Nominal Load Conditions Research Articles

Evaluation of contact fatigue life of a wind turbine carburized gear considering gradients of mechanical properties

Case hardening processes such as carburizing are extensively applied in heavy-duty gears used in wind turbines, ships, high-speed rails, etc. Contact fatigue failure occurs commonly in engineering practice, thus reduces reliabilities of those machines. Rolling contact fatigue life of a carburized gear is influenced by factors such as the gradients of mechanical properties and profile of initial residual stress. In this regard, the study of contact fatigue life of carburized gears should be conducted with the consideration of those aspects. In this study, a finite element elastic–plastic contact model of a carburized gear is developed which takes the gradients of hardness and initial residual stress into account. Initial residual stress distribution and the hardness profile along the depth are obtained through experimental measurements. The effect of the hardness gradient is reflected by the gradients of yield strength and fatigue parameters. The modified Fatemi–Socie strain-life criterion is used to estimate the rolling contact fatigue life of the heavy-duty carburized gear. Numerical results reveal that according to the Fatemi–Socie fatigue life criterion, rolling contact fatigue failure of the carburized gear will first initiate at subsurface rather than surface. Compared with the un-carburized gear, the rolling contact fatigue lives of the carburized gear under all load conditions are significantly improved. Under heavy load conditions, the carburized layer significantly reduces the fatigue damage mainly due to the benefit to inhibit the accumulation of plasticity. Influence of the residual stress is also investigated. Under the nominal load condition, compared with the residual stress-free case, the existence of the tensile residual stress causes remarkable deterioration of the rolling contact fatigue life while the compressive residual stress with the same magnitude leads to a moderate growth of the rolling contact fatigue life. As the load becomes heavier when plasticity becomes notable, the influence of the initial residual stress on the life is somewhat weakened.

A Hardware Implemented Autocorrelation Technique for Estimating Power Spectral Density for Processing Signals from a Doppler Wind Lidar System.

A signal processing technique utilizing autocorrelation of backscattered signals was designed and implemented in a 1.5 µm all-fiber wind sensing Coherent Doppler Lidar (CDL) system to preprocess atmospheric signals. The signal processing algorithm’s design and implementation are presented. The system employs a 20 kHz pulse repetition frequency (PRF) transmitter and samples the return signals at 400 MHz. The logic design of the autocorrelation algorithm was developed and programmed into a field programmable gate array (FPGA) located on a data acquisition board. The design generates and accumulates real time correlograms representing average autocorrelations of the Doppler shifted echo from a series of adjustable range gates. Accumulated correlograms are streamed to a host computer for subsequent processing to yield a line of sight wind velocity. Wind velocity estimates can be obtained under nominal aerosol loading and nominal atmospheric turbulence conditions for ranges up to 3 km.

System Design and Measurements of a 115-kV/3.5-ms Solid-State Long-Pulse Modulator for the European Spallation Source

In this paper, the results of a 2.88-MW solid-state long-pulse modulator, which has been designed for the new linear collider at the european spallation source in Lund, are summarized. The presented modulator generates an output voltage pulse of 115 kV with a pulselength of 3.5 ms. The main components of the modular system, which include a series–parallel resonance converter, a high-voltage high-frequency transformer, and an output rectifier, are discussed. The modulator design is verified by measurements performed with a full-scale prototype, which is operated under nominal load conditions. A detailed output voltage ripple evaluation is given, and in addition, the system design has been proven by a 7- and 2/3-h heat run test. All specifications are well within the given limits, and the maximum occurring temperatures in the transformer stay below 110 °C even under worst case assumptions. The system achieves a pulse efficiency (determined by the pulse shape) of 96.78%, an overall electrical system efficiency of 91.9%, and a combined pulse system efficiency of 89%.

Design and Investigation of Low-cost PM Flux Switching Machine for Geared Medium-speed Wind Energy Applications

There has been increasing innovation in research to harness nonconventional electrical machines e.g., stator-mounted electrical machines like the flux switching machines (FSMs), in various applications such as wind power generation and electric traction. Hence, the need to further minimize the cost of electrical machines by avoiding designs using scarce rare-earth permanent magnets (PMs), especially if such machines were to be designed using PM materials. The approach in this article is to investigate the cost savings if a hitherto optimally designed 10 kW FSM which uses rare-earth PM is rather designed on less expensive ferrite PM for a medium-speed geared wind generator drive. Using 2D finite-element analyses and comparing the performance of two optimal benchmarks, it is shown that the ferrite design is heavier, leading to 61.2% reduction in the torque density compared to the rare-earth design. However, due to the high cost of rare-earth PMs, the active material cost of the ferrite design is found to be 35% cheaper. It is also found that due to deep saturation effects, the rare-earth design is more susceptible to demagnetization risks even within nominal load conditions, whereas the ferrite demagnetizes rapidly only beyond the nominal load.

