Permanent magnet vernier machines (PMVMs) gained a lot of interest over the past couple of decades. This is mainly due to their high torque density enabled by the magnetic gearing effect. This study will provide a thorough review of recent advances in PMVMs. This review will cover the principle of operation and nature of magnetic gearing in PMVMs, and a better understanding of novel PMVM topologies using different winding configuration as well as different modulation poles and rotor structures. Detailed discussions on the choice of gear ratio, slot-pole combinations, design optimisation and role of advanced materials in PMVMs will be presented. This will provide an update on the current state-of-the art as well as future areas of research. Furthermore, the power factor issue, fault tolerance as well as cost reduction will be discussed highlighting the gap between the current state-of-the art and what is needed in practical applications.

Abstract:In this paper, a six-phase fault-tolerant modular permanent magnet synchronous machine (PMSM) with a novel 24-slot/14-pole combination is proposed as a high-performance actuator for wheel-driving electric vehicle (EV) applications. Feasible slot/pole combinations of the fractional-slot concentrated winding six-phase PMSM are elicited and analyzed for scheme selection. The novel 24-slot/14-pole combination is derived from the analysis and suppression of the magnetomotive force (MMF) harmonics. By making use of alternate-teeth-wound concentrated winding configuration, two adjacent coils per phase and unequal teeth widths, the phase windings of the proposed machine is magnetically, thermally isolated, which offers potentials of modular design and fault tolerant capability. Taking advantage of the leakage component of winding inductance, 1.0 per unit short-circuit current is achieved endowing the machine with short-circuit proof capability. Optimal design of essential parameters aiming at low eddy current losses, high winding factor and short-circuit-proof ability are presented to pave the way for a high-quality system implementation.

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Insulating materials and insulation systems design have been gaining more attentions as more electrical machines tend to operate in harsher environments for various applications. Harsh environments include high temperature, humidity, erosion, low air pressure, etc. This paper discusses recent advances in insulation systems for electrical machines. Insulation tests as well as test standards that have been used to evaluate insulation systems and detect insulation failures will be discussed. Insulating materials used for a wide range of industrial applications such as wind turbine generators, aerospace hybrid/electric powertrain, and hydro generators have been summarized. For the emerging high-altitude, highvoltage aerospace applications, partial discharge and its impact on insulation systems will be discussed. Finally, polymer nanocomposite materials with excellent thermal conductivity and dielectric strength are highlighted as an outlook. SECTION I. Introduction Due to the continued growth of renewable energys, the number of electrical machines used worldwide has significantly increased. Insulation system is a very critical component of electrical machines. Insulation breakdown can lead to failures, which eventually result unpredicted downtime and negative financial impacts and in some applications can be a safety hazard. For some specific industries where uninterruptible operation is required, the unpredicted downtime is unacceptable. The unpredicted downtime for an offshore oil plant would be $25,000/h [1]. The dielectric strength of electrical insulation materials has been gradually improved over the years. By introducing new materials in the past 20 years for instance, the dielectric strength of ground-wall insulation nearly doubled [2]. Electrical, mechanical, thermal, and ambient stresses can cause insulation degradation which consequently leads to insulation failure [3], [4]. Insulation failure causes short circuits in the stator winding and consequently high currents could pass through the defected stator winding [5]. A survey on 1141 induction motors with power ratings above 200 hp shows that around 30% of motor failures are due to insulation failures [6]. With advances in sensors, digital signal processing, diagnosis methods and test standards, insulation failures can be detected [7], [8]. Through online estimation of material degradation and lifetime in early stages, insulation systems of electric machines can be protected from further aging while unpredicted downtime could be avoided through scheduled maintenance. Even though insulation system is a passive component in an electrical machine which does not produce torque, insulation build/thickness (which represents the key thermal resistance in electrical machine)can have significant impact on the machine cooling and hence electrical loading and torque production. There have been several efforts to minimize insulation thickness for design compactness, low manufacturing cost and high efficiency [3]. The two main functions of insulation systems in electr...

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This paper presents a five-phase fault-tolerant permanent-magnet synchronous machine (PMSM) used for electric vehicles. In multiphase fault-tolerant PMSMs equipped with fractional-slot concentrated windings, excessive magneto-motive force (MMF) harmonics can lead to thermal demagnetization of the permanent magnets (PMs). In order to reduce the lower-order harmonics, the origins of the 2-pole harmonic in conventional winding configurations are investigated, and an unequal-turn winding configuration is applied to cancel the lower-order harmonics. The main electromagnetic performances of the unequal-turn winding configuration are investigated and compared with conventional winding topologies. Based on the principle of maintaining constant instantaneous power, the fault-tolerant control strategies for open-circuits of up to two phases are developed. All of the investigations are verified by finite element analysis (FEA) results.

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