# Stall Torque Performance Analysis of a YASA Axial Flux Permanent Magnet Synchronous Machine

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## Abstract

**:**

## 1. Introduction

- Slot/pole combination: This aspect influences both the fundamental winding factor and back-emf constant, which directly impact the torque. It also influences the number of adjacent slots belonging to the same phase. As heat has to flow from the phase with the highest losses to phases with lower losses, it can be expected that the number of adjacent slots influences the thermal performance under uneven loss distribution.
- Thermal end-winding interconnection: From previous studies, it is known that the end-winding at the inner diameter is often the hottest area of a YASA AFPMSM [2,10]; therefore, a good thermally conducting ring which interconnects all end-windings can redistribute the heat from the phase with the highest losses to the other phases.
- Equivalent winding body thermal conductivity: Since in a YASA AFPMSM there is no iron stator yoke which has a good thermal connection with all slots, the equivalent thermal conductivity of the winding body can have a significant influence on the heat transfer between phases.

**Figure 1.**(

**a**) The motor in a force controlled gripper has to generate high torque at (quasi)-standstill [14]. (

**b**) Overview of the studied parameters in this work that potentially have a significant impact on the stall torque performance.

## 2. Design Parameters Affecting Tangential Heat Transfer

#### 2.1. Loss Distribution

#### 2.2. Slot/Pole Combination

#### 2.3. Thermal End-Winding Interconnection

#### 2.4. Equivalent Winding Body Thermal Conductivity

## 3. Materials and Methods

#### 3.1. Experimental Setup

^{®}TCPA 300 current amplifier. A dSPACE MicroLabBox

^{®}platform was used for data-acquisition. All signals were sampled at 1 kHz.

#### 3.2. 3D Thermal FE Model

#### 3.2.1. Geometry

#### 3.2.2. Thermal Interfaces

#### 3.2.3. Anisotropic Material Modelling

#### 3.2.4. Boundary Conditions

#### 3.2.5. Transient Model Calibration

#### 3.2.6. Steady-State Temperature Distribution

## 4. Results

#### 4.1. Experimental Results

#### 4.1.1. Influence of Slot/Pole Combination

#### 4.1.2. Influence of Thermal End-Winding Interconnection

#### 4.1.3. Influence of Equivalent Winding Body Thermal Conductivity

#### 4.2. Experimental Data Analysis through Simulation

#### 4.2.1. Influence of Gap between End-Winding and End-Winding Interconnection Ring

#### 4.2.2. Analysis of Equivalent Thermal Conductivity of Winding Body

#### 4.2.3. Influence of Cyclic Loading

## 5. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 2.**Prototype Yokeless and Segmented Armature Axial Flux Permanent Magnet Synchronous Machine used throughout this work; it has two rotors and a single stator. For visualization purposes only a single rotor is shown here. Its specifications are given in Table 1.

**Figure 3.**Winding diagrams and the number of adjacent coils belonging to the same phase n for the feasible slot/pole combinations from Table 2.

**Figure 4.**Yokeless and Segmented Armature Axial Flux Permanent Magnet Synchronous Machine: (1) Thermal end-winding interconnection ring (2) Concentrated winding tooth coil (3) Aluminium housing (4) Epoxy impregnation (5) Permanent magnet rotor.

**Figure 5.**Experimental test setup. (left) Detailed view of prototype stator (1) thermal end-winding interconnection (2) PT100 temperature sensor on aluminium housing (right) Overview of setup: (3) cooling fans (4) thermal insulation wool (5) Expanded Polystyrene Insulation (XPS) (6) prototype stator between insulation (7) PT100 temperature sensor signal conditioning board (8) DC power supply (9) dSPACE MicroLabBox

^{®}real time control and data processing unit (10) Tektronix

^{®}TCPA 300 current amplifier.

**Figure 7.**Prototype tooth coils. Left: enamelled copper wire tooth coil, Right: anodised aluminium foil tooth coil.

**Figure 9.**Concentrated winding tooth coil: (1) Aluminium thermal end-winding interconnection ring; (2) Laminated iron core; (3) Mica inter coil insulation sheet; (4) Mica slot liner; (5) Winding body; (6) Aluminium-oxide thermal pad; (7) Aluminium housing; (8) Epoxy potting.

