This paper proposes a six-phase surface-mounted permanent-magnet machine with a 24-slot/10-pole fractional slot winding, which not only eliminates the air-gap flux subharmonics, but also minimizes ...the effect of slot harmonics, which highly affect both the core and magnet losses. The six-phase winding design also offers an improved drive train availability for electric vehicle applications due to its inherent high fault-tolerant capability. When compared with a three-phase design, the proposed winding offers approximately 3.5% improvement in torque density, a significant reduction in both the core and magnet losses, and an improved overall efficiency. The proposed winding is deduced based on the stator shifting concept of two 12-slot/10-pole stators with single tooth windings. The coil span of the resulting machine will be two slots, which stands as a compromise between single tooth and distributed windings. The concept of stator shifting is first presented, and then, a prototype machine is designed and simulated using the 2-D finite-element analysis to validate the proposed concept. A comparative study is also carried out to compare six-phase and three-phase designs with the same slot/pole combination and also with the 18-slot/10-pole combination, which was recently shown to be a competitive alternative.
The application of multiphase machines in high-power applications is now a recognized alternative, thanks to their higher fault-tolerance when compared with standard three-phase systems. Although ...multiphase machines with multiples of three-phase winding sets have been favored in many applications, literature has shown that machines with a prime number of phases, such as five-phase machines, generally outperform other phase orders, especially in terms of machine torque density. One of the major problems when dealing with a prime number of phases either in academic research or industrial applications is the special stator design. This paper provides a general technique to rewind standard off-the-shelf three-phase stator frames with any general prime phase order while preserving the same copper volume. The proposed winding layout is derived using the star of slots theory and the concept of stator winding shifting. Finite-element simulations are used to investigate examples with different phase orders, while experimental investigation is carried out to corroborate the proposed concept by rewinding a standard three-phase stator as a five-phase machine.
Two-level voltage-source converters and half-bridge modular multilevel converters are among the most popular types of HVDC converters. One of their serious drawbacks is their vulnerable nature to ...dc-side faults, since the freewheeling diodes act as a rectifier bridge and feed the dc faults. The severity of dc-side faults can be limited by connecting double thyristor switches across the semiconductor devices. By turning them on, the ac current contribution into the dc side is eliminated and the dc-link current will freely decay to zero. The main disadvantages of this method are: high dv/dt stresses across thyristors during normal conditions, and the absence of bypassing for the freewheeling diodes during dc faults since they are sharing the fault current with thyristors. This paper proposes combining and connecting the double thyristor switches across the ac output terminals of the HVDC converter. The proposed protection scheme provides advantages, such as lower dv/dt stresses and lower voltage rating of thyristor switches in addition to providing full segregation between the converter semiconductor devices and ac grid during dc-side faults. A simulation case study has been carried out to demonstrate the effectiveness of the proposed scheme.
The nine-phase six-terminal induction machine has recently been proposed as a promising contender to the conventional six-phase asymmetrical winding machine in terms of torque density, phase current ...quality, stator winding simplicity, and fault-tolerant capability. However, the relatively lower dc-link voltage utilization of a single neutral arrangement in multiphase machines with multiple three-phase windings represents, in general, a technical challenge when compared to windings with isolated star points. Therefore, this paper proposes a new pseudo six-phase winding layout suitable for medium-voltage high-power induction machines, which employs quadruple three-phase stator winding sets, while providing the same terminal behavior of a nine-phase six-terminal winding. Additionally, like the traditional six-phase winding, two possible neutral arrangements can be configured. The proposed winding configuration provides the same dc-link voltage utilization as in conventional dual three-phase winding machines with isolated neutrals. The effect of the circulating zero-sequence current component experienced with a single neutral arrangement can also be avoided. A 1.5 Hp prototype induction machine is used to experimentally validate the proposed six-phase winding layout under both healthy and fault conditions.
Turbulent, single-phase forced convection of water in circular microchannels with diameters of 0.76 and 1.09 mm has been investigated. The data show that the Nusselt numbers for the microchannels are ...higher than those predicted by traditional large channel correlations. Based on the data obtained in this investigation, along with earlier data for smaller diameter channels, a generalized correlation for the Nusselt number for turbulent, single-phase, forced convection in circular microchannels has been developed. The diameter, Reynolds number, and Prandtl number ranges are 0.102–1.09 mm, 2.6 × 10
3−2.3 × 10
4, and 1.53–6.43, respectively. With a confidence level of greater than 95%, differences between experimental and predicted Nusselt number values are less than ± 18.6%.
