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This paper reports on a research project commissioned by LEONARDO Aircraft S.p.A. to the DAEEE for improvement of diagnostic and protection systems of avionics generators. The arc faults arising in worn brushes and slip-rings supplying the propeller anti-ice rotating loads on board aircrafts are of concern here. Intermittent arc faults may arise, causing current and torque ripples potentially exciting torsional resonances leading to the breakage of the coupling joint (‘mechanical fuse’) between turbine and generator and, hence, to a permanent out of service of the latter. Common non-selective protections are not effective against these types of faults. The study of alternative solutions requires accurate simulations of the complex electromechanical system. This work moves from the construction of the system model, for time-stepping FEM-circuit co-simulations by Maxwell/Simplorer software. Then, fixed-frequency intermittent faults matching experimental tests are analyzed. The causes of failure are definitively determined and solutions to overcome the problem are proposed. A companion paper shows an original online and sensorless method for speed and torque estimation and detection of harmful torsional resonances resulting from intermittent arc faults.

This work shows experimental evidences of abnormal end-ring torsional vibrations in fabricated cage induction motors and their role as root cause of bar breakages. A case-study of cage torsional resonances in railway traction motors is presented, which led to bar breakages with a peculiar failure pattern. A particular cage resonant mode (the second natural mode with both the rings vibrating in phase-opposition with respect to the rotor iron stack) is shown as responsible of excessive bar bending, fatigue, and bar fractures, through direct measurements of ring-stack oscillations in working motors. Cage torsions were excited by large harmonic torques due to inverter supply in the case considered. The mechanism of resonance-induced bar solicitations and fatigue is discussed, and the difference of failure pattern with respect to more common bar breakage mechanisms (overloads, heavy start-ups, etc.) is evidenced. Some theoretical models for modal frequency calculation are presented, which may help to identify the origin of vibrations and the fault root-cause in faulted drives. Dynamic simulations of the drive have been carried out for airgap torque calculation and bar length optimization.

The study of electric machines subject to rotor eccentricity is usually performed using a winding and permeance function approach. In this paper, an alternative method is investigated based on conformal mapping. The geometry of a an eccentric round-rotor machine is mapped into a uniform air-gap topology by means of Moebius conformal map. The mathematical details of the method are provided along with explicit formulas to compute the air-gap field and inductances of the eccentric machine. It shown how, unlike conventional winding-function theory, the proposed method yields closed-form formulas which do not involve series expansions nor require numerical integrations and are thus particularly fit for implementation into fast simulation programs. Such expressions are assessed by comparison with Finite Element Analysis (FEA).

This paper investigates the hydraulic system drive used onboard ships to actuate stabilizing fins. The hydraulic drives are often characterized by low efficiencies, heavy maintenance, frequent oil leaks and large weight and size. To overcome these draw-backs, in this paper a concept design of a full-scale permanent magnet linear synchronous motor directly coupled to the shipboard fin stabilizer, is presented, used as a servo-motor. The innovative electric drive is based on prototype of high thrust linear motor built in a previous research. A complete Simulink model, where electric, mechanical and hydraulic domains can be connected, has been realized. Simulations has been performed to evaluate the hydraulic drive efficiency and in order to assess the servo-assistance capabilities. The results show the feasibility of the proposed solution for a future practical application. This research has been carried out in the framework of the Italian Defence Research National Program for the development of ‘dual use’ technologies.

The study of electric machines subject to rotor eccentricity is usually performed using a winding and permeance function approach. In a companion paper, an alternative method is investigated based on conformal mapping. The geometry of a an eccentric round-rotor machine has been mapped into a uniform air-gap topology by means of Moebius conformal map. In this paper, the approach is used to derive explicit formulas to compute the unbalanced magnetic pull of the eccentric machine. It shown how, unlike conventional winding-function theory, the proposed method yields closed-form formulas which do not involve series expansions nor require numerical integrations and are thus particularly fit for implementation into fast simulation programs. Such expressions are assessed by comparison with Finite Element Analysis (FEA).

Alternatives to hydraulic drives used on board ship are under study for the Italian Navy in order to reduce the use of the oil which cause environmental problems and high cost. The hydraulic system drive used onboard ship are affected by low efficiencies and generally require intensive maintenance, very high encumbrance. So that the Italian Navy has commissioned a research project for the construction and installation of an innovative oil-free electric drive based on a permanent magnet linear actuator. The new electric motor is more efficient, fault-tolerant, reduces the encumbrance and ensures a greater control.

