# Rotor Position Alignment of FSTPI Based PMSM Drive Using Low Frequency Signal Injection
## 1 Introduction
Current and torque ripple minimization incorporating SVPWM are also reported in [8] for FSTP inverters where the compensation techniques are employed for PMSM drives to reduce the unbalancing and current and torque distortion in FSTP inverters. However, the main focus in this work is to reduce the oscillation observed in current and torque waveforms due to the nature of FSTP inverters. Therefore, in this study, no details have been given on the encoder type or about the initial rotor position detection methodology. A study in the literature given in [15] discusses the initial rotor position alignment issue of the FSTP inverters. The solution to this problem explained in [8], which is locking the rotor with line-to-line voltage is possible but does not acquire zero angle alignment. In this paper, the proper rotor position alignment methodology to a predetermined zero angle position is developed for the FSTP driven PMSM drive. It is shown that the rotor position can be aligned to zero electrical degree position properly using low-frequency \(d\)-axis synchronous reference frame voltage in which the desired constant virtual zero position is applied to the co-ordinate transformation (inverse Park transformation) without using classical closed loop \(dq\)-axes current regulators. Thus, correct zero position alignment is achieved in a short amount of time for the FSTP inverter based PMSM drive at start-up. The proposed initial zero angle rotor position alignment method is simple and therefore is eligible for real-time implementation of cost effective four-switch PMSM drives. Definition of Problem and the Proposed Signal Injection-Based Initial Zero Angle Rotor Position Alignment Method for FSTP Inverter  
The simplest approach to align/lock the rotor to a known initial position in the FSTP inverter fed PMSM drive before starting is performed by applying a constant voltage vector to the stator phases to produce a stationary magnetic field and the rotor aligns itself naturally onto this stator field. However, one phase (phase \(a\)) is mounted to the mid-point of the capacitors for the FSTP topology, therefore under constant voltage input no current will flow through phase \(a\) which does not allow aligning the rotor position to the desired zero angle. Fluctuations in the two capacitor voltages \(V_{C_{1}}\) and \(V_{C_{2}}\) exist that produce a deviation of the non-zero voltage vectors from their previous position, as shown in Figure 2. This can easily be overcome in the SSTP inverter where all phases can be controlled by the corresponding switches on each of the inverter legs. In the proposed alignment scheme at the startup process for FSTP inverter based PMSM drives, the pulse width modulated low-frequency \(d\)-axis voltage reference signal is injected to phase \(a\) through phase \(b\) and \(c\) allowing current to be flown in phase \(a\) and therefore proper rotor position alignment to zero angle is achieved. Figure 1: PMSM drive using four-switch three-phase (FSTP) VSI with stator and rotor flux linkage vectors: (**a**) Phase–\(a\) is wired to the middle of the two DC-link capacitors; (**b**) rotor alignment process.