# Performance Comparison of High-Speed Motors for Electric Vehicle

^{1}

^{2}

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## Abstract

**:**

^{−1}, are designed for use with EV traction motors, and the study clarifies which the type of motor is most suitable for application in high-speed motors of EVs in terms of their mechanical and electrical characteristics.

## 1. Introduction

^{−1}. It is predicted that the maximum speed of automobile motors will increase in the future due to reductions in size and weight, and it is necessary to evaluate the characteristics of each motor in the higher speed range from both electrical and mechanical points of view. In this research, motors that achieve output powers of 85 kW and maximum speeds of 52,000 min

^{−1}are proposed as EV traction motors, and the study reveals which motor type is most suitable for the realization of the output power and the maximum speed in terms of mechanical and electrical characteristics. The PMSM, SRM, and IM are designed to achieve the performances required for use in a high-speed drive. The performances of these designed motors are evaluated using FEA, and the advantages and disadvantages of their use in a high-speed drive are clarified.

## 2. Target Performances and Design Flow

^{−1}, and maximum speed is 52,000 min

^{−1}. The maximum phase current, maximum DC voltage, and the current density are constant at 356 Arms, 365 V, and less than 15 A/mm

^{2}, respectively. The stack length is a constant 100 mm, and the motor diameter can be changed by the design to be less than 200 mm. As shown in the specification, the maximum speed of 52,000 min

^{−1}is very high compared with a general EV traction motor. It is possible to greatly downsize a motor’s volume using high-speed rotation while obtaining high output power. To achieve these specifications and the smallest motor volume, the PMSM, SRM, and IM are designed, and the performances such as maximum torque, maximum output power, loss, and efficiency are evaluated at each rotation speed.

^{−1}, which is determined using centrifugal force analysis. In the design of the electric characteristics, the number of turns, the shape of the stator, and the pole number are determined to obtain the maximum torque of 70 Nm and to retain the induced voltage as less than the DC voltage. In this study, PMSM and IM are assumed to be driven sinusoidally by the vector control with a sensor in all speed region, while SRM is assumed to be driven by hysteresis control at the low-speed region and voltage single pulse control at high-speed region. Actually, the driving by the sinusoidal wave in the high-speed region requires the high switching frequency in the inverter. However, the purpose of this paper is to evaluate the motor characteristics under ideal conditions, and the effects of drive conditions of the inverter are not considered.

## 3. Design of PMSM

^{−5}Ωm. The PMSM is assumed to be driven by a sinusoidal wave. The detailed design process is stated as follow.

#### 3.1. Mechanical Design of PMSM

^{−1}is evaluated using FEA. JMAG-designer is used as the simulation tool. Figure 4 shows the comparison of Mises stress in the IPMSM and SPMSM. The two types of magnet arrangements (Model A and Model B) in the IPMSM and SPMSM, which are designed with constant rotor diameters of 70 mm, are considered. Surface permanent magnet type and interior permanent magnet type are abbreviated as SPM type and IPM type, respectively. For the magnet and core, sintered magnet (NEOMAX-42) and magnetic steel sheet (35H230) materials are used. Then, the Young’s modulus and Poisson ratio of the magnetic steel sheet are 210,000 MPa and 0.3, respectively. Those of the magnet are 120,000 MPa and 0.3, respectively. As shown in Figure 4, in Model A and Model B of the IPM type, the high Mises stress is concentrated at the edge of flux barrier. On the other hand, the Mises stress of the SPM type is concentrated at the surface permanent magnet on the rotor, and it is much lower than that of the IPM type. Although these models of IPM types are not optimized to reduce the Mises stress, it is obvious that SPM types are more suitable for decreasing Mises stress compared with IPM types. Figure 5a shows Mises stress distribution for the rotor diameter in the SPM type. As shown in Figure 5a, the Mises stress can be reduced by decreasing the rotor diameter. Figure 5b shows the maximum Mises stress for the rotor diameter. As shown in Figure 5b, considering the limit of the yield stress of 300 MPa, the rotor diameter is determined to be less than 70 mm. Moreover, the rotor magnet can be reinforced with a retaining sleeve made of carbon-fiber-reinforced plastic (C-FRP).

