Nuwantha Fernando

Impact of Soft Magnetic Material on Design of High-Speed Permanent-Magnet Machines

This paper investigates the effect of two soft magnetic materials on a high-speed machine design, namely, 6.5% silicon steel and cobalt–iron alloy. The effect of design parameters on the machine performance as an aircraft starter-generator is analyzed. The material properties which include B-H characteristics and the losses are obtained at different frequencies under an experiment and used to predict the machine performance accurately. In the investigation presented in this paper, it is shown that machines designed with 6.5% silicon steel at a high core flux density has lower weight and lower losses than the cobalt–iron alloy designs. This is mainly due to the extra weight contributed by the copper content especially in the end-windings. Due to the high operating frequencies, the core losses in the cobalt–iron machine designs are found to outweigh the copper losses incurred in the silicon steel machines. It is also shown that change in stack length/number of turns has a considerable effect on the copper losses at starting, however has no significant advantage on rated efficiency which happens to be in a field-weakening operating point. It is also shown that the performance of the machine designs depends significantly on material selection and the operating point of the core. The implications of the variation of design parameters on the machine performance is discussed and provide insight into the influence of parameters that effect overall power density.

A field modulated electromagnetic clutch with slip control

This paper presents the control of an electromagnetic clutch. The considered clutch is particularly based on the modulated field principle and has two shafts with speed ratio of 1.14 at synchronous conditions. The clutch engagement and disengagement and torque transmission is controlled by manipulation of the stator currents and the slip speed between the two shafts. The dynamic model of the electromagnetic clutch is found to be nearly identical to that of a permanent magnet synchronous machine, however the electrical speed is replaced by the slip speed. The relationship between the torque transmission between the input and output shafts and the electrical power input is explained. A clutch control strategy based on the d-q current control is presented. The test bed setup with the experimental prototype electromagnetic clutch is presented and the simulation results for this prototype is presented. It is shown that the electrical power demanded by the clutch is high only during engagement and disengagement transient periods and the steady state power is only required to compensate the electrical losses in the clutch. Experimental results demonstrate the controllability of the clutch with the proposed strategy.

Marine propulsion PM motor control under inverter partial fault

Fault tolerance is an essential feature in marine and naval electric propulsion drives. Therefore, there is a growing research interest in developing fault tolerant technologies for marine applications. This paper investigates the control and operation of an inverter fed PM propulsion motor under a specific power electronic failure case. Compared to the technologies reported in literature, this paper formulates a special control strategy for the motor operation during the situation where only one high side switch and one low side switch in two different phases are operational in a standard three-phase six-switch inverter. The proposed control strategy assumes that the remaining switches are failed in open circuit while the corresponding anti-parallel diodes are functional. The mathematical background of the two quadrant control strategy is formulated. The operation is analyzed in the context of marine propulsion power/speed requirements. Simulations for a 50 kW three-phase fault tolerant motor is presented for the operation with the proposed dual switch control strategy. Controllability of motor torque for speeds up to 45% rated speed corresponding to approximately 9% rated propulsion power output is achieved without overrating the device current ratings.

A coordinated control of grid connected PMSG based wind energy conversion system under grid faults

This paper aims to develop a coordinated controller to enhance the low voltage ride through (LVRT) performance of a permanent magnet synchronous generator (PMSG) based variable speed wind energy conversion system (WECS). This coordinated control comprises of a pitch angle control, a flux weakening control and a static synchronous compensator (STATCOM) control. The pitch angle control is adjusted to smooth the WECS output power fluctuations or to enhance the LVRT performance. The flux weakening controller is used to reduce the overvoltage of DC-link capacitor, and the STATCOM control is used to ensure continuous operation of WECS during symmetrical and asymmetrical grid faults. The proposed coordinated control method is compared with the conventional LVRT strategy based on the braking chopper (BC) via time series simulation results and effectiveness of the proposed control is verified.

Design of a Stator for a High-Speed Turbo-Generator With Fixed Permanent Magnet Rotor Radius and Volt–Ampere Constraints

This paper investigates high-speed surface PM machine design for portable turbo-generator applications. The rotor radius is fixed to achieve certain optimal characteristics of the magnet retention mechanism. The objective of this paper is to design and select the stator. The stator designs are populated by different slot/pole combinations, winding arrangements, and over a range of possible stack lengths. However, the design is constrained by physical diameter, stack length, and electrical volt–ampere constraints. A large number of preliminary machine designs do not satisfy the specified turbo-generator torque/speed requirements under the given volt–ampere constraints and therefore it is ineffective to perform finite element analysis on all preliminary design variations. In order to establish the feasibility of a machine design to fulfill the specified turbo-generator torque/speed requirements, the concept of inductance-limits is presented and then linked with stator design parameters. By application of this analysis strategy, a confined optimal set of stator designs are obtained and are subjected to detailed finite element simulations. A final machine design is selected and fabricated. Experimental results are presented for the validation of the final machine design.

