# Rotor Position Alignment of FSTPI Based PMSM Drive Using Low Frequency Signal Injection
## 1 Introduction  
In many applications such as in light electric vehicles and consumer products, low power is desired. The utilization of a cost-effective three-phase inverter is crucial to decrease the cost of the drive system. This can be achieved by eliminating two semi-conductors in the inverter topology making it four-switch three-phase inverter, as shown in Figure 1a [1; 2; 3; 4]. For sensorless operations where the initial rotor position detection is not available or starting from an unknown rotor position is not possible, aligning or locking the rotor to a predetermined position is inevitable for proper starting of PMSM [5]. Additionally, for sensed operation of PMSM drives where incremental optical encoder is used as the position sensor, the initial rotor position is not known, therefore the alignment procedure is mandatory [6]. The precise rotor position must be identified before starting the motor and after the DC-link is at full capacity. The PMSM has to be energized by a selected voltage vector or sequence of voltage vectors to align the rotor to the predefined position. In studies using sensorless control and incremental encoders insensor based AC drives, methods for determination of initial rotor position and alignment to zero angle have not been investigated for the FSTP inverter. Locking the rotor to a known position (especially zero angle), as shown in Figure 1b at start-up is easily performed in classical six-switch three-phase (SSTP) inverters using conventional techniques, however zero angle alignment with classical methods do not work in case of an FSTP inverter due to the fact that the DC-link voltage is at full capacity and one phase (phase \(a\) which is set to align with zero position and \(d\)-axis rotor flux linkage vector) of the PMSM is mounted to the midpoint of the split capacitors in the DC-link, as shown Figure 1a such that all the four non-zero voltage vectors aligned with the \(\alpha\beta\)-axes. Therefore, simplified space vector pulse width modulation (SVPWM) generation in FSTP based inverter is achieved [7; 8; 9]. In the classical rotor alignment procedure of the PMSM drive with vector control scheme, enough \(d\)-axis constant current is injected to overcome the load, inertia and friction in the vector controlled scheme whilst \(q\)-axis current reference is set to zero with zero position input applied to the co-ordinate transformations such as Park and inverse Park prior to startup, therefore the rotor flux is aligned with the stator flux which is set to zero degree position [10]. The main disadvantages of such conventional techniques are that they are mainly dependent on PI regulators and can be easily applicable to six-switch inverter based PMSM drives without causing severe degradation at the startup procedure. Moreover, with the classical rotor position alignment method, dynamic performance strongly relies on the bandwidth of the current control loop and the parameters of the \(d\)- and \(q\)-axes current regulators. High and low frequency pulse signal injection methods are investigated for the sensorless control of PM machines in classical six-switch three-phase inverters [11; 12; 13]. However, these studies are intended to estimation of initial rotor position. In the FSTP inverter fed PMSM drive, classical alignment procedure with \(d\)-axis current regulator with zero position applied to the co-ordinate transformations that is used for three-phase six-switch (SSTP) inverter generates improper voltage vectors that does not perform alignment to zero angle when phase \(a\) (\(d\)-axis zero angle position) is attached to the mid-point of the split capacitors. In [14], a simple sector determination scheme for a normalized SVPWM is proposed for the FSTP inverter without voltage or position sensors such that the complex calculations are eliminated in the controller. Although, the sector determination algorithm is developed for SVPWM, no discussion has been given on obtaining a zero angle initial rotor position alignment.