#### 3.2. Electrical Design of PMSM

_{d}, and i

_{q}are output torque, the number of pole pairs, magnet flux linkage, and the q-axis current, respectively. Additionally, the magnet flux linkage is expressed as follows:

_{d}, v

_{q}

_{,}and V

_{dc}are the d-axis voltage, q-axis voltage, and DC voltage source, respectively. Then, the inductance condition to achieve the base speed and the output power is given by:

_{b}and L

_{b}are the base angular velocity and inductance (there is a relationship L

_{b}= L

_{d}= L

_{q}due to SPMSM). In Equation (4), the maximum torque per ampere control is assumed under base speed; therefore, the d-axis current is 0 A. Moreover, the inductance condition to achieve the maximum speed and the output power is given by:

_{m}, I

_{m}, and i

_{d}are the maximum angular velocity, the current vector amplitude, and the d-axis current, respectively. The functions of a, b, and c can be calculated using the parameters of a phase current of 356 Arms and a d-axis current at a beta angle of 78 degrees. From Equations (4) and (6), the inductance range of L

_{b}to achieve the condition of phase voltage at the base speed and maximum speed is shown in Figure 7. As shown in Figure 7, the number of turns, seven turns/slot, which satisfied Equations (4) and (6), is selected. A stator diameter that retained the slot area is used, which achieves less than 15 A/mm

^{2}of the current density.

#### 3.3. Torque and Phase Voltage at Required Speed of PMSM

^{−1}and 52,000 min

^{−1}, respectively. As shown in Figure 8a, the designed PMSM can achieve the maximum torque of 70 Nm at 11,500 min

^{−1}. As shown in Figure 9b, the phase voltage is also suppressed to less than half of the DC voltage of 365 V at 52,000 min

^{−1}, and the output power of 85 kW is obtained. However, a lot of the d-axis current is energized by the flux weakening control in 52,000 min

^{−1}to cancel out the magnet flux and to sustain the induced voltage.

## 4. Design of SRM

#### 4.1. Mechanical Design of SRM

^{−1}. As shown in Figure 11a, high Mises stress is generated around the shaft and the edge of the teeth. As shown in Figure 11b, considering the limit of the Mises stress is under 300 MPa, the rotor diameter has to decrease to less than 114 mm.

#### 4.2. Electrical Design of SRM

^{2}of the current density.

#### 4.3. Torque and Phase Voltage at Required Speed of SRM

_{dc}to achieve the required torque for each operation speed.

^{−1}. As shown in Figure 13, the hysteresis current control is used in 11,500 min

^{−1}, and the output torque is about 70 Nm, while the phase current is 345 Arms. Figure 14 shows their waveforms at 52,000 min

^{−1}. As shown in Figure 14, the voltage single pulse control is used in 52,000 min

^{−1}. In the high-speed region, the current can be raised, and the torque can be obtained by setting the turn-on angle to an early timing as rotation speed increases. The turn-off angle is also set to a timing earlier than the aligned position of rotor and stator to avoid negative torque.

## 5. Design of IM

#### 5.1. Mechanical Design of IM

^{−1}. As shown in Figure 16, high mises stress is generated around the shaft. Considering the limit of Mises stress is under 300 MPa, the rotor diameter has to decrease to less than 80 mm. Therefore, the rotor diameter is set to 80 mm.

#### 5.2. Electrical Design of IM

#### 5.3. Torque and Phase Voltage at Required Speed of IM

^{−1}and 52,000 min

^{−1}, respectively. As shown in Figure 18, the designed IM can achieve the maximum torque of 70 Nm under conditions with a phase current of 356 Arms and slip of 0.16. However, as shown in Figure 17, the output torque is 8.1 Nm, the required torque of 15.6 Nm and 85 kW cannot be obtained at 52,000 min

^{−1}. The phase voltage is more than 200 V, and the voltage source required is 420 V

_{dc}.

