Although the concept of variable-speed drives, based on utilization of multiphase machines, dates back to the late 1960s, it was not until the mid- to late 1990s that multiphase drives became serious contenders for various applications. These include electric ship propulsion, locomotive traction, electric and hybrid electric vehicles, ldquomore-electricrdquo aircraft, and high-power industrial applications. As a consequence, there has been a substantial increase in the interest for such drive systems worldwide, resulting in a huge volume of work published during the last ten years. An attempt is made in this paper to provide a brief review of the current state of the art in the area. After addressing the reasons for potential use of multiphase rather than three-phase drives and the available approaches to multiphase machine designs, various control schemes are surveyed. This is followed by a discussion of the multiphase voltage source inverter control. Various possibilities for the use of additional degrees of freedom that exist in multiphase machines are further elaborated. Finally, multiphase machine applications in electric energy generation are addressed.

Multiphase Electric Machines for Variable-Speed Applications

The area of multiphase variable-speed motor drives in general and multiphase induction motor drives in particular has experienced a substantial growth since the beginning of this century. Research has been conducted worldwide and numerous interesting developments have been reported in the literature. An attempt is made to provide a detailed overview of the current state-of-the-art in this area. The elaborated aspects include advantages of multiphase induction machines, modelling of multiphase induction machines, basic vector control and direct torque control schemes and PWM control of multiphase voltage source inverters. The authors also provide a detailed survey of the control strategies for five-phase and asymmetrical six-phase induction motor drives, as well as an overview of the approaches to the design of fault tolerant strategies for post-fault drive operation, and a discussion of multiphase multi-motor drives with single inverter supply. Experimental results, collected from various multiphase induction motor drive laboratory rigs, are also included to facilitate the understanding of the drive operation.

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Multiphase induction motor drives - : a technology status review

Fractional-slot concentrated-winding (FSCW) synchronous permanent magnet (PM) machines have been gaining interest over the last few years. This is mainly due to the several advantages that this type of windings provides. These include high-power density, high efficiency, short end turns, high slot fill factor particularly when coupled with segmented stator structures, low cogging torque, flux-weakening capability, and fault tolerance. This paper is going to provide a thorough analysis of FSCW synchronous PM machines in terms of opportunities and challenges. This paper will cover the theory and design of FSCW synchronous PM machines, achieving high-power density, flux-weakening capability, comparison of single- versus double-layer windings, fault-tolerance rotor losses, parasitic effects, comparison of interior versus surface PM machines, and various types of machines. This paper will also provide a summary of the commercial applications that involve FSCW synchronous PM machines.

Fractional-Slot Concentrated-Windings Synchronous Permanent Magnet Machines: Opportunities and Challenges

An analytical model has been developed for analyzing the radial vibration force in fractional-slot permanent-magnet machines. It is compared extensively by finite-element analyses and used to investigate the influence of the following: 1) stator slotting; 2) tangential field component; 3) radius in the air gap for computation; 4) load condition, etc. The major findings include the following: 1) even on an open circuit, the low harmonic component (e.g., the second for a 10-pole/12-slot machine) of the radial force exists due to the slotting effect, although the amplitude is relatively low, while the slotless analytical model cannot predict this phenomenon; 2) on a load, the slotless analytical model is accurate enough for the radial force analysis since the low-order harmonic component of the radial force is mainly due to the interaction between the magnet field and the armature-reaction field and is largely determined by the combination of the pole and slot numbers; 3) it is much more reliable to calculate the radial force in the middle of the air gap rather than close to the stator bore; and 4) the simple formula accounting only for the radial field component in the middle of the air gap is accurate enough for the radial force calculation.