This paper deals with the electromagnetic, mechanical, and cooling solutions for an innovative direct-drive suited for steering gears on board ships. The work is comprised in a research project funded by the Italian Navy. The research aims to realize a full-electric and oil-free drive for actuation of hydraulic loads in the perspective of “more green ships”. The electrical drive proposed is based on a fractional slot permanent magnet linear motor sized and optimized for high force and low speed operation, as required by hydraulic loads on board ships. The capabilities and efficiencies of the electric drive in servo-assistance to the hydraulic motor were evaluated in previous works of the same authors. In this paper the electrical, mechanical and thermal design and the feasibility of the installation are shown.

Torsional resonances may incur in induction drives due to large harmonic torques produced by the inverter. The torsion can affect the rotor-load system, but more sneaky resonances may arise in the rotor end-rings of fabricated-cage induction motors. Abnormal cage torsions may lead to bar breakage due to high-cycle fatigue. Since the high-frequency speed oscillations are difficult to detect, torsional resonances may go unnoticed and lead to drive fault on the long run. This paper proposes a non-invasive technique for early detection of high-frequency speed fluctuations in case of mechanical resonance, by using motor current signature analysis. The rotor slot harmonics (usually researched for rotor eccentricity detection or for drive speed estimation) are originally used here to detect drive speed fluctuations. Simulations of the motor-converter system carried out by winding function approach show the feasibility of this kind of motor condition monitoring.

Hydraulic drives used on board ship to empower rudders, stabilizing fins, anchor winches, etc., are often characterized by low efficiencies, heavy maintenance, frequent oil leaks and large weight and size. To overcome these drawbacks, oil-free inverter-fed alternative drives can be conceived. In this paper a concept design of a full-scale permanent magnet linear synchronous motor directly coupled to a twin rudder on board ship is presented, used as a servo-motor. The servo-assistance capabilities are simulated and evaluated by using a complete model of the electro-mechanic-hydraulic system in Simulink environment. The results show the effectiveness of the electromagnetic drive servo-assistance. This research has been carried out in the framework of the Italian Defence Research National Program for the development of `dual use' technologies.

Round conductive bars embedded in semi-closed slots are frequently used in the construction of electric machinery, for instance in the squirrel cage of induction motors. Frequently, such as in the study of induction motor steady-state performance at rated slip, it is useful to estimate the slot leakage inductance of these conductors under the hypothesis of no eddy currents and no magnetic saturation. This is usually done through simple approximated analytical formulas available in the literature. In this paper, an improved explicit ready-to-use leakage inductance expression for circular bars embedded in semi-closed slots is derived by solving Poisson's equation in the slot domain. The precision of the proposed formulation is assessed against Finite Element Analysis (FEA) for various slot geometries and is shown to always give very accurate results, with errors below 2%. Conversely, approximated simplified formulas available from the literature are demonstrated to possibly give large errors, which exceed 20% for some explored slot geometries.

Squirrel-cage induction motors are the most widely used type of electric machinery in industrial applications thanks to their rugged, cheap and robust construction. For the fast prediction of induction motor steady-state performance without the need for time-consuming finite-element analyses, the equivalent circuit parameters of the machine need to be calculated, including rotor slot leakage inductances. This paper, in particular, proposes a new improved analytical formula to determine the rotor slot leakage inductance in squirrel-cage induction motors with rectangular-shaped bars. The formula is obtained by solving Poisson's equation in the slot domain. Results are assessed by comparison against Finite Element Analysis (FEA). The precision of the presented methodology is also compared to that of approximated analytical models available from the literature. The comparison shows that the accurate formula proposed is in excellent agreement with FEA with errors below 2%, while the simplified model generally leads to errors above 10% which increase as the height-to-width bar ratio decreases.

The use of linear machines (LMs) is increasing in research and industry fields. All kinds of LMs are studied: synchronous, induction, flux switching. Research is leading to overcome the known drawbacks of all these machines. Anyway the use of innovative direct-drive solutions are under study to obtain more efficient energy conversion, such as for marine field. As alternative to classical systems, linear force can be used to obtain a more efficient plant. Hydraulic drives used on board ship to empower rudders, stabilizing fins, anchor winches, etc., are often characterized by low efficiencies, heavy maintenance, frequent oil leaks and large weight and size. To overcome these drawbacks, an oil-free inverter-fed alternative drives can be conceived. In this paper a concept design of full-scale permanent magnet linear synchronous motor directly coupled to a twin rudder on board ship is presented, used as a servo-motor. A complete model of the electro-mechanic-hydraulic system in Simulink environment was built. Several simulations are performed to evaluate the servo-assistance capabilities. The results show the effectiveness of the electromagnetic drive servo-assistance and an increased overall efficiency of the steering gear. This research has been carried out in the framework of the Italian Defence Research National Program for the development of `dual use' technologies.

