# FE-Aided Synchronization Analysis of Line-Start Synchronous Reluctance Motors
## 1 Introduction  
Induction motors have by far the vast majority of the market share of electric motors. Besides cost effectiveness, their line-start ability may be the major reason for their popularity, where a single speed is applicable. However, efficiency regulations of electric motors are being strictly applied around the world to reduce energy consumption broadly, and these regulations will be further tightened. This means that induction motors (IMs) in the small-mid power range might lose their majority in the future market. The most attractive alternative up to now has been permanent magnet (PM) synchronous motors with embedded rotor cage for single-speed applications. Undoubtedly, the LSPM exhibits high efficiency thanks to the PM; however, the use of a PM is a cost sensitive issue. To eliminate the use of PMs, reluctance torque can be utilized. A line start synchronous reluctance motor (LS-SynRM) has multiple flux barriers instead of buried PMs to maximize rotor saliency. Although LS-SynRMs may not achieve efficiency as high as that of an LSPM of an equivalent volume, it is still attractive due to the near absence of rotor copper loss, which typically accounts for 25% of the total loss in IMs [1]. LS-SynRMs are fairly mature; however, they have never been considered or commercialized as IM counterparts [2]. A technical article was presented by one of the leading motor manufacturers [3]; however, that device did not enter the motor market. Relatively few studies using manufactured LS-SynRMs can be found, relative to the number using LSPMs. This may be due to the fact that the motor is quite difficult to manufacture. The typical rotor of an LSPM is composed of small rotor slots with one layer of buried PMs [4, 5, 6, 7]. In contrast, a LS-SynRM utilizes rotor slots as part of a flux barrier, such that the regular rotor slot shapes of IMs are generally not feasible for use in this motor. This has resulted in rotor slots of various shapes and positions. A 7.5 kW, 4-pole LS-SynRM with flux barrier filled with aluminum is presented in [8]. Two prototype motors were manufactured and experimented upon. In these, the fluxbarriers of one rotor are completely of die-cast aluminum. This motor was compared experimentally with one partly filled with aluminum. The directly measured efficiency of the completely aluminum filled one achieved efficiency of 95%. A 4 kW, 4-pole LS-SynRM in which the rotor bars are inserted in small round slots with a flux barrier of hyperbolic shape is described in [9]. Two prototype motors were manufactured and each motor was equipped with copper and aluminum rotor conductors using a brazing process. The prototype motors recorded directly measured efficiency of 92%. The new motors achieved about 2% higher efficiency than the reference IM. A 3.7 kW, 4-pole LS-SynRM in which the rotor conductors were irregularly filled in the flux barriers is presented in [10]. It was built using an aluminum die-casting process. The prototype motor achieved slightly over 92% efficiency according to the IEC 60034-30-1 test standard. The efficiency of the reference IM was 90%. A 2.2 kW, 4-pole LS-SynRM with a flux barrier filled with copper bars is reported in [11]. Harmonic impacts due to rotor-cage currents were analyzed. A prototype was manufactured; however, the efficiency determined by experiments was not stated. A 1.88 kW, 2-pole LS-SynRM with square rotor slots parallel to the \(d\)-axis is reported in [12]. A prototype LS-SynRM with a die-cast aluminum rotor cage was manufactured. It achieved around 90% efficiency in direct measurement, whereas the reference IM recorded directly measured efficiency of around 88.5%. From the literature studies, it can be concluded that LS-SynRMs of small power rating can exhibit about 2% higher efficiency than IMs of similar volume. This indicates that a LS-SynRM of small power rating may achieve IE4-class efficiency with the same frame size as an IE3-class IM.