High-speed motors offer advantages such as small size, light weight, and high power density. For high-speed loads, direct drive with a high-speed motor eliminates the need for mechanical transmission devices, avoiding losses, mechanical vibration, and noise caused by transmission devices. This results in smaller equipment size and improved operating efficiency and accuracy. High-speed motors are widely used in spindles, power tools, pumps, and high-speed compressors. Currently, induction motors are commonly used for high-speed direct drive both domestically and internationally. While simple in structure, they suffer from high rotor losses, low efficiency, and short lifespan, and also exhibit slip and difficulty in maintaining a stable speed. Square-wave driven brushless DC motors use electromotive force commutation and phase-locked control. Although this eliminates the need for sensors, they suffer from high harmonic losses and large torque fluctuations, generally limiting their use to low-power drives. Permanent magnet synchronous motors offer advantages such as high efficiency, low noise, reliable operation, small torque fluctuations, and good control characteristics, making them ideal for high-speed operation. However, traditional permanent magnet synchronous motors require rotor position sensors. High-speed motors require sufficient mechanical strength and rigidity for their rotors. Therefore, their structures are more compact than those of conventional motors, with the shaft designed to be as short as possible to improve rigidity. This presents significant challenges in installing position sensors on the high-speed motor shaft, both in terms of motor structure and sensor assembly and debugging. While the three-phase voltage or current of the motor can be used to estimate the rotor position, the large PWM carrier wave superimposed on the phase voltages makes filtering difficult, and achieving electrical isolation between the control circuit and the main circuit is challenging. This paper takes a 30,000 r/min, 3.3 kW high-speed permanent magnet synchronous motor as an example to study its electromagnetic and mechanical design. A novel and simplified rotor position sensing method is adopted, using a locked Hall element as the sensor. High-precision rotor signals can be obtained through position signal conversion and compensation. Simultaneously, the speed signal can be obtained from the position signal processing circuit. Based on this position sensing method, a field-oriented vector control is implemented using the CRPWM current control method. Electromagnetic Design Due to the use of electronic commutation and variable frequency power supply, brushless DC motors have a greater leeway in the selection of the number of poles. However, the current frequency in the stator winding of the motor is: f = pn / 60 (1) where n is the speed of the motor; p is the number of pole pairs. It can be seen that for a given speed, the more poles there are, the higher the alternating frequency of the stator current, which will increase the iron loss of the stator core and the copper loss of the stator winding. In addition, with the increase of the output frequency, the carrier ratio of the pulse width modulation will decrease, and the stator current harmonics will increase, which will increase the stator and rotor losses. However, when 2p = 2, the magnetic flux density of the stator and rotor yoke is higher, and the winding ends are longer. If the stator winding uses integer slots, there will often be a large cogging positioning torque, which will affect the low-speed performance of the motor system. Due to the limitations of processing conditions, it is not easy to use skewed slots or skewed poles in the motor. In order to reduce the positioning torque and reduce the tooth harmonics in the no-load back EMF, this paper adopts a fractional slot distributed winding. Considering the above factors, the number of poles is set to 2p=4, the number of stator slots to 18, and the number of slots per pole per phase to q=1.5. For ease of installation, the motor adopts a radial magnetic circuit surface magnet rotor structure. The magnets are made of neodymium iron boron permanent magnet material with high magnetic energy product. The tile-shaped magnets are pasted on the rotor surface, and the pole arc coefficient α=1. The schematic diagram of the motor structure is shown in Figure 1. In high-speed permanent magnet synchronous motors, the influence of high-frequency iron losses increases with the increase of the power supply frequency. To reduce iron losses and avoid magnetic circuit saturation, the air gap magnetic flux density should not be too high. The air gap magnetic flux density of the motor designed in this paper is 0.5T. The back electromotive force waveform and its spectrum of the high-speed synchronous motor studied in this paper are shown in Figures 2 and 3. The spectrum diagram shows that the back EMF mainly contains the 3rd harmonic. However, due to the use of a three-phase half-bridge inverter drive circuit without a neutral line, there are no harmonics multiples of 3 in the line EMF, and therefore no 3rd harmonic current is generated. The back EMF also contains a 5th harmonic with a relative amplitude of less than 1%. Even