**Figure 11.**Transient temperature evolution at selected locations (see Figure 10) in a stator with round enamelled copper wire for a 24 slot, 22 or 26 pole combination. (

**a**) Simulated temperatures|stator without end-winding ring. (

**b**) Measured temperatures|stator without end-winding ring. (

**c**) Simulated temperature|stator with end-winding ring. (

**d**) Measured temperatures|stator with end-winding ring.

**Figure 12.**Transient temperature evolution at selected locations (see Figure 10) in a stator with anodised aluminium foil winding for a 24 slot, 22 or 26 pole combination. (

**a**) Simulated temperatures|stator without end-winding ring. (

**b**) Measured temperatures|stator without end-winding ring. (

**c**) Simulated temperatures|stator with end-winding ring. (

**d**) Measured temperatures|stator with end-winding ring.

**Figure 13.**Steady-state temperature distribution in the stator with round enamelled copper wire without thermal end-winding interconnection, with a 24 slots, 22 or 26 poles combination.

**Figure 14.**Measured ratio of the losses that can be dissipated in under stall torque conditions over the losses that can be dissipated in case of uniform loss distribution for various slot/pole combinations (see Figure 3) and for the cases with and without thermal end-winding interconnection.

**Figure 15.**Measured torque derating factor ${T}_{\mathrm{stall}}/{T}_{\mathrm{uniform},\mathrm{s}24\mathrm{p}22}$ for various slot/pole combinations and for the case with and without thermal end-winding interconnection.

**Figure 16.**Measured torque derating factor ${T}_{\mathrm{stall}}/{T}_{\mathrm{uniform},\mathrm{s}24\mathrm{p}22}^{\mathrm{x}}$ with $\mathrm{x}\in \left\{\mathrm{Al},\mathrm{Cu}\right\}$ for different conductor types, for $n=4$, and for the case with and without thermal end-winding ring.

**Figure 17.**Simulated (

**a**) torque derating factor ${T}_{\mathrm{stall}}/{T}_{\mathrm{uniform},\mathrm{s}24\mathrm{p}22}$ and (

**b**) maximum stall torque for various slot/pole combinations and for the case without thermal end-winding interconnection and with a thermal end-winding interconnection with a minimum gap between the end-winding and the ring. (

**a**) Simulated torque derating factor ${T}_{\mathrm{stall}}/{T}_{\mathrm{uniform},\mathrm{s}24\mathrm{p}22}$. (

**b**) Simulated maximum stall torque (@ ${T}_{\mathrm{hotspot}}=150{\phantom{\rule{3.33333pt}{0ex}}}^{\xb0}$C).

**Figure 18.**Simulated (

**a**) torque derating factor ${T}_{\mathrm{stall}}/{T}_{\mathrm{uniform},\mathrm{s}24\mathrm{p}22}^{\mathrm{x}}$ with $\mathrm{x}\in \left\{\mathrm{Al},\mathrm{Cu}\right\}$ and (

**b**) maximum stall torque, for different values of ${f}_{\mathrm{wi}}^{\mathrm{Al}}$, for different conductor types, for a 24 slots and 22 or 26 poles combination, and for the case with and without thermal end-winding ring. (

**a**) Simulated torque derating factor ${T}_{\mathrm{stall}}/{T}_{\mathrm{uniform},\mathrm{s}24\mathrm{p}22}$. (

**b**) Simulated maximum stall torque (@ ${T}_{\mathrm{hotspot}}=150{\phantom{\rule{3.33333pt}{0ex}}}^{\xb0}$C).

**Figure 19.**Simulated hotspot temperature variations for a load cycle with a duty cycle of 50% for a case with uniform losses (e.g., when the motor is rotating when producing torque) and for a case with non-uniform losses (e.g., when the motor is at standstill when producing torque).

