Low Speed Shaft Research Articles

Double-notch single-pumper fluid mounts

Passive fluid mounts are widely used in the automotive and aerospace applications to isolate the cabin from the engine noise and vibration. In the case of aerospace fixed wing applications, when fluid mounts are used, they are placed in between the engine and the fuselage, and the notch frequency of each fluid mount is tuned to either N1 frequency (engine low speed shaft imbalance excitation frequency) or to N2 frequency (engine high speed shaft imbalance excitation frequency) at the cruise condition. Since current passive fluid mount designs have only one notch, isolation is only possible at N1 or at N2, but not both. Here, in this paper, a double-notch passive fluid mount design will be presented, which has two notch frequencies, and therefore can provide vibration and noise isolation at two frequencies. In this paper, the new fluid mount design concept and its mathematical model and simulation results will be presented.

Rotor-Bearing Analysis for Turbomachinery Single- and Dual-Rotor Systems

Finite element analysis models were developed to investigate the dynamic characteristics of single- and dual-rotor-bearing turbomachinery systems. When an inertial coordinate system was used, the dynamic models of the rotor-bearing systems included gyroscopic moments, rotary inertias, and bending and shear deformations. The models were analyzed to predict the natural frequencies, to produce critical speed maps, and to estimate the bearing stiffnesses. These rotor-bearing system analyses were then applied to both single-rotor and dual-rotor system applications. In the single-rotor system application, a small turbojet engine and its rotor components were used as a basis for the model. Both theoretical and experimental analyses were used to study this engine rotor-bearing system. Modal testing and a dynamic engine test were used to verify the analytical results, including the predicted critical speed map and the bearing stiffnesses. Very good agreement was found between the analyses and the test data. In the dual-rotor application, the effects of the speed ratio of the high-speed to low-speed shafts of the dual-rotor system on the critical speeds was studied. It was demonstrated that this speed ratio could be used as one of the dual-rotor system design parameters. Finally, it was noted that the interrotor bearing stiffness between the high-speed and the low-speed shafts of the dual-rotor system affected the mode shapes of the shafts within the system, in addition to the rotor system critical speeds.

Soft-stall control for variable-speed stall-regulated wind turbines

A variable-speed, fixed-pitch wind turbine control strategy was investigated to evaluate the feasibility of constraining rotor speed and power output without the benefit of active aerodynamic control devices. A strategy was postulated to control rotational speed by specifying the demanded generator torque. By controlling rotor speed in relation to wind speed, the aerodynamic power extracted by the