# Comparative Study and Overview of Field-Oriented Control Techniques for Six-Phase PMSMs
## 1 Introduction  
Multiphase electric machines have been frequently discussed in recent years by professionals worldwide, where the term \"multiphase\" refers to machines with more than three phases. The results of various authors have described an improvement in efficiency, the harmonic spectrum of MMF, a lower phase current and torque ripple, and, most significantly, parameters related to the system's fault tolerance and redundancy [1, 2, 3, 4, 5]. Multiphase machines are used in high-power applications for a reduction in phase current stress, e.g., huge cargo ships, mills, and wind power plants [6, 7]. All these advantages are, of course, dependent on many variables, especially based on the particular design of a machine. Various numbers of phases are used for applications involving 5-, 6-, 9-, 12-, and 15-phase machines. The most preferred ones are six-phase machines. They are simple to control, are easy to use, and can be driven by two pairs of well-known three-phase converters. Today, six-phase electrical machines dominate in the field of critical applications, where a higher level of redundancy, safety, and fault-tolerant behavior is required [8, 9, 10]. Permanent\(-\)magnet synchronous machines (PMSMs), in general, are used in a large set of applications, and the automotive industry is no exception. PMSMs can be found in window lifts, water pumps, seat and mirror positioning elements, fans, and other auxiliary drives; the most well-known PMSM is the automotive body segment. The body segment represents the lower corner case of PMSM use. Other applications such as electrical starters, brakes, e-turbo, power steering, and main traction drive units represent middle and upper corner cases of PMSM use. This kind of electrical machine is often controlled by a well-known field-oriented control. Field-oriented control can be accomplished using a six-phase PMSM via several methods. In various applications and research works, two main approaches can be recognizedaccording to the number of current controllers used for torque\(-\) and flux\(-\)producing components of a whole machine. One approach is where the six-phase machine is reflected as one entity, where the torque/flux control works in one orthogonal system. In [11], a vector space decomposition (VSD) method was introduced as a control of torque/flux components in one orthogonal subspace, with current harmonics elimination. Modulation techniques for this control topology were discussed in [12], while realization of the VSD method with control in a stationary reference frame was proposed in [13]. A method with only two current sensors for the field-oriented control of a six-phase machine was shown in [14]. In [5], the post-fault behavior was investigated for a one-phase fault in a six-phase induction machine. The same transformations as for healthy conditions were used, whereby \(z1z2\) reference currents in the VSD method were recalculated according to coefficients on the basis of the criteria of maximal torque or minimal losses. An investigation of VSD harmonics control was presented in [15], where asymmetries were created through the addition of external passive components. In [16], general VSD transformations were proposed. The algorithm works for multiphase machines with an optional number of phases. Another approach considers the six-phase machine as two three-phase machines in parallel, and the control is divided into two orthogonal systems, one for each set. In [17], such an approach was used, where mutual coupling between two sets of three-phase windings was investigated. A decoupling for this phenomenon was proposed in [18]. A sensorless technique with separated three-phase winding control was shown in [19]. In [20; 21; 22; 23], a method was presented for multiphase machines with \(n\times 3\)-phase windings (\(n=2\), \(3\), \(4\),... ). Both techniques have been realized in the past three decades by several authors with various high-level additional algorithms for drive quality improvement.