## 6. Performance Comparison of Designed Motors

#### 6.1. Motor Volume

#### 6.2. Mechanical Strength

#### 6.3. Output Characteristics

^{−1}and 52,000 min

^{−1}, although the magnet flux has to be strongly weakened by flux weakening control to suppress the induced voltage at the maximum rotational speed of 52,000 min

^{−1}. In the SRM, the output torque is 67.8 Nm at 11,500 min

^{−1}, and it is 14.2 Nm at 52,000 min

^{−1}. Although the output power is slightly lower than the required torque for the output power of 85 kW, the required torque can be obtained by optimizing the turn-on angle and turn-off angle. In the IM, the output torque is 71 Nm at 11,500 min

^{−1}, and it achieves the required maximum torque. However, the output torque at 52,000 min

^{−1}is much lower than the required torque for the output power of 85 kW, since the voltage is limited by increasing the rotor flux due to the second current.

#### 6.4. Loss and Efficiency

^{−1}and the maximum speed of 52,000 min

^{−1}are evaluated. Figure 22 shows losses of joules in the rotor and stator and core loss. As shown in Figure 22, the joule loss and the core loss in the PMSM are low, since the input current and the number of turns are small under the small-sized motor volume, owing to utilization of the permanent magnet. On the other hand, the joule loss in the stator of the PMSM is higher than that of the SRM at 52,000 min

^{−1}, since a lot of d-axis current is needed by the flux weakening control. The SRM also achieves low levels of joule loss in the stator, since the input current and number of turns are almost same for the PMSM instead of the rotor diameter increasing. On the other hand, the core loss increases compared with the PMSM. The IM generates high levels of joule loss, since the joule loss in the rotor accounts for a large percentage (the stator joule loss and the rotor joule loss are 1.1 kW and 23 kW at 11,500 min

^{−1}, respectively, and the stator joule loss and the rotor joule loss are 0.23 kW and 7.8 kW at 52,000 min

^{−1}, respectively). The core loss of the IM is also higher than that of the PMSM.

^{−5}Ω m. Actually, the number of layers and the material of retaining sleeve should be determined in consideration of the trade-off with manufacturing cost.

_{a}, K, C

_{d}, ρ, R, and L are the windage loss, salient-pole correction factor, skin friction coefficient, air density, radius of high-speed rotor, and motor stack length, respectively. In the SRM, the windage loss increases considerably at 52,000 min

^{−1}due to the salient-pole structure of the rotor. The windage loss can be reduced by using the shroud or cylindrical rotor structure [16]. Figure 25 shows the comparison of the efficiency. As shown in Figure 25, the PMSM can achieve the highest efficiency of 97% at 11,500 min

^{−1}and 86% at 52,000 min

^{−1}. The SRM achieves the efficiency of 95% at 11,500 min

^{−1}and 81% at 52,000 min

^{−1}. The efficiencies of the IM are 77.8% at 11,500 min

^{−1}and 80.3% at 52,000 min

^{−1}.

## 7. Conclusions

^{−1}, and the performances were evaluated using FEA. Table 7 shows the comparison of these motor performances. As shown in Table 7, the PMSM was advantageous in terms of the downsizing of the motor volume and motor efficiency. However, it is necessary to reduce the eddy current loss of the magnet at 52,000 min

^{−1}by increasing the number of magnet layers and reducing the harmonic flux by applying distributed winding. They have to be designed considering the trade-off between increased cost and larger motor size. On the other hand, the SRM was advantageous in terms of the high mechanical strength of the rotor, and it is suitable for high-speed rotation. The motor volume of the SRM was larger than that of the PMSM. Although the efficiency of the SRM was lower than that of the PMSM at the high load condition of 11,500 min

^{−1}, the efficiency at 52,000 min

^{−1}can be improved by decreasing windage loss using the shroud or cylindrical rotor structure. In this study, the efficiency of the IM at 11,500 min

^{−1}was considerably lower due to the joule loss in the rotor. On the other hand, the IM had the advantage of no magnet eddy current loss, and lower iron loss and wind loss than the SRM, so it has potential as a high-speed traction motor if the joule loss can be reduced.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 7.**Inductance condition which achieves the demand torque at base speed 11,500 min

^{−1}and maximum speed 52,000 min

^{−1}.