The finite reluctance approach is very well suited for fast modeling and dynamic simulation of permanent magnet synchronous machines with inverter feeding. This paper shows validating simulations, in both steady-state and transient conditions, carried out for a permanent magnet linear motor designed for on board ship load actuation. The basics of the method are resumed. The simulations obtained by using the finite reluctance approach are compared with FEM simulations, showing good agreement. Keywords—Dynamic model, dynamic simulation, linear machine, magnetic network, reluctance.

The finite reluctance approach is based on a simple synergy between a basic magnetic domain decomposition rule and the voltage balance at the terminals of any winding acting in the domain. The finite reluctance approach is thought as a tool for fast dynamic simulations of any kind of machine, but it is particularly adapt for irregular flux geometries as in fractional-slot surface-mounted permanent magnet machines with large slots and high slot flux leakages, for which only the finite element method has found good usage. This approach is based on the definition of a complementary mesh interlaced with the reluctance mesh, on the concept of material cell, and on unique rules to carry out both reluctances and magneto-motive forces. At this scope, definitions of mesh nodes, co-nodes, and auxiliary nodes are shown. The iron saturation can be easily taken in account, as well as motion effects. The circuit flux linkages are elected as state variables for integration, and they are directly linked to the loop fluxes in the magnetic network. The finite reluctance approach has been applied in this paper for dynamic modeling of an inverter-fed permanent-magnet linear synchronous motor.

One of the key tasks in the modeling of electric machines is the analytical expression of self and mutual phase inductances as a function of the rotor position. Winding and permeance functions are a powerful tool for this purpose. This paper investigates the expression of the air-gap permeance function for a round rotor machine subject to pronounced eccentricity. An expression is sought in the form of a Fourier series expansion in order to make it suitable for modeling purposes. Although derived through non trivial algebraic passages, the final expression proposed is proved to be accurate and computationally efficient as the theoretically infinite sums involved in it exhibit a very rapid convergence to their asymptotical values.

This paper studies the cage torsional vibrations in a traction drive comprising a variable-frequency voltage-source inverter and a fabricated-cage induction motor. End-ring vibrations may arise due to harmonic torques, with exciting frequencies matching the cage mechanical resonance frequency. It is shown that abnormal cage torsions may lead to bar breakage due to high-cycle fatigue. An actual case-study is presented, with cage vibration measurements. The measurements are matched with dynamic simulations of the complete electro-mechanical system comprising a vibrating-cage motor and the static converter. This paper shows full details of the dynamic model. Modal frequency calculation is also carried out. The model may help to identify the origin of vibrations and the fault root-cause in faulted drives, or may support the designer in preventing harmful cage resonances at the drive design stage.

The electromagnetic structure of the phase-shift autotransformer (PSAT) is introduced. The operation of the PSAT in connection with a double 3-steps rectifier is discussed. The algorithm for deduction of the main design quantities of such machine is described. The circuit model of the PSAT is arrived at; the circuit analysis allows deducing the quantities from which the machine design can be performed and that are the inputs for the thermal analysis. The results of as thermal analysis on a machine designed and built by the Authors are shown.

A fault-tolerant machine used for safety-critical tasks must a) guarantee at least a reduced-performance operation in case of partial machine fault and b) guarantee avoidance of drive mechanical jam/stall in case of total machine fault, to allow for the intervention of the back-up systems. Classical hydrostatic transmissions used on board ships for critical tasks such as rudder and stabilizing fin steering gears fulfill both the requirements a), b) above, but recent proposals for substitution with full-electric drives (rotary motors coupled with multistage reduction gears) usually do not. Especially the requirement b) needs particular attention and increased complexity when dealing with geared drives. This paper proposes a linear permanent-magnet direct drive fulfilling both the requirements above, for (but not limited to) rudder/fin steering gears. The absence of gears grants the requirement b), whereas the full-modular structure satisfies a), with independently fed stator modules and multiple inverters. This paper addresses some fault scenarios including electrical failures, in the machine winding (short-circuited coils) and in the inverters (trip of one or more units). The performance degradation is studied and assessed for the cases considered by both simulations and measurements on a prototype.