Multi magnetic material laminated cores: Concept and modelling

Modern developments in advanced material technology have produced new magnetic materials capable of dealing with high frequencies. However, such metals typically have low mechanical strength and they are found to be brittle. In order to manufacture electromechanical devices with such new materials, it has to be reinforced with a mechanically strong structure. One option is to incorporate stronger core materials, such as Si-steel sandwiched together with the high performance material resulting in a multi-magnetic material laminated core (MMLC). The MMLCs also allow the opportunity shape the effective magnetic properties to certain extent. In this paper, the homogenization of MMLC BH characteristics and a strategy to calculate the core loss properties of an MMLC constructed with two materials is developed using the individual properties of the materials.

Improved Synchrophasor Models for Power System Dynamic Stability Evaluation Based on IEEE C37.118.1 Reference Architecture

Synchrophasors are an emerging power system wide-area dynamic performance monitoring and control technology. The IEEE standard C37.118.1 outlines specifications for the construction of synchrophasor units, in which two reference synchrophasor models are proposed for protection (P-type) and measurement (M-type) purposes. This paper develops improved synchrophasor models for power system dynamic stability/performance evaluation based on the reference architecture proposed in the IEEE standard C37.118.1, considering frequency error (FE) and the total vector error (TVE) as the main performance indices. The quadrature demodulation filters are designed explicitly considering the impact of harmonic distortion and out-of-band interference, and thus optimized quadrature demodulation filters improve estimation performance during adverse conditions. Furthermore, a special emphasis is placed on the input low-pass filter to attenuate high-frequency components above the Nyquist rate. The improved synchrophasor models are rigorously tested to ensure compliance with performance requirements stipulated in the IEEE standard. Results show that improved synchrophasor models significantly enhance the FE while ensuring compliance requirements for the TVE.

Degradation of mechanical properties of class-H winding insulation

Thermal ageing is one of the key factors that result in insulation degradation leading to lower dielectric strength and thereafter occurrence of partial discharge in high voltage machines. However, in low voltage high current machines, the occurrence of partial discharge is minimal and degradation of dielectric strength alone will not cause the breakdown of the insulation. This paper hypothesizes that the breakdown of insulation in low voltage/high current machines occur as a result of insulation cracking among other factors. In this paper, it is shown that thermal degradation leads to degradation of mechanical properties of Polyamide-imide insulation, particularly reduction of the ultimate tensile strength and leads to enhanced brittleness. It follows that once a crack in the insulation is formed, the breakdown strength at the point of cracking will be lower. Multiple such cracks may lead to the formation of a breakdown path thereafter leading to aggravated heating and further accelerated deterioration of the insulation. In this paper, the methodology for testing the mechanical properties of insulation material is presented. A model to account for the variation of such properties due to thermal degradation is developed.

Magnetic materials for electrical machine design and future research directions: A review

The selection of the appropriate core magnetic material for an electrical machine is one of the main design decisions that will significantly influence the power density and efficiency of the machine. The selection of the material is function of the torque/speed requirements and structure of the machine itself. Recent research have investigated different material options for electrical machine design and the basis of this paper is to provide a review and comparison of the research outcomes. The review is presented under three sections, viz. application of soft magnetic alloys, soft magnetic composites and amorphous magnetic materials. Based on the research outcomes, the most suitable applications for these different material categories have been identified. Future research directions of electrical machine design that may benefit the efficiency and power density are identified and associated research challenges are highlighted. The core loss parameters of state-of-the-art material in literature are compared and discussed.

Flux switching machines: A review on design and applications

This paper reviews recent trends in the application of flux switching machines (FSMs) in industry, the variation in FSM designs and control strategies to achieve enhanced performance. FSMs offer advantages such as high torque density, high speed capability, ease of control and low vibration and acoustic noise compared with competing technologies. These features enable FSMs to be applied in transportation, renewable energy and aerospace applications. However, the high cogging torque is found to be a negating feature of the FSMs that can be exacerbated by manufacturing tolerance issues. Design techniques developed to achieve lower cogging torque, in addition to lower back-EMF harmonics and higher torque density is reviewed in this paper. In addition, the design for fault tolerance, field weakening and high speed operation and the use of special magnetic material for FSMs have been reviewed. Control strategies developed for torque ripple minimization, fault tolerant operation and strategies to estimate and control FSM field flux has also been reviewed.