Dynamics analysis of the pitch control reducer for MW wind turbine

An analytic dynamics model was presented for the three-stage planetary transmission in the pitch control reducer for MW wind turbine based on the lumped-parameter method. The mechanical characteristic of the contact components was analyzed using the stiffness factor method. All the stiffness sub-matrices were combined to form the overall stiffness matrix of the three-stage transmission. According to the analytic model and the parameters of the pitch control gearbox, the movement differential equations were solved to investigate the natural frequencies and the vibration modes. Then, the undamped and damping forced vibration response were studied. A test rig was set up to measure the vibration displacement of the ring at the second stage and the output shaft under the nominal load condition, the comparison of the analytic forced vibration response with the experimental results validates the effectiveness of the lumped-parameter dynamics model for the pitch control reducer. This paper provides a reference for the dynamics optimization of multistage planetary transmission.

Experimental investigation of hydrogen addition in the intake air of compressed ignition engines running on biodiesel blend

This study investigates the effects on the performance of a diesel generator set at constant electric power of 2.43 kW (approximately 60% nominal engine load condition) and rotation at 3600 rpm operating with a 7% biodiesel-diesel blend (B7) and being doped with hydrogen into the intake air. Hydrogen was injected continuously into the engine intake manifold at different mass concentrations of 2, 6, 8 and 10% of the total fuel mass (B7 + hydrogen) which represents energy fractions of 5, 15, 20 and 24% of total fuel energy. To this, a small L-shaped tube installed at the center of the pipe was used. It is anticipated that the intake air fluctuations at that location, due to opening and closing the intake valve, allows a rapid mixture of hydrogen and air. Due to the addition of hydrogen the total amount of energy in the fuel (B7 + hydrogen) introduced into the engine was increased, so that the engine speed tends to increase, but this was prevented by the governor of the fuel injection pump which decreases the amount of B7 injected until the working frequency of the generator was 60 Hz. The test results showed a reduction in the specific fuel consumption as a function of the increase of hydrogen concentrations. Likewise, CO2, CO and HC emissions decreased proportionately as the hydrogen concentration was increased. On the other hand, the emissions of nitrogen oxides (NOx) increased due to the increase in the average temperature inside the cylinder. There was also an increase in the peak pressure and in the heat release rate inside the cylinder, since B7 ignition delay was reduced due to the increase in hydrogen content.

Experimental fatigue crack growth behavior and predictions under multiaxial variable amplitude service loading histories

This study presents experimental crack growth data for naturally initiated fatigue cracks in tubular specimens of 2024-T3 aluminum alloy subjected to both uniaxial and multiaxial variable amplitude flight loading spectra. Experimental crack growth behavior is compared to predictions based on two state-of-the-art analysis codes: UniGrow and FASTRAN. UniGrow is based on the idea that residual stress distributions surrounding the crack tip are responsible for causing load sequence effects in variable amplitude crack growth, while FASTRAN attributes these effects to varying degrees of plasticity induced closure in the crack wake. For variable amplitude fatigue tests performed under pure axial nominal loading conditions, both UniGrow and FASTRAN analyses were found to produce conservative growth life predictions despite good agreement with constant amplitude crack growth data. For variable amplitude torsion and combined axial-torsion crack growth analyses, however, the conservatism in growth life predictions was found to reduce. This is attributed to multiaxial nominal stress state effects, such as T-stress and mixed-mode crack growth, which are not accounted for in either UniGrow or FASTRAN, but were observed in constant amplitude crack growth tests to increase experimental crack growth rates. Additionally, by comparing experimental crack growth lives between tests performed using full and edited versions of the same loading history, it was found that a 94% reduction in loading history length resulted in differences in experimental crack growth life of less than a factor of 5. Since cracks in this study were initiated naturally, the effect of initial crack geometry assumptions on crack growth predictions was also investigated.