This paper analyzes a different approach for actuation of shipboard loads such as rudders, stabilizing fins, or other linear drives, which are usually powered by oil-fed actuators. Hydrostatic transmissions are classically used when the highest torques/forces with low speeds are demanded. However, the oil used as force-transmission fluid is troublesome due to environmental issues. Oil production, use, and final disposal is costly and presents environmental risks. Also the maintenance onboard ship is intensive and hazardous for the operators. A full electric solutions is ulteriorly studied in this paper, based on a prototyped linear machine using permanent magnets and inverter feeding. Copper, friction, and iron losses are the main limiting factors in developing the required thrust at low speed, which is, ultimately, the main obstacle to a gear-less or direct-drive full-electric application. This paper shows incremental work on the prototype, through a dedicated circuit model accounting for all the machine losses and providing an affordable model of machine overall efficiency. The model will be used in further works to asses the machine performances in typical onboard service duties.

An approach to machine modeling by using finite reluctances is proposed. The finite reluctance approach is well suited for fast modeling and dynamic simulation of electrical machines in general and of fractional-slot surface-mounted permanent magnet machines in particular. The latter usually have very large air gap and slots, with relevant leakage and large tangential components of flux density in air gap, slot, and magnets as well. Modeling tools as winding functions are not accurate enough in this case, and thus time-stepping finite elements are used, which are slow and with limited time-step resolution (i.e. low simulation frequency). The approach used in this paper uses magnetic networks built on the principle of the cell theory. Network reluctances and magnet MMFs are defined by using unique rules. This paper shows that, besides faster dynamic simulations, the finite reluctance approach allows the graphical representation of the magnetic vector and scalar fields inside the machine, in a way very similar to the more cumbersome finite element method, but with a much smaller computational effort. The results from both the methods are presented and compared for a case study of permanent magnet linear synchronous motor.

The ships have long been an easy play-ground for hydraulic technologies. When high torques and low speeds are demanded on board ship, both technical and traditional reasons usually lead to hydraulic solutions. In fact, the conventional electrical machines are better suited for low-torque, high-speed applications, and coupling gears must be used to reduce the speed. However, oil-free direct-drive alternatives can be conceived, by new concepts of low-speed permanent-magnet actuators fed by low-frequency inverters. This paper shows the experimental results obtained with a linear permanent-magnet motor designed for direct coupling to rudder stocks or stabilizing fins. The prototype weights 700kg and features a thrust-to-weight ratio of 5. Even better results can be obtained by using composite materials for the machine frame. The machine features a full structural redundancy and inherent fault tolerance, thanks to the modular construction. The present research has been carried out in the framework of the Italian Defence's Research National Program.

Synchronous reluctance (SynRel) motors with rotor flux barriers are gaining increasing attractiveness in automotive applications thanks to their cheap, rugged and magnet-free rotor construction. When equipped with a split-phase stator winding and supplied from multiple inverters, these machines can exhibit further merits as traction motors in regard to enhanced fault tolerance compared to conventional three-phase solutions. Since SynRel motors are usually designed through iterative optimization techniques, it is highly desirable to have accurate and fast methods to predict their performance without the need for time-consuming finite element analysis (FEA) simulations. An analytical procedure is set forth in this paper to analytically model and simulate a SynRel motor with a split-phase stator winding through a magnetic equivalent circuit (MEC) technique. MEC parameters are computed from analytical formulas describing the air-gap magneto-motive force distribution and the magnetic field inside flux barriers. As an output, the air-gap flux density of the SynRel motor can be computed through the presented technique at any operating point. Results are positively assessed by comparison with FEA simulation on a sample SynRel motor including magnetic saturation effects.

In this work performances of an electric hybrid version of a telehandler working machine are studied with a numerical model developed in Matlab/Simulink®. The algorithm, focused on the electric driveline performances, has been developed in a modular approach. Within each module a specific dynamic aspect of the working machine is studied. The hybrid/electric model is validated in different operational conditions by means of full scale experiments of the vehicle on test bench. The so validated model can be used to simulate different hybrid architectures reducing the number of prototype versions and test bench experiments. A comparison between performances of two different architectures of the electric hybrid telehandler is presented in the paper. The former with a conventional mechanical traction driveline and the other with in wheel motors. The comparison has been carried out, taking into account only the electric traction behaviour, in terms of performances and energy consumption.