Power generation through wind is expected to play a major role in the world's future energy portfolio. Nevertheless, wind power integration remains a challenging research area due to the special ...characteristics of wind power generation. Specifically, offshore wind has received significant attention worldwide due to the vast generation potential available. The electrical infrastructure of offshore wind farms is thus of significant importance. The multi-terminal HVDC solution represents a preferable solution and has shown promise in solving wind farm interconnection problems. Droop control techniques have been proposed as a means to regulate the DC voltage and facilitate the automatic coordination between different converters without the need for fast communication between units. Different methodologies have been developed to select the droop gains to satisfy the system performance specifications. In this work, a control design methodology is proposed for power sharing among the multi-terminal HVDC feeders providing that the power transmission efficiency is optimized. A simulation study on a 400-kV/1000-MW four-terminal HVDC transmission topology is conducted to ensure the validity of the proposed methodology.
The nine-phase six-terminal induction machine (IM) has been recently proposed as a promising contender to the conventional asymmetrical six-phase type in terms of torque density, stator winding ...simplicity, and fault-tolerant capability. The stator is composed of nine phases, which are connected in a fashion to only provide six stator terminals. Therefore, this connection combines the high performance of a nine-phase winding with the terminal behavior of a six-phase machine. This paper introduces the machine mathematical model based on the vector space decomposition (VSD) modeling approach. The required current and voltage sequence transformation matrices are derived such that the machine is mathematically regarded as an equivalent six-phase IM with only three decoupled subspaces. This way, the same VSD-based controller structures conventionally applied to six-phase-based systems can be preserved. A 1.5-hp prototype IM is used to experimentally validate the machine model under both healthy and open-phase conditions.
Void fraction and two-phase frictional pressure drop in microchannels were experimentally investigated. Using air and water, experiments were conducted in transparent circular microchannels with 1.1 ...and 1.45
mm inner diameters and in microchannels with semi-triangular (triangular with one corner smoothed) cross-sections with hydraulic diameters 1.09 and 1.49
mm. Gas and liquid superficial velocities were varied in the 0.02–80
m/s and 0.02–8
m/s ranges, respectively, and void fractions were calculated by analyzing photographs taken from the test sections with circular cross-section.
Measured void fractions were compared with several correlations. The homogeneous flow model provided the best prediction of the experimental void fractions in bubbly and slug flow patterns. The homogeneous flow model and all other tested empirical correlations significantly over predicted the void fractions in annular flow pattern, however.
A one-dimensional model, based on the numerical solution of mass and momentum conservation equations was applied for the calculation of test section pressure drops, using various two-phase friction models. For bubbly and slug flow patterns, the two-phase friction factor based on homogeneous mixture assumption provided the best agreement with experimental data. For annular flow the homogeneous mixture model and other widely used correlations significantly over predicted the frictional pressure drop.
This paper investigates the performance of synchronous reluctance motors when the stator is equipped with a combined star-delta winding layout. The conventional star winding is used as a benchmark in ...this study to compare different possible single-layer winding layouts. Among these different winding layouts, those which maximize the fundamental magneto-motive force component are selected. A simple mathematical formula is then derived to calculate the equivalent winding factor for different shares between star and delta subwindings. It has been proved that for the same copper volume and line current magnitude, the star-delta connection can offer an enhancement in the torque density of approximately 5.2% over the conventional star case under rated conditions. However, this gain is affected by the employed number of poles and stator slots. On the other hand, the effect of the winding layout on either power factor or core loss can be merely neglected over a wide range of speeds and currents. Nevertheless, the machine efficiency under a combined star-delta connection is relatively improved under light loading as a result to the machine torque/current ratio enhancement. The theoretical findings are experimentally validated using two identical 5.5 kW prototype machines, having star and combined star-delta winding connections.
One of the main merits offered by multiphase machines is their high fault-tolerant capability. Literature has demonstrated that the performance of multiphase induction machines under fault conditions ...is affected by the employed stator winding connection. In open-loop controlled five-phase machines, the star connection is favorable under healthy conditions while the pentagon connection is favorable and yields a lower derating factor under the open phase condition. In this paper, a new combined star/pentagon single layer winding layout that combines the advantages of both star and pentagon connections is proposed for a five-phase induction machine. Although the proposed winding is intrinsically an asymmetrical ten-phase machine, the proposed connection allows for only five-phase terminals. Moreover, the proposed winding not only yields better flux distribution compared to a conventional single layer winding, but also provides a complete cancellation of the third-order harmonic flux component caused by the induced third sequence currents due to the saturation effect and/or under unbalanced operation. Hence, the machine losses are decreased, which improves the overall machine efficiency. For the healthy case, the machine is similar to a conventional star connected five-phase machine. However, with one phase open, the proposed connection results in a lower derating factor compared to conventional connections for both open-loop and optimal current control techniques. A 1-kW prototype machine is used for experimental verification.