Simulation and optimization of steam operated double effect water-LiBr absorption heat pump

The absorption heat pump technology plays an invaluable role in energy structure optimization and sustainable development. In this research, the steam operated double effect water-LiBr absorption heat pump is introduced and the performance is studied at nominal and part load conditions. Mathematical models are developed and verified by experiments, and the results indicate that the maximum deviation is about 13% at heating condition and 9% at cooling condition. An optimization analysis is presented with mathematical models to optimize the absorption heat pump. The optimal combination of multi-factorial internal parameters, which are the eight main heat transfer temperature differences, are obtained from the thermoeconomic optimization analysis. The results will be much helpful in the design and optimization of double effect water-LiBr absorption heat pumps.

Fatigue crack growth behavior under multiaxial variable amplitude loading

This study compares both uniaxial and multiaxial variable amplitude experimental crack growth data for naturally initiated fatigue cracks in tubular specimens of 2024-T3 aluminum alloy to predictions based on two state-of-the-art analysis codes: UniGrow and FASTRAN. For variable amplitude fatigue tests performed under pure axial nominal loading conditions, both UniGrow and FASTRAN analyses were found to produce mostly conservative growth life predictions, despite good agreement with constant amplitude crack growth data. For variable amplitude torsion and combined axial-torsion crack growth analyses, however, the conservatism in growth life predictions was found to reduce. This was attributed to multiaxial nominal stress state effects, such as T-stress and mixed-mode crack growth, which are not accounted for in either UniGrow or FASTRAN, but were found in constant amplitude fatigue tests to increase experimental crack growth rates. Since cracks in this study were initiated naturally, different initial crack geometry assumptions were also investigated in the analyses

Effect of Phosphate-Buffered Solution Corrosion on the Ratcheting Fatigue Behavior of a Duplex Mg-Li-Al Alloy

This work reports the uniaxial ratcheting and fatigue behavior of a duplex Mg-Li-Al alloy under the influence of phosphate-buffered solution corrosion. Microstructural observations reveal pitting and filament corrosion defects, which impair the load-bearing capacity of the alloy and cause stress concentration, thus leading to an accelerated accumulation of ratcheting strain and shortened fatigue life under the same nominal loading conditions. Comparing Smith model, Smith-Watson-Topper model, and Paul-Sivaprasad-Dhar model, a ratcheting fatigue life prediction model based on the Broberg damage rule and the Paul-Sivaprasad-Dhar model was proposed, and the model yielded a superior prediction for the studied magnesium alloy.

Notch deformation and stress gradient effects in multiaxial fatigue

This paper investigates notch mechanics under multiaxial fatigue loading conditions with respect to notch stress–strain estimation rules and stress gradient effects. A pseudo stress-based plasticity modeling technique, incorporating a structural yield surface concept, was applied to predict local stress distributions for a 2024-T3 aluminum alloy notched tubular specimen, as well as for two different AISI 1141 steel alloy notched shaft specimen geometries. Stress–strain predictions were generated for several nominal loading conditions and compared to nonlinear FEA solutions. Most predictions were found to be within ±15% error for loading levels relevant to typical fatigue loading applications. To evaluate stress gradient models, fatigue life predictions were performed for each specimen geometry using Neuber’s rule with fatigue notch factor, as well as several different interpretations of Theory of Critical Distances (TCD) approaches. The effect of critical distance value on life predictions was also studied. The Fatemi–Socie critical plane damage parameter was used to calculate fatigue lives. While the TCD approaches were found to provide improved multiaxial fatigue data correlation when compared to fatigue notch factor, negligible differences between the different TCD approaches were observed. Life prediction trends were found to be consistent regardless of notch geometry and material, thus suggesting some generality of the findings.

Signal to Noise Ratio Characterization of Coherent Doppler Lidar Backscattered Signals

An eye-safe coherent Doppler Lidar (CDL) system for wind measurement was developed and tested at the Remote Sensing Laboratory of the City College of New York (CCNY). The system employs a 1542 nm fiber laser to leverage components’ availability and affordability of the telecommunication industry. A balanced detector with a bandwidth extending from dc to 125 MHz is used to eliminate the common mode relative intensity noise (RIN). The system is shot noise limited i.e., the dominant component of received signals’ noise is the shot noise. Wind velocity can be measured under nominal aerosol loading and atmospheric turbulence conditions for ranges up to 3 km while pointing vertically with 0.08 m/s precision.