**Figure 8.**Torque waveform and phase voltage waveform in rotation speed of 11,500 min

^{−1}, input current of 356 Arms, and beta angle of 0 degrees.

**Figure 9.**Torque waveform and phase voltage waveform in rotation speed of 52,000 min

^{−1}, input current of 356 Arms, and beta angle of 78 degrees.

**Figure 13.**Rotation speed of 11,500 min

^{−1}, turn-on angle of −5 degrees, and turn-off angle of 85 degrees.

**Figure 14.**Rotation speed of 52,000 min

^{−1}, turn-on angle of −40 degrees, and turn-off angle of 51 degrees.

Output power [kW] | 85 |

Maximum torque [N m] | 70 |

Voltage source [V] | 365 |

Maximum current [Arms] | 356 |

Current density [A/mm^{2}] | 15 |

Maximum diameter [mm] | 200 |

Stack length of core [mm] | 100 |

Number of poles | 4 |

Stator slots | 6 |

Number of turns [turn/slot] | 7 |

Resistance of winding [Ω] | 0.0018 |

Winding type | Concentrated |

Air gap [mm] | 0.5 |

Magnet thickness [mm] | 5 |

Current density [A/mm^{2}] | 10 |

Material of magnetic steel sheet | 35H230 |

Material of permanent magnet | NEOMAX-42 |

Number of magnet segmentations | 16 |

Number of poles | 8 |

Stator slots | 12 |

Number of turns [turn/slot] | 5 |

Resistance of winding [Ω] | 0.003 |

Winding type | Concentrated |

Air gap [mm] | 0.5 |

Magnet thickness [mm] | 5 |

Current density [A/mm^{2}] | 12.7 |

Material of magnetic steel sheet | 35H230 |

Number of poles | 4 |

Stator slots | 24 |

Number of bars | 36 |

Number of turns [turn/slot] | 4 |

Resistance of winding [Ω] | 0.003 |

Winding type | Distributed |

Air gap [mm] | 0.5 |

Current density [A/mm^{2}] | 11 |

Material of magnetic steel sheet | 35H230 |

Material of bar | Aluminum |

Secondary resistance [Ω] | 0.071 |

Primary leak inductance [μH] | 26.2 |

Secondary leak inductance [μH] | 26.2 |

Mutual inductance [μH] | 278 |

PMSM | SRM | IM | |
---|---|---|---|

Size [mm] | φ174 × L100 | φ200 × L100 | φ180 × L100 |

Rotor outer diameter [mm] | 70 | 114 | 80 |

Motor volume [p.u.] | 0.76 | 1.0 | 0.81 |

Current density [A/mm^{2}] | 10 | 12.7 | 11 |

Motor size | Mises Stress at Maximum Speed | Motor Efficiency | |
---|---|---|---|

PMSM | Φ174 mm × L100 mm | 287 MPa (Rotor diameter: 70mm) | 11,500 min^{−1}: 97%52,000 min ^{−1}: 86% |

SRM | Φ200 mm × L100 mm | 291 MPa (Rotor diameter: 114mm) | 11,500 min^{−1}: 95%52,000 min ^{−1}: 81% |

IM | Φ180 mm × L100 mm | 258 MPa (Rotor diameter: 80mm) | 11,500 min^{−1}: 78%52,000 min ^{−1}: 79% |

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**MDPI and ACS Style**

Aiso, K.; Akatsu, K.
Performance Comparison of High-Speed Motors for Electric Vehicle. *World Electr. Veh. J.* **2022**, *13*, 57.
https://doi.org/10.3390/wevj13040057

**AMA Style**

Aiso K, Akatsu K.
Performance Comparison of High-Speed Motors for Electric Vehicle. *World Electric Vehicle Journal*. 2022; 13(4):57.
https://doi.org/10.3390/wevj13040057

**Chicago/Turabian Style**

Aiso, Kohei, and Kan Akatsu.
2022. "Performance Comparison of High-Speed Motors for Electric Vehicle" *World Electric Vehicle Journal* 13, no. 4: 57.
https://doi.org/10.3390/wevj13040057