In today’s large ships a wide use is made of hydraulic machinery to actuate high-torque low-speed onboard loads like rudders, stabilizing fins and bow thrusters. Such mechanical drives are very cumbersome and suffer from serious maintenance and reliability issues. In this paper, a novel all-electric direct-drive oil-free drive solution is proposed based on an inverter-fed permanent magnet linear synchronous motor (PMLSM). The PMLSM is coupled to shipboard rotary loads through an innovative kinematic chain, consisting of a tiller and a prismatic-rotoidal joint, that performs force to torque conversion. The advantages of the PMLSM solution are presented in the paper in terms of compactness, robustness, reliability, efficiency and maintenance. Furthermore, the design optimization of a technology demonstrator which is presently under construction is addressed.

Many shipboard loads requiring high torques at very low speeds, like capstans, bow thrusters, rudders, stabilizing fins, etc., are presently driven by hydrostatic actuators. This paper presents an innovative inverter-fed electric linear motor prototype conceived to replace hydrostatic drives with large expected benefits in compactness, maintenance, efficiency and reliability. The linear motor prototype has been designed through special optimization algorithms based on finite element analysis to maximize the force density. The electromagnetic and mechanical structure has been also selected to achieve superior reliability by means of structural simplicity, robustness and redundancy. Along with design criteria, the paper outlines the prototype technology and the construction issues and provides some testing results collected during the experimental performance assessment.

Surface Permanent Magnet (SPM) machines with slotless stator design are used is some applications where torque pulsations and/or the additional losses due to slotting effects need to be eliminated. These machines have a geometric structure allowing for an accurate analytical solution for the magnetic fields, which is desirable especially for optimization purposes. This paper derives a compact but exact analytical formula for the load torque computation in slotless SPM machines. The formula has the advantage of well highlighting the dependence of the load torque from machine construction details. It can be therefore useful in the design and dimensioning stage as a fast alternative to time-consuming Finite Element (FE) analysis.

An innovative design of a device embedding the functions of both power transformer and shunt reactor is presented. The device is thought for installation onboard substation platforms of long HV/EHV AC cable connected offshore wind plants. The compact design saves much space on the hosting offshore platform, which can be sized smaller and cheaper. Machine sizing as well as cost saving considerations are exposed in this paper.

This work shows a method to quantify rotor eccentricities in synchronous machines by exploiting the unbalance caused in the split-phase currents. The paper first develops a machine model comprehensive of eccentricities and parallel circuits in the stator, by using symmetrical components. Then, the model is used for formal calculation of the unbalanced currents. Finally, the equations are reversed to obtain eccentricity degrees from current measurements. Practical formulas are given for fault assessment, only requiring machine line voltage and synchronous reactance. The method can be applied on load. This paper provides full details of the theory underlying the method. The theory also clarifies some aspects about split-phase currents, not deepened before. It is proven that the air gap flux modulation due to eccentricities, acting through additional 2(p ±1) -pole flux waves in 2p-pole machines, stimulates additional currents, which circulate in the stator and turn into 2(p ±1)-pole rotating space vectors in the complex domain. Vector trajectories have shape and amplitude dictated by eccentricity type and degree, respectively. This study is limited to 2p-pole machines with p ≥ 2. The theory is corroborated by simulations of a practical 1950-kVA generator in this paper. Experimental proofs and simulations of a laboratory 17-kVA machine are provided in a sequel of this paper.

A 1130kW squirrel-cage induction motor used for railway traction is analyzed in this paper in case of eccentric rotor. Since the motor has parallel-connected windings, the air-gap unbalance makes rise unbalanced currents in the stator branches, and the split-phase current signature analysis can be attempted for fault detection. The split-phase currents are better analyzed when transposed in the complex domain, by using symmetrical components. Moreover, the rotor cage currents must be carefully taken in account in the model, since they develop a powerful damping effect on the signature of static eccentricity. This paper shows that the faulted machine can be represented by the superimposition of five equivalent circuits corresponding to five centered-rotor virtual machines, useful for fault-related split-phase current calculation. The mathematical modeling shown in this paper is useful to carry out a diagnostic method, as shown in a companion paper. The latter also reports confirmatory simulations of the faulted 1130kW motor, based on winding function approach.