harmonics may be generated due to uneven rotational speed and uneven magnet material, with a relative amplitude of less than 0.5%. Mechanical Design Figure 2: Back EMF Waveform Diagram Figure 3: Back EMF Spectrum Diagram In high-speed motors, not only must the selected bearings be suitable for high-speed operation, but the fit between the bearing housing and the bearing bush must also be appropriate—neither too tight nor too loose—and a certain preload should be applied to the bearings. The motor rotor should have sufficient mechanical strength and good dynamic balance to avoid damage to the rotor due to centrifugal force during high-speed operation. Simultaneously, the rotor surface should be smooth to reduce noise and mechanical losses during high-speed operation. The magnets are bound with fiberglass tape to enhance mechanical strength. The number of binding turns can be expressed as: (2) where C is the safety factor; Z is the centrifugal force (kg); FT is the strength of the binding strap (kg). Rotor position detection In a permanent magnet synchronous motor, a Hall element is placed at an appropriate position on the stator. When the rotor rotates at a constant speed, the Hall element will output a square wave signal whose rising and falling edges correspond to the zero crossing point of the rotor magnetic field strength B0. After processing the square wave signal, a digital rotor position signal θ is obtained. This digital quantity is used to look up a table. After D/A conversion and amplification, a sinusoidal reference voltage is obtained, as shown in Figure 4. This paper adopts the position conversion circuit proposed in reference [5]. The difference between the Hall output and the actual rotor position can be compensated. At the same time, the advance angle can also be set. Figure 4 Relationship between Hall output and corresponding rotor position Drive system This paper uses CRPWM to realize magnetic field orientation appropriate control. The system structure block diagram is shown in Figure 5. The high-speed synchronous motor drive system consists of a high-speed permanent magnet synchronous motor (PMSM), a starting circuit, a Hall sensor position signal conversion and compensation and speed measurement circuit, a three-phase current command synthesis circuit, a current regulator, a speed regulator, an SPWM circuit, and a power drive section. Figure 5 shows the block diagram of the high-speed permanent magnet synchronous motor drive system. Since this motor lacks self-starting capability, a starting circuit must be designed. The system studied in this paper adopts a synchronous starting method, for which a synchronous pulse generator was designed. This starting circuit has a simple principle and has been experimentally proven to be reliable. Ignoring higher harmonics, the deviation voltage caused by the dead time can be equivalent to a rectangular wave deviation voltage with polarity opposite to the current direction. Due to the very small winding inductance of the high-speed permanent magnet synchronous motor, the deviation voltage will generate a large harmonic current in the winding. To suppress the harmonic current, dead-time compensation technology is used in this paper. To further control the current, a proportional current regulator is used. To achieve appropriate field-oriented control, Id should be set to 0. Under different speed and load conditions, the optimal lead angle for achieving magnetic field-oriented vector control can be expressed as: (3) Where L and Ra are the phase inductance and phase resistance of the winding; Ke is the phase potential coefficient; ωr is the angular frequency; Kp is the gain of the current proportional regulator; KS is the equivalent gain of the SPWM inverter; Kf is the current feedback coefficient; and Iq is the phase cross-axis current. Experimental Results This paper uses a precision hysteresis dynamometer to conduct experimental research on the high-speed permanent magnet synchronous motor system. The phase current waveforms measured at 10000 r/min and 1.1 Nm are shown in Figure 6, and their spectrum is shown in Figure 7. It can be seen from the experimental results that the low-order harmonic content in the current is very small, so the low-frequency torque pulsation is very small; while due to the small inductance of the motor winding, the high-order harmonics caused by PWM are relatively large. Figure 6 Phase A current and reference voltage Figure 7 Phase A current spectrum At 10,000 r/min, the efficiency of the entire system was tested, and the efficiency of the system with an output torque of 1.1 Nm reached 80%. Conclusion The high-speed permanent magnet synchronous motor designed in this paper has a reliable structure, is suitable for high-speed operation, and exhibits low harmonic content in the line back EMF. Using a locked Hall element as a position sensor is feasible. The speed signal can be obtained from the rotor position signal processing circuit. The high-speed permanent magnet synchronous motor drive system designed in this paper operates reliably. Due to the adoption of dead-time compensation and a proportional current regulator, the low-order harmonic content in the winding current is low, and low-frequency torque ripple is minimal.