Design optimization on conductor placement in the slot of permanent magnet machines to restrict turn-turn short-circuit fault current

In Permanent Magnet (PM) machines, a turn-turn Short-Circuit (SC) fault is the most critical fault to eradicate. The fault introduces high SC current in the shorted turn which may consequently lead to secondary faults unless the fault is appropriately controlled. This paper proposes feasible conductors’ placement in a slot of PM machine to minimize such turn-turn fault current. In order to minimize the fault current, the conductor arrangement in a slot is optimized using multi-objective Genetic Algorithm (GA) incorporating with both analytical and Finite Element (FE) numerical tool. The possible combinations of conductors’ placement are set as variables and optimized for a given machine which is designed for safety critical applications. It is shown that the fault current associated to a single turn fault can be significant for the random winding placement even though the remedial strategies are put in place. It is also shown that the fault current can be limited significantly by rearranging the winding placement in a way to share slot-leakage fluxes. This is confirmed via experiment on E-core. Influences of the winding arrangement on both frequency dependent resistances and windings capacitances are experimented. It is demonstrated that adopting the winding arrangement that shares the slot-leakage flux effectively benefits to minimize the AC losses in addition to improved fault tolerance. But it increases the turn-turn capacitances whose effect however can be neglected as the resonance frequency occurs beyond the operational frequency range of the machines of interes

Design and modelling of permanent magnet machine's windings for fault-tolerant applications

The research described in this thesis focuses on the mitigation of inter-turn short-circuit (SC) faults in Fault tolerant Permanent Magnet (FT-PM) machines. An analytical model is proposed to evaluate the inter-turn SC fault current accounting for the location in the slot of the short-circuited turn(s). As a mitigation strategy to SC faults at the design stage, a winding arrangement called VSW (Vertically placed Strip Winding) is proposed and analysed. The proposed analytical model is benchmarked against finite element (FE) calculation and validated experimentally. The results demonstrate that the proposed winding arrangement in the slot improves the fault tolerance (FT) capability of the machine by limiting the inter-turn SC fault current regardless the fault location in the slot. Electromagnetic and thermal studies are conducted to verify the merits and drawbacks of the proposed winding compared to the conventional winding using round conductors (RCW). The study shows that the proposed winding scheme, in addition to being fault-tolerant, has an improved bulk radial conductivity, can achieve a good fill factor, but has a significantly higher frequency-dependent AC copper loss. To predict the AC losses an analytical model based on an exact analytical 2D field solution is proposed. This model consists of first solving the two-dimensional magneto-static problem based on Laplace’s and Poisson’s equations using the separation of variables technique. Then, based on that solved solution, by defining the tangential magnetic field (Ht) at the slot opening radius, Helmholtz’ equation is solved in the slot sub-domain. Subsequently, an FE and MATLAB® coupled parametric design is undertaken to maximise the VSW wound machine’s efficiency whilst maintaining its FT capability. The proposed analytical models for prediction of the SC fault current and AC copper losses are integrated into the coupled optimisation. It is shown that the effective losses of the VSW can be minimised through the parametric design while maintaining the required level of machine performance. Using an existing FT-PM machine of which the rotor is kept unchanged two stators were designed, manufactured and wound with RCW and VSW respectively and experimental tests are carried out to validate the analytical models and the new winding concept

Design and modelling of permanent magnet machine's windings for fault-tolerant applications

The research described in this thesis focuses on the mitigation of inter-turn short-circuit (SC) faults in Fault tolerant Permanent Magnet (FT-PM) machines. An analytical model is proposed to evaluate the inter-turn SC fault current accounting for the location in the slot of the short-circuited turn(s). As a mitigation strategy to SC faults at the design stage, a winding arrangement called VSW (Vertically placed Strip Winding) is proposed and analysed. The proposed analytical model is benchmarked against finite element (FE) calculation and validated experimentally. The results demonstrate that the proposed winding arrangement in the slot improves the fault tolerance (FT) capability of the machine by limiting the inter-turn SC fault current regardless the fault location in the slot. Electromagnetic and thermal studies are conducted to verify the merits and drawbacks of the proposed winding compared to the conventional winding using round conductors (RCW). The study shows that the proposed winding scheme, in addition to being fault-tolerant, has an improved bulk radial conductivity, can achieve a good fill factor, but has a significantly higher frequency-dependent AC copper loss. To predict the AC losses an analytical model based on an exact analytical 2D field solution is proposed. This model consists of first solving the two-dimensional magneto-static problem based on Laplace’s and Poisson’s equations using the separation of variables technique. Then, based on that solved solution, by defining the tangential magnetic field (Ht) at the slot opening radius, Helmholtz’ equation is solved in the slot sub-domain. Subsequently, an FE and MATLAB® coupled parametric design is undertaken to maximise the VSW wound machine’s efficiency whilst maintaining its FT capability. The proposed analytical models for prediction of the SC fault current and AC copper losses are integrated into the coupled optimisation. It is shown that the effective losses of the VSW can be minimised through the parametric design while maintaining the required level of machine performance. Using an existing FT-PM machine of which the rotor is kept unchanged two stators were designed, manufactured and wound with RCW and VSW respectively and experimental tests are carried out to validate the analytical models and the new winding concept

