residue stresses accompanied with welding. Due to higher frequencies, there is less improvement compared to other driving cycles.

Fig. 17.

Iron loss while operating the WLTC-c3 driving cycle.

Fig. 18.

Iron loss while operating the city driving cycle.

Fig. 19.

Ironloss while operating the highway driving cycle.

6. Conclusions

To analyze the effects of different welding schemes on the loss dissipation (efficiency) of an electric drive train, the ring core samples were electromagnetically measured. Loss dissipation attributed to a reference sample (ring packaged with a conventional glue), a spot welded and conventional line welded sample are compared and analyzed. A more visible deterioration of the magnetization was observed for all welded ring cores at flux densities between 0.6 T–1.5 T and at all frequencies during the electromagnetic measurements. For the evaluation of the results, a model incorporating the machine data with the application (golf-vehicle) and driving cycle characteristics is utilized. This modeling approach helps in mapping out of the knowledge on component level to system-wide investigations. Little or no influence of welding on the dissipated eddy current loss energy is observed, because the connection of only the outer surface has an insignificant influence on the induced eddy current. A higher performance improvement of the drive train was achieved at low and average speeds, due to the effects of average grain size at the welded area. Reduction of the excess loss component leads to a higher level of improvement due to its dependency on the grain size of the electrical steel grade. It can be seen, that a reduction of the hysteresis loss component with spot-welding in comparison with line-welding was due to the effects of residue stresses associated with welding. The improvement of the overall efficiency of the motor through spot-welding is due its lower iron loss compared to line-welding. The use of spot-welding procedure leads to less degradation compared to conventional line-welding procedure. This research shows, that a system-wide improvement of the motor can be achieved with welding procedural change (spot-welding) instead of line welding.

Footnotes

Acknowledgements

The work is funded by the Deutsche Forschungsgemeinschaft (DFG) 432930813 in the research group project “Elektromagnetische Bewertung und Quantifizierung von Schweißprozessen zur Paketierung von Elektroblechen”. Simulations were performed with computing resources granted by RWTH Aachen University under project rwth0922.

References

Arshad

W.M.

Ryckebush

Magnussen

Lendenmann

Soulard

Eriksson

Incorporating lamination processing and component manufacturing in electrical machine design tools, in: Conference Record of the 2007 IEEE Industry Applications Conference. 2007. 42nd IAS Annual Meeting, 2007, pp. 94–102.

Krings

Nategh

Wallmark

and Soulard

, Influence of the welding process on the performance of slotless PM motors with SiFe and NiFe stator laminations, IEEE Transactions on Industry Applications50(1) (2014), 296–306.

Schoppa

Schneider

and Wuppermann

C.-D.

, Influence of the manufacturing process on the magnetic properties of non-oriented electrical steels, Journal of Magnetism and Magnetic Materials215–226 (2000), 74–78.

Krichel

Olschok

and Reisgen

, Laser in vacuum spot welding of electrical steel sheets with 3.7% Si-content, in: LiM – Lasers in Manufacturing Conference, June 21–24, 2021, 2021.

Leuning

Steentjes

Gerhards

Reisgen

and Hameyer

, Analysis of a novel laser welding strategy for electrical steel laminations, in: 7th International Electric Drives Production Conference EDPC 2017, Nürnberg, Germany.

Eggers

Steentjes

and Hameyer

, Advanced iron-loss estimation for nonlinear material behavior, IEEE Transactions on Magnetics48(11) (2012), 3021–3024.

Bertotti

, General properties of power losses in soft ferromagnetic materials, IEEE Transactions on Magnetics24(1) (1988), 621–630.

Lamprecht

Hömme

and Albrecht

, Investigations of eddy current losses in laminated cores due to the impact of various stacking processes, in: Electric Drives Production Conference (EDPC), 2012 2nd International, 2012, pp. 1–8.

Imamori

Steentjes

and Hameyer

, Influence of interlocking on magnetic properties of electrical steel lamination, IEEE Trans. Mag.53(11) (2017), 1–4.

10.

Leuning

Steentjes

and Hameyer

, Effect of grain size and magnetic texture on iron-loss components in NO electrical steel at different frequencies, Journal of Magnetism and Magnetic Materials469 (2019), 373–382.

11.

Leuning

Steentjes

Weiss

H.A.

Volk

and Hameyer

, Magnetic material deterioration of non-oriented electrical steels as a result of plastic deformation considering residual stress distribution, IEEE Trans. Mag.54(11) (2018), 1–5.

12.

Luming

Songling

Xiaofeng

Keren

and Su

, Magnetic field abnormality caused by welding residual stress, Journal of Magnetism and Magnetic Materials261(3) (2003), 385–391.

13.

Pfeifer

and Kunz

, Bedeutung von Kornstruktur und Fremdkörpereinschlüssen für die Magne-tisierungseigenschaften hochpermeabler Ni-Fe-legierungen, Journal of Magnetism and Magnetic Materials4(1–4) (1977), 214–219.

14.

Schade

Ramsayer

R.M.

and Bergmann

J.P.

, Laser welding of electrical steel stacks investigation of the weldability, in: Electric Drives Production Conference (EDPC), 2014 4th International, 2014, pp. 1–6.

15.

Schade

T.e.a.

, Electrical steel stacks for traction motors – fundamental investigation of the weldability, in: Shaping the Future by Engineering: Proceedings; 58th IWK, Ilmenau Scientific Colloquium, Vol. 58, 2014.

16.

Schmidt

van der Giet

and Hameyer

, Improved iron-loss prediction by a modified loss-equation using reduced parameter identification range, in: 20th Int. Conf. on Soft Magnetic Materials (SMM20), Kos, Greece, 2011, p. 421.

17.

Leuning

Steentjes

and Hameyer

, Effect of magnetic anisotropy on Villari effect in non-oriented FeSi electrical steel, International Journal of Applied Electromagnetics and Mechanics55(S1) (2017), S23–S31.

18.

Wang

Zhang

and Li

, Laser welding of laminated electrical steels, Journal of Materials Processing Technology230 (2016), 99–108.

19.

Ukwungwu

Krichel

Schauerte

Leuning

Olschok

Reisgen

Electromagnetic assessment of welding processes for packaging of electrical sheets, in: 2020 10th International Electric Drives Production Conference (EDPC), 2020, pp. 1–6.