Development and Operational Analysis of an All-Fiber Coherent Doppler Lidar System for Wind Sensing and Aerosol Profiling

A 1.5- $\mu\mbox{m}$ all-fiber coherent Doppler lidar system for wind sensing was developed and characterized. The system design and implementation is presented. The system employs a 20-kHz pulse repetition rate transmitter and samples return signals at 400 MHz. A field-programmable gate array is programmed to generate and accumulate real-time periodograms representing average power spectra of the Doppler shifted echo from a series of more than 60 range gates, where each range gate is 48 m. The accumulated periodograms are streamed to a host computer for subsequent processing to yield the line-of-sight wind velocity. Wind velocity estimates with a precision value of 0.08 m/s can be made under nominal aerosol loading and nominal atmospheric turbulence conditions for ranges up to 3 km. Wind velocities and aerosol profiles are obtained while scanning the pointing direction of the lidar, thus producing both horizontal and vertical representations of the wind vectors.

Model validation of a hexapod walker robot

SUMMARYOur complete dynamical simulation-model realistically describes the real low-cost hexapod walker robot Szabad(ka)-II within prescribed tolerances under nominal load conditions. This validated model is novel, described in detail, for it includes in a single study: (a) digital controllers, (b) gearheads and DC motors, (c) 3D kinematics and dynamics of 18 Degree of Freedom (DOF) structure, (d) ground contact for even ground, (e) sensors and battery model. In our model validation: (a) kinematical-, dynamical- and digital controller variables were simultaneously compared, (b) differences of measured and simulated curves were quantified and qualified, (c) unknown model parameters were estimated by comparing real measurements with simulation results and applying adequate optimization procedures. The model validation helps identifying both model's and real robot's imperfections: (a) gearlash of the joints, (b) imperfection of approximate ground contact model, (c) lack of gearhead's internal non-linear friction in the model. Modeling and model validation resulted in more stable robot which performed better than its predecessors in terms of locomotion.

Efficient Methods for Predicting Low Pressure Steam Turbine Exhaust Hood and Diffuser Flows at Design and Off-Design Conditions

The exhaust hood of a steam turbine is an important area of turbomachinery research as its performance strongly influences the power output of the last stage blades (LSB). This paper compares results from 3D simulations using a novel application of the nonlinear harmonic (NLH) method with more computationally demanding predictions obtained using frozen rotor techniques. Accurate simulation of exhausts is only achieved when simulations of LSB are coupled to the exhaust hood to capture the strong interaction. One such method is the NLH method. In this paper, the NLH approach is compared against the current standard for capturing the inlet circumferential asymmetry, the frozen rotor approach. The NLH method is shown to predict a similar exhaust hood static pressure recovery and flow asymmetry compared with the frozen rotor approach using less than half the memory requirement of a full annulus calculation. A second option for reducing the computational demand of the full annulus frozen rotor method is explored where a single stator passage is modeled coupled to the full annulus rotor by a mixing plane. Provided the stage is choked, this was shown to produce very similar results to the full annulus frozen rotor approach but with a computational demand similar to that of the NLH method. In terms of industrial practice, the results show that for a typical well designed exhaust hood at nominal load conditions, the pressure recovery predicted by all methods (including those which do not account for circumferential uniformities) is similar. However, this is not the case at off-design conditions where more complex interfacing methods are required to capture circumferential asymmetry.

Influence of the closing rotor slots on the additional losses in the induction motor

Purpose – The purpose of this paper is to present the distribution of the magnetic field and additional losses analysis of the induction motors (IM) with opened and closed rotor slots. Design/methodology/approach – In the field-circuit approach the distribution and changes of magnetic flux density in the motor are computed using a time-stepping finite element method. The additional losses in each element are evaluated at different frequencies. Findings – An approximate analytical formulation is derived for rapid losses computation confirmed by the results of field-circuit method. For high-voltage motors due to the size ratios of the core and relatively deep stator and rotor slots major role in causing loss of higher harmonics play a fundamental slot harmonics. Higher harmonics order bigger than 100 cause only small part of total higher harmonics core losses. Closed rotor slots construction influenced significantly on no-load losses mainly due to reduction of losses at slot upper part. For nominal load condition that influence is not so strong according to the saturation of slot tips by rotor leakage flux. Nevertheless, core losses at load are several times higher as at no-load. Research limitations/implications – In future research authors will take into account motors feed from PWM inverter, working in the frequency range up to 400 Hz. Practical implications – The results of investigation will be used in more detailed design of IMs especially for motors with closed rotor slots. Originality/value – The methods presented in the paper was not used before. Also results of additional losses in the motor core calculation, especially according motors with closed slots at no load and load conditions are new.