Parameter | Symbol | Value | Unit |
---|---|---|---|

Three-phase inverter DC bus voltage | ${V}_{\mathrm{DC}}$ | 48 | V |

Maximum speed | ${\mathsf{\Omega}}_{\mathrm{max}}$ | 300 | rpm |

Number of pole pairs | ${N}_{\mathrm{p}}$ | 13 | / |

Number of slots | ${Q}_{\mathrm{s}}$ | 24 | / |

Number of phases | ${n}_{\mathrm{ph}}$ | 3 | / |

Number of turns per tooth coil | ${n}_{\mathrm{turns}}$ | 35 | / |

Outer diameter stator iron core | ${D}_{\mathrm{o}}$ | 138.5 | mm |

Inner diameter stator iron core | ${D}_{\mathrm{i}}$ | 98.5 | mm |

Axial length stator iron core | ${h}_{\mathrm{stat}}$ | 15 | mm |

Axial slot length | ${h}_{\mathrm{slot}}$ | 10 | mm |

Total axial length (incl. housing) | ${l}_{\mathrm{tot}}$ | 62.5 | mm |

Slot width | ${b}_{\mathrm{slot}}$ | 6 | mm |

Airgap thickness | ${h}_{\mathrm{air}}$ | 1.5 | mm |

Magnet height | ${h}_{\mathrm{mag}}$ | 5 | mm |

Rotor yoke height | ${h}_{\mathrm{mag}}$ | 6 | mm |

${\mathit{Q}}_{\mathit{s}}$ | Number of Poles (p) | ||||||||
---|---|---|---|---|---|---|---|---|---|

16 | 18 | 20 | 22 | 26 | 28 | 30 | 32 | ||

24 | $\xi $ | 0.866 | 0.933 | 0.9495 | 0.9495 | 0.933 | 0.866 | ||

${k}_{\varphi}^{\prime}$ | 0.324 | 0.353 | 0.358 | 0.354 | 0.346 | 0.329 | |||

${k}_{\varphi}={k}_{\varphi}^{\prime}\xb7\xi $ | 0.281 | 0.329 | 0.340 | 0.336 | 0.323 | 0.285 |

Enamelled Copper Wire | Symbol | Value | Unit |
---|---|---|---|

(Grade I, IEC 60317-13) | |||

Number of turns | ${n}_{\mathrm{turns}}$ | 35 | / |

Nominal outer diameter | ${d}_{\mathrm{Cu},\mathrm{o}}$ | 0.8425 | mm |

Conductor diameter | ${d}_{\mathrm{Cu},\mathrm{i}}$ | 0.8 | mm |

Winding length (incl. terminals) | ${l}_{\mathrm{Cu}}$ | 276 | cm |

measured DC resistance (@ 25 °C) | ${R}_{\mathrm{DC},\mathrm{Cu}}$ | 94.54 ± 0.37 ^{1} | $\mathrm{m}\mathsf{\Omega}$ |

Height laminated iron core | ${h}_{\mathrm{core}}$ | 20 | mm |

Weight of tooth coil | ${m}_{\mathrm{Cu}+\mathrm{SiFe}}$ | 31.3 | g |

Resistivity copper | ${\rho}_{\mathrm{Cu}}$ | 1.72 $\phantom{\rule{3.33333pt}{0ex}}\times \phantom{\rule{3.33333pt}{0ex}}{10}^{-8}$ | $\mathsf{\Omega}$m |

Resistance temperature coeff. | ${\alpha}_{\mathrm{Cu}}$ | 3.93 $\times \phantom{\rule{3.33333pt}{0ex}}{10}^{-3}$ | K${}^{-1}$ |

Fill factor | ${f}_{\mathrm{Cu},\mathrm{coil}}$ | 49 | % |

Dielectrical strength | ${E}_{\mathrm{max}}$ | 87 | ${V}_{\mathrm{RMS}}$/$\mathsf{\mu}$m |

(IEC 60317-0-1) | |||

Price/kg | 16.64 | EUR/kg | |

Anodised aluminium foil | |||

Number of turns | ${n}_{\mathrm{turns}}$ | 35 | / |

foil width | ${h}_{\mathrm{Al}}$ | 10 | mm |

total foil thickness | ${t}_{\mathrm{Al},\mathrm{tot}}$ | 86 | µm |

thickness ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ layer | ${t}_{\mathrm{AlOx}}$ | 4.6 | µm |

Foil length (excl. terminals) | ${l}_{\mathrm{Cu}}$ | 250 | cm |

Cu terminal length (dia. 0.9 mm) | ${l}_{\mathrm{term}}$ | 40 | cm |

measured DC resistance (@ 25 °C) | ${R}_{\mathrm{DC},\mathrm{Al}}$ | 95.83 ± 0.6 ^{1} | $\mathrm{m}\mathsf{\Omega}$ |

Height laminated iron core | ${h}_{\mathrm{core}}$ | 20 | mm |

Weight of tooth coil | ${m}_{\mathrm{Al}+\mathrm{SiFe}}$ | 25.8 | g |

Resistivity aluminium | ${\rho}_{\mathrm{Al}}$ | 2.74 $\phantom{\rule{3.33333pt}{0ex}}\times \phantom{\rule{3.33333pt}{0ex}}{10}^{-8}$ | $\mathsf{\Omega}$m |

Resistance temperature coeff. | ${\alpha}_{\mathrm{Al}}$ | 4.03$\phantom{\rule{3.33333pt}{0ex}}\times \phantom{\rule{3.33333pt}{0ex}}{10}^{-3}$ | K${}^{-1}$ |

Fill factor | ${f}_{\mathrm{Al},\mathrm{coil}}$ | 75 | % |

Dielectrical strength | ${E}_{\mathrm{max}}$ | 26.5 | ${V}_{\mathrm{RMS}}$/$\mathsf{\mu}$m |

(ISO 2376) | |||

Price/kg | 685 ^{2} | EUR/kg |

^{1}mean and standard deviation over 24 tooth coils.