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 basis 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

Analysis of vertical strip wound fault-tolerant permanent magnet synchronous machines

This paper investigates the behavior of a vector- controlled, fault-tolerant, permanent magnet motor drive system adopting a vertically placed strip winding (VSW) which can limit inter-turn short-circuit (SC) fault current to its rated value regardless of the position in the slot containing the shorted turns. The drives’ dynamic behavior is simulated using a per-phase equivalent circuit model with the winding inductances and resistances analytical calculated based on the machine geometry and fault location. A simplified thermal model is also grafted into the system model to effectively simulate the dynamic behavior of the machine during healthy, inter-turn SC fault and post-fault controlled scenarios. The SC fault current limiting capability, the additional losses and thermal behavior of the winding are studied and compared with conventional winding adopting round conductors winding (RCW). The proposed winding design is verified with Finite Element (FE) analysis and then validated experimentally. Results show that the VSW inherently limits the SC current, reduces its dependence on the position of the fault within the slot but results in an increase in AC losses

Design optimization of a short-term duty electrical machine for extreme environment

This paper presents design optimisation of a short term duty electrical machine for extreme environments of high temperature and high altitudes. For such extreme environmental conditions of above 80⁰C and altitudes of 30km, thermal loading limits are a critical consideration in machines, especially if high power density and high efficiency are to be achieved. The influence of different material on the performance of such machines is investigated. Also the effect of different slot and pole combinations are studied for machines used for such extreme operating conditions but with short duty. In the research, A Non-dominated Sorting Genetic Algorithm (NSGAII) considering an analytical electromagnetic model, structural and thermal model together with Finite Element (FE) methods are used to optimise the design of the machine for such environments achieving high efficiencies and high power density with relatively minimal computational time. The adopted thermal model is then validated through experiments and then implemented within the Genetic Algorithm (GA). It is shown that, generally, the designs are thermally limited where the pole numbers are limited by volt-amps drawn from the converter. The design consisting of a high slot number allows for improving the current loading and thus, significant weight reduction can be achieved

Impact of slot/pole combination on inter-turn short-circuit current in fault-tolerant permanent magnet machines

This paper investigates the influence of the slot/pole (S/P) combination on inter-turn short-circuit (SC) current in fault-tolerant permanent magnet (FT-PM) machines. A 2-D sub-domain field computational model with multi-objective genetic algorithm is used for the design and performance prediction of the considered FT-PM machines. The electromagnetic losses of machines, including iron, magnet, and winding losses are systematically computed using analytical tools. During the postprocessing stage, a 1-D analysis is employed for turn-turn fault analysis. The method calculates self-and mutual inductances of both the faulty and healthy turns under an SC fault condition with respect to the fault locations, and thus SC fault current, considering its location. Eight FT-PM machines with different S/P combinations are analyzed. Both the performance of the machine during normal operation and induced currents during a turn-turn SC fault are investigated. To evaluate the thermal impact of each S/P combination under an inter-turn fault condition, a thermal analysis is performed using finite element computation. It is shown that low-rotor-pole-number machines have a better fault tolerance capability, while high-rotor-pole-number machines are lighter and provide higher efficiency. Results show that the influence of the S/P selection on inter-turn fault SC current needs to be considered during the design process to balance the efficiency and power density against fault-tolerant criteria of the application at hand

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 analysed. 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 depend 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 provides insight into the influence of parameters that effect overall power density

Non-linear circuit based model of PMSM under inter-turn fault: a simple approach based on healthy machine data

The paper proposes a fast dynamic mathematical model to evaluate the performances of saturated permanent magnet synchronous machines (PMSM) under stator winding’s inter turn fault. The parameters of the model can be determined using only manufacturer’s data of the healthy machine. Two surface mounted PMSM have been considered to investigate the validity of the proposed approach; with distributed and concentrated winding. It has been shown that the proposed model predicts the fault current with a reasonable accuracy compared to the non-linear Finite Elements analyses and to the experimental results. This model can be incorporated in a global simulation environment of power electronic of electrical device since the computation time is very short

High speed solid rotor permanent magnet machines: concept and design

This paper proposes a novel solid rotor topology for an Interior Permanent Magnet (IPM) machine, adopted in this case for an aircraft starter-generator design. The key challenge in the design is to satisfy two operating conditions which are: a high torque at start and a high speed at cruise. Conventional IPM topologies which are highly capable of extended field weakening are found to be limited at high speed due to structural constraints associated with the rotor material. To adopt the IPM concept for high speed operation, it is proposed to adopt a rotor constructed from semi-magnetic stainless steel, which has a higher yield strength than laminated silicon steel. To maintain minimal stress levels and also minimize the resultant eddy current losses due to the lack of laminations, different approaches are considered and studied. Finally, to achieve a better tradeoff between the structural and electromagnetic constraints, a novel slitted approach is implemented on the rotor. The proposed rotor topology is verified using electromagnetic, static structural and dynamic structural Finite Element (FE) analyses. An experiment is performed to confirm the feasibility of the proposed rotor. It is shown that the proposed solid rotor concept for an IPM fulfils the design requirements whilst satisfying the structural, thermal and magnetic limitations