*4.2. Speed Regulation Characteristics*

Figure 17 shows the output torque of the HTR-ASD under different slip speeds and excitation currents. From Figure 17, it can be clearly seen that the speed and torque of the HTR-ASD can be easily adjusted by changing the excitation current. Additionally, the output torque is proportional to the square of the current, which is similar to the characteristics of the voltage regulated speed control of an asynchronous motor. Through a comparison between the analytical calculations and simulations, the accuracy of the analytical analysis in this study is verified. Figure 18 shows the operating characteristics at different prime mover speeds. When the speed of the prime mover changes, its characteristics are like those of the variable frequency speed regulation of an asynchronous motor.

**Figure 17.** The output torque of HTR-ASD under different slip speed and excitation currents.

**Figure 18.** The mechanical properties of the HTR-ASD.

When the inductance parameters of the machine are calculated, the transient torque response of the HTR-ASD can be obtained by the s-function simulation model, which is much quicker than the FEM. Figure 19 shows the transient torque response of the HTR-ASD calculated by the two methods under different slip speeds and an excitation current of 6A. As can be seen from Figure 19, the results of the two methods are basically consistent. Furthermore, the higher the slip speed, the longer the response time because of the larger current.

**Figure 19.** Transient torque response of the HTR-ASD under different slip speeds and an excitation current of 6A.

#### **5. Conclusions**

In this study, a new type of adjustable speed drive, named the HTR-ASD, is proposed. Its unilateral air gap magnetic field is unipolar, while its synthetic air gap magnetic field is bipolar. The HTR-ASD has an absence of brushes, permanent magnets, and mechanical flux regulation devices, leading to an obvious advantage of high reliability. The speed and torque can be adjusted only by adjusting the excitation current with dc power. Meanwhile, the rotating parts of the device are composed of a solid homopolar-type rotor and simple

squirrel-cage structure. This simple and robust structure makes it more suitable for working in harsh environments. In order to calculate its steady and dynamic characteristics, the equation and equivalent circuit of the HTR-ASD are analyzed in a rotor d-q frame. Additionally, the air gap permeance functions are developed to simplify the analysis of the air gap magnetic density and the calculation of the parameters, which may make the design flow more efficient. Finally, a protype is designed and simulated by the FEM, and analytical analyses are carried out to verify and evaluate the HTR-ASD's performance. The comparison between the FEM and the analytical analyses are developed, and the results show great agreement in relation to accuracy. In addition, the results of the operating characteristics indicate that this machine is very suitable for use as an adjustable speed drive.

**Author Contributions:** Conceptualization, S.G. and K.Y.; methodology, Z.Y.; software, P.L., H.L., and G.W.; validation, S.G. and X.X.; formal analysis, S.G. and Z.Y.; investigation, K.Y.; resources, K.Y. and P.L.; data curation, Z.Y. and X.X.; writing—original draft preparation, S.G. and X.X.; writing—review and editing, S.G. and X.X.; visualization, X.X.; supervision, X.X.; project administration, X.X.; funding acquisition, P.L., H.L. and G.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (52007072), the National Natural Science Foundation of China (51821005) and the interdisciplinary program of Wuhan National High Magnetic Field Center (WHMFC 202110).

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
