## 1 Introduction
Another question is parameter variation, especially the precision of current and voltage measurement, which influences the final quality of estimation. Many of these aspects can be compensated [20; 21]. In [22], the inductance compensation method is presented to achieve enhanced torque control performance against parameter variation. The adverse effects of the inverter harmonics were minimized in [23; 24] with voltage error compensation techniques. The low-speed limit evaluation of back-EMF tracking was discussed in [25]. The non-model-based strategies offer parameter sensitiveness and work effectively in zero and low-speed regions. However, these methods basically require any type of signal injection, high-frequency (HF) voltage [26; 27; 28; 29; 30; 31; 32], or HF current [33] injection to utilize the rotor saliency. Some methods can also be used for the surface PMSM (SPMSM), where the \(d\) and \(q\) axis of the rotor are geometrically symmetrical [30; 31; 32]. The HF voltage injection causes the saliency reflected current, including the rotor position information. Demodulation processes are then used depending on various voltage and current injection types to retrieve the rotor speed and position. Band-pass and low-pass filters are widely used to obtain a rotor position error signal in the demodulation process. However, these filters inevitably have drawbacks, such as a time delay of the rotor speed or position estimation and a limitation of current controllers' bandwidth [29]. Square-wave injection methods [34; 35; 36; 37] are used to solve these issues. They offer an extended range of injected voltage frequency, and accordingly, the bandwidth of the current controllers can be significantly enhanced. As a result, dynamic control performance can be improved. However, there are some adverse effects of the signal injection methods, for example, increased losses, torque ripples, and acoustic noises. Furthermore, the maximum inverter output voltage at higher speeds could limit the additional injected signal. Consequently, it is recommended to use the magnetic saliency-based method with signal injection at a standstill and low-speed range and a back-EMF-based technique above a certain threshold speed in the medium- and high-speed region [38] to provide sensorless control of PMSM in a wide speed range. In some papers, strategies also known as hybrid techniques have been proposed to provide a full-speed-range sensorless drive. They usually represent a combination of back-EMF-based and magnetic saliency-based techniques. In [39; 40], a transition algorithm is proposed to achieve a smooth operation between the two estimation methods. Furthermore, the combination of the model-based dynamic feedforward control and current injection is proposed in [33]. In comparison with FOC, scalar control is simpler and could be an ideal low-cost alternative in some applications where high dynamic performance is not crucial, such as fans, compressor drives, or pumps. It is a strategy in which a motor terminal voltage \(V\) changes in proportion to the applied frequency \(f\), and hence, it can also be expressed as \(V/f\) control. It maintains constant machine flux and thus constant torque production capability up to the rated machine speed. Using \(V/f\) control, a stator resistance voltage drop can be neglected at higher frequencies. However, the voltage drop cannot be ignored at lower frequencies. In this region, boost voltage can be applied to the machine to compensate for the voltage drop. This allows the motor to develop the necessary torque even at lower speeds [41]. The other disadvantage of scalar control is a possible loss of synchronism, leading to instability issues. For this purpose, a stabilizing loop may be applied to modulate reference frequency and thus maintain synchronism. This correction provides system damping and reduces the speed oscillations, stabilizing the system [42; 43; 44]. Some authors have presented different approaches to \(V/f\) control.