Preliminary Design Analysis of Low Speed Interior Permanent Magnet Machine with Distributed and Concentrated Windings

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Abstract – The paper presents a systematic comparison of four topologies of the interior permanent magnet machine (IPMM) designed for low speed applications. This comparative study investigates the suitability of the concentrated winding and distributed winding in the stator and the flat-shaped or V-shaped magnets in the rotor poles. The paper also studies the inductance characteristics of the designs using finite element analysis. Various steps taken to minimize the cogging torque and torque ripple in the studied machines were also discussed in details. Keywords : AC standstill saliency test, Cogging torque, Concentrated winding, Distributed winding, Direct-drive, Interior Permanent Magnet Machine, PM machine, Torque ripple 1. Introduction I n direct drive wind turbine systems, intermediate gear boxes are eliminated to increase the efficiency and reliability of the whole system, as well as reducing maintenances down-time. The generator used in such systems needs to have a low rated speed. This demand for an increase in the number of pole pairs of the generator. A review done in [1], [2] on direct-drive wind turbines suggests that permanent magnet (PM) machines are the most favorable for direct-drive wind turbine. While the most common type of PM machines used in the wind industry is the radial flux surface mounted magnet (SPM) type, an interior PM synchronous machine (IPMSM) offers a number of advantages over the SPM type. The most commonly cited advantages are: the presence of reluctance torque ( ) as shown in (1) in an IPM machine is beneficial in achieving a higher torque with lower magnet volumes and IPMSM has a mechanically and magnetically robust rotor. It should be noted that the presence of saliency in the IPSM results in the reluctance torque and also contributes toward the constant power speed range. A recent study carried out in [3] found that the use of an IPM machine in a small scale direct drive wind turbine can provide quicker return of investment compared to a SPM machine, especially in sites where average wind speed is low. According to [3] the return of investment can be achieved by taking advantage of provision for a constant power speed range available in IPMMs. But cogging torque in an IPMM is generally greater compared to a SPM machine due to the greater variation of reluctance between the stator and rotor [4]. This makes reduction of the cogging torque crucial in a low speed IPM machines [5]. The IPMSM with radially magnetized V-shaped (V-S) magnets has a better flux concentration and higher saliency ratio than its counter-part flat-shaped (F-S) magnet designs which makes it more attractive for traction drives [6]. The torque components of an IPM machine is (1) where, = magnet alignment torque, = cogging torque, and reluctance torque. The majority of IPM machines are designed with distributed windings (DW). Recently, the concentrated winding (CW) for the IPMSM is getting attention from the research community due to its high slot-fill factor, high tolerance to phase fault, simplified manufacturing process, non-overlapping coils resulting in shorter end-windings and reduced copper usage. In spite of having these advantages, the CW was not widely used in the past due to its characteristics in producing EMF and MMF waveforms with unacceptably high total harmonic distortion. However, in recent years, it has been shown that through appropriate choice of slot and pole combinations [7], the CW stator has the ability to produce sinusoidal EMF waveform with high a winding factor, and smooth average torque. In this paper, four design topologies of IPMSM were considered for investigation which includes (a) conventional distributed winding in the stator and flat-shaped IPM structure in the rotor( DW FS-IPMM), (b) distributed winding in the stator and V-shaped IPM structure in the rotor (DW VS-IPMM), (c) concentrated winding in the stator and flat-shaped IPM structure in the rotor (CW FS-IPMM), (d) concentrated winding in the stator and V-shaped IPM structure in the rotor(CW VS-IPMM). Fig. 1 shows the cross-section of the four topologies and no-load flux distribution. The objective of this paper is to present a detailed comparison of the major performance characteristics of all the four topologies illustrated in Fig.1. The selection process of the major dimensions and winding structure of the proposed IPMM is described in Section 2. Section 3 presents the cogging torque and torque ripple minimization of all the four investigated designs.