Speed control of permanent magnet synchronous motors (PMSMs) without position sensors is preferable in some applications due to cost and mounting space concerns. For applications in which the operating speed is relatively stable, the dynamic performance and efficiency demands of PMSMs are not severe during the startup process. Under such circumstances, a sensorless controller based on back electromotive force (EMF) estimation combined with a simple startup strategy is a suitable solution. In this paper, an I-f (current-frequency) startup method and a sliding mode observer are adopted to develop a sensorless control system, and the startup process which employs the I-f method is analysed in detail. Based on the analysis, this paper proposes a simple and robust startup strategy with an adaptive compensator to realize the smooth transition from I-f control to EMF-based control. The proposed strategy allows quick startup from standstill under different load conditions without initial position estimation. Furthermore, the strategy does not introduce any additional control coefficients, and none of the original control coefficients need to be adjusted. The strategy is suitable for both surface and interior PMSMs, and a test bench based on an interior PMSM is built to validate its properties. The experimental results show that the motor can smoothly accelerate from standstill to its rated speed in less than 2 s in both loaded and non-loaded situations, and the transition between the two sensorless control methods does not cause any large speed or current fluctuations.

Extended back electromotive force (EEMF)-based position sensorless field-oriented control (FOC) is widely utilized for ultra-high-speed surface-mounted permanent magnet synchronous motors (UHS-SPMSMs) driven fuel cell air compressors in medium-high speed applications. Unfortunately, the estimated position is imprecise due to too small EEMF under low speed operation. Hence, current-to-frequency (I-f) control is more suitable for startup. Conventional I-f methods rarely achieve the tradeoff between startup acceleration and load capacity, and the transition to sensorless FOC is mostly realized in the constant-speed stage, which is unacceptable for UHS-SPMSM considering the critical requirement of startup time. In this article, a new closed-loop I-f control approach is proposed to achieve fast and efficient startup. The frequency of reference current vector is corrected automatically based on the active power and the real-time motor torque, which contributes to damping effect for startup reliability. Moreover, an amplitude compensator of reference current vector is designed based on the reactive power, ensuring the maximum torque per ampere operation and higher efficiency. Furthermore, the speed PI controller is enhanced by variable bandwidth design for smoother sensorless transition. These theoretical advantages are validated through experiments with a 550 V, 35 kW UHS-SPMSM. The experimental results demonstrated the enhanced startup performance compared with conventional I-f control.

Overmodulation method is an effective way to improve the voltage utilization for the PMSM drives, thus achieving higher efficiency and power capacity. However, the voltage jump between the output voltage vectors (OVVs) causes the deterioration of both phase voltage and current performance in the high overmodulation region. In order to eliminate the voltage jump, a novel variable-weight superposition theory (VWST) is proposed. Different from other methods, the phase angle of the reference voltage vectors (RVVs) is introduced to construct the variable-weight factor. This elaborate weight factor is assigned to the jump component with the variation of the phase angle. Eventually, the voltage jump is eliminated, and the total harmonic distortion (THD) of both phase voltage and current is decreased, which means smaller current ripple and torque jitter. In addition, the linearity of the modulation index is guaranteed by a convert block based on the Fourier analysis. With the help of the proposed method, the voltage jump can be eliminated, and better performance of phase voltage and current can be achieved. The effectiveness of the proposed method is verified by PMSM bench experiments.

The voltage utilization in the PMSM control can be improved through the voltage angle control (VAC) method, thereby enhancing the control efficiency and output power capability. However, the traditional VAC method can only be applied to a high-speed region, and it is still necessary to switch to another control mode in the low-speed region, resulting in a significant torque jitter during the switching process. Meanwhile, the deviation between the actual current operating point and the optimal one is caused by the dead-time effect in the traditional VAC method, causing more energy loss. To solve these problems, a global-speed-region VAC method considering dead-time effect compensation is proposed in this article. Firstly, the application area of the proposed method is extended to both the high-speed and low-speed region on the basis of analyzing the variation function of the modulation index (MI), achieving global non-mode-switching control and eliminating the torque jitter. Additionally, the relationship between the dead time and MI is quantitatively analyzed, and a global-speed-region VAC method is proposed to compensate the dead-time effect and reduce the energy loss. Finally, the effectiveness of the proposed method is verified in PMSM bench experiments.