^{2}Note that this is the cost for a small order quantity of 2 kg; for larger order quantities, the cost will be lower.

Parameter | Value | Unit |
---|---|---|

${k}_{\mathrm{Cu}}$ | 385 | W/mK |

${k}_{\mathrm{Al}}$ | 237 | W/mK |

${k}_{\mathrm{Ep}}$ | 0.37 | W/mK |

${k}_{\mathrm{AlOx},\mathrm{film}}$ | 1.6 | W/mK |

${k}_{\mathrm{AlOx}}$ | 20 | W/mK |

${k}_{\mathrm{FeSi}}$ | 28 | W/mK |

${f}_{\mathrm{wi}}^{\mathrm{Cu}}$ | 0.49 | [/] |

${f}_{\mathrm{afol}}$ | 0.89 | [/] |

${f}_{\mathrm{wi}}^{\mathrm{Al}}$ | 0.75 | [/] |

${f}_{\mathrm{core}}$ | 0.98 | [/] |

${f}_{\mathrm{pad}}$ | 0.88 | [/] |

${k}_{1,\mathrm{wi}}^{\mathrm{Cu}}$ | 189 | W/mK |

${k}_{2,\mathrm{wi}}^{\mathrm{Cu}}$ | 1.08 | W/mK |

${k}_{1,\mathrm{afol}}$ | 212 | W/mK |

${k}_{2,\mathrm{afol}}$ | 14.3 | W/mK |

${k}_{1,\mathrm{wi}}^{\mathrm{Al}}$ | 159 | W/mK |

${k}_{2,\mathrm{wi}}^{\mathrm{Al}}$ | 1.37 | W/mK |

${k}_{1,\mathrm{core}}$ | 27.4 | W/mK |

${k}_{2,\mathrm{core}}$ | 0.37 | W/mK |

${k}_{1,\mathrm{pad}}$ | 17.6 | W/mK |

${k}_{2,\mathrm{pad}}$ | 2.72 | W/mK |

n | Thermal End-Winding Connection (Yes/No) | Al/Cu | ${\mathit{R}}_{\mathbf{hotspot}}$ (K/W) | $\frac{{\mathit{Q}}_{\mathbf{stall}}}{2\xb7{\mathit{Q}}_{\mathbf{uniform},\mathit{s}24\mathit{p}22}}$ |
---|---|---|---|---|

4 | No | Cu | 7.89 | 0.534 |

4 | Yes | Cu | 7.29 | 0.578 |

2 | No | Cu | 6.44 | 0.655 |

2 | Yes | Cu | 6.34 | 0.665 |

1 | No | Cu | 4.86 | 0.866 |

1 | Yes | Cu | 4.90 | 0.860 |

4 | No | Al | 7.33 | 0.536 |

4 | Yes | Al | 6.43 | 0.611 |

uniform losses | Cu | 8.42 | ||

uniform losses | Al | 7.86 |

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## Share and Cite

**MDPI and ACS Style**

Van Damme, J.; Vansompel, H.; Crevecoeur, G.
Stall Torque Performance Analysis of a YASA Axial Flux Permanent Magnet Synchronous Machine. *Machines* **2023**, *11*, 487.
https://doi.org/10.3390/machines11040487

**AMA Style**

Van Damme J, Vansompel H, Crevecoeur G.
Stall Torque Performance Analysis of a YASA Axial Flux Permanent Magnet Synchronous Machine. *Machines*. 2023; 11(4):487.
https://doi.org/10.3390/machines11040487

**Chicago/Turabian Style**

Van Damme, Jordi, Hendrik Vansompel, and Guillaume Crevecoeur.
2023. "Stall Torque Performance Analysis of a YASA Axial Flux Permanent Magnet Synchronous Machine" *Machines* 11, no. 4: 487.
https://doi.org/10.3390/machines11040487