Analytical Modeling and Finite-Element Computation of Radial Vibration Force in Fractional-Slot Permanent-Magnet Brushless Machines

Synchronous permanent magnet (PM) machines have been gaining a lot of interest over the years. This is due to their several advantages including high power density, high efficiency and high reliability. One of the key concerns about PM machines especially in safety-critical applications (such as the more-electric aircraft) has been the issue of fault-tolerance since the machine cannot be de-excited. A lot of work has been done both on the machine side as well as the power converter side (including power converter configuration and remedial control strategies). This study will provide a thorough review and summary of what has been covered in literature up-to-date. The study will highlight the tradeoffs (including weight, cost and reliability) involved in the various proposed methods and strategies with more emphasis on the machine side. The methods discussed in this study include active control methods form converter side, memory motors, doubly-salient and flux-switching machines, use of auxiliary windings, mechanical flux-weakening methods, use of shunts and shields, thermal protection and transverse flux machines. The study will also include some comments about where the research in this area is heading in the future.

Fault-tolerant permanent magnet machines: a review

An electrical machine comprises a combined magnetic gearbox and electrical generator. A first set of permanent magnets (30) are arranged on a rotor (16) to produce a spatially variable first magnetic field. A second set of permanent magnets (32) are arranged on a rotor (40,41,43), stationary pole pieces (36) are positioned between the first set of permanent magnets (30) and the second set of permanent magnets (32) to interfere with the first magnetic field. Rotation of the rotor (16) relative to the pole pieces (36) produces a second magnetic field which rotates the second set of permanent magnets (32). A stator (42) has windings (46) to transduce a changing second magnetic field produced by the rotation of the second set of permanent magnets (32) into an electrical voltage. The electrical machine is useful for a wind turbine generator. Alternatively the arrangement may be modified to produce an electrical motor.

Compact electrical machine

A new approach is described for selecting pole and slot numbers for fault-tolerant permanent-magnet machines so that there is inherently negligible coupling between phases (regardless of other design detail). The preferred slot and pole number combinations thereby help to ensure that a fault in one phase does not undesirably affect the remaining unfaulted phases. Other well-known criteria for fault-tolerant operation, including high phase inductance, also have to be met. It is demonstrated how particular slot and pole combinations can be used to eliminate low pole number armature MMF, thereby reducing the vibration and stray loss present during normal operation.

Favourable slot and pole number combinations for fault-tolerant PM machines

The paper discusses the effect of single turn short-circuits in the windings of fault tolerant permanent magnet machines. It is shown that previously recognised methods for dealing with shorted turns do not work for larger bar-wound machines, and a new method for protecting the windings against single turn faults is therefore proposed. The new method depends on rapid detection of the fault and injection of a current (of appropriate magnitude and phase) into the faulted winding. The paper gives a theoretical analysis supported by finite element simulations of the fault conditions and illustrates these with a case study taken from the aerospace industry.

Implications of shorted turn faults in bar wound PM machines

A component of an electric machine 46 , the component comprising a core 10 ; two or more teeth 14 extending radially therefrom; at least one electromagnetic winding 12 , each winding 12 around at least one of the teeth 14 ; wherein the core 10 having a cooling arrangement including at least one cooling insert 48 located between adjacent windings 12 and whereby in use a cooling fluid being arranged to flow through the at least one cooling insert 48 to facilitate heat transfer and dissipation from the electromagnetic windings 12 . The cooling inserts providing improved cooling, structural support to the windings, and electrical insulation between the windings and the core.

Cooling arrangement of an electrical machine

A synchronous electrical machine comprising a plurality of phases, detecting means arranged to detect a fault in at least one of the phases of the synchronous electrical machine, isolating means arranged to isolate the at least one phase of the synchronous electrical machine with the fault, phase shift means arranged to produce a controlled phase shift between the voltage and the current within the remaining phases of the synchronous electrical machine to adjust the phase angle and magnitude of the second harmonic powers produced by the remaining phases of the synchronous electrical machine such that the vector sum of the second harmonic power vectors of the remaining phases of the synchronous electrical machine is zero.

John James Anthony Cullen

Papers

Compact electrical machine

Favourable slot and pole number combinations for fault-tolerant PM machines

Implications of shorted turn faults in bar wound PM machines

Cooling arrangement of an electrical machine

Synchronous electrical machine