6배압 정류기를 이용한 고전압 전원장치에 관한 연구

This paper presents design, fabrication, and performance evaluation of a high voltage power supply for Carbon Nano Tube-based planar light sources. The proposed power supply employs an LLC resonant half-bridge converter and a sixfold voltage-multiplying rectifier. Steady-state characteristics of the voltage-multiplying rectifier are analyzed and used to derive the input-to-output voltage conversion ratio of the power supply. The input-to-output frequency response characteristics of the LLC tank circuit are analyzed and utilized in designing a proto-type power supply which produces a 15 KV output using a 400 V input source. The high-voltage transformer is fabricated using a sectional bobbin structure with an epoxy impregnation, in order to provide sufficient insulation for high voltage operations. The performance of the proposed power supply is confirmed with stable and reliable operations at the 15 KV output from no load to nominal load conditions. The proposed power supply is well suited as an electric ballast required stable operations of Carbon Nano Tube-based planar light sources.

Temperature Reduction in the Clamping Bolt Zone of Shunt Reactors: Design Enhancements

This paper presents design improvements to lower the temperature in the clamping bolt (CB), in a single-phase extra-high-voltage (EHV) shunt reactor. Laboratory temperature measurements, under overload and nominal load conditions, were performed in the middle of the two top main air gaps inside one of the slots drilled in the CB using commercial fiber-optic temperature sensors. 3-D finite-element (FE) simulations were performed to calculate fringing losses in the CB. Subsequently, the fringing losses were employed as a heat source for static-steady thermal analysis using 3-D FE simulations. Convective heat-transfer coefficients were selected with a methodology that leads to a close match between measured and simulated temperatures. Two practical design enhancements to diminish temperatures in the CB are analyzed using measured and simulated data. These alternatives improve design and reduce potential failures and, hence, increase the EHV shunt reactor lifetime.

Control of AGC in Interconnected Power System with Diverse Sources of Power Generation

In this paper, automatic generation control (AGC) of two area interconnected power system having diverse sources of power generation is studied. A two area power system comprises power generations from hydro, thermal and gas sources in area-1 and power generations from hydro and thermal sources in area-2. All the power generation units from different sources are equipped with speed governors. A continuous time transfer function model of the system for studying dynamic response for small load disturbances is presented. A proportional-integral-derivative (PID) automatic generation control scheme is applied only to power generations from thermal and gas sources and power generation from hydro source is allowed to operate at its scheduled level with only speed governor control. The two area power system is simulated for different nominal loading conditions. Particle Swarm Optimization (PSO) is used to obtain the optimal PID gains for various cases using integral squared error plus integral time absolute error (ISE+ITAE) performance index for fitness evaluation. Some of the transient responses are shown for different nominal loading conditions due to step load disturbances in the system.

A Very Low-Profile Dual Output LLC Resonant Converter for LCD/LED TV Applications

The concept of low-profile power supply becomes a current issue because of the recent trends in LCD/LED TV consumer market. Realizing a low-profile power supply with high power density is a challenging work due to the size limitation of passive components. In this paper, a very low-profile dual output LLC resonant converter proposed by considering the criteria of 10 mm component height limit without any drop-in cuts on the surface of printed circuit board. The simulation model of the converter is applied to the SPICE tool in order to estimate the performance and borders of the passive components. On the basis of theoretical approach and simulation results, two low-profile transformers are utilized in order to acquire a prototype with 200 W output power (24 V/6 A and 12 V/4.5 A). LLC series resonant converter prototype with the dimension of 140 mm × 291 mm × 10 mm achieves 89.4% efficiency at nominal load conditions. The input voltage range is between 330 to 400 V and the nominal input voltage is specified as 400 V. Operating frequencies are obtained as 65 and 81 KHz at 330 and 400 V, respectively, under the nominal load conditions. Thermal and electromagnetic interference performances of the proposed converter are included in the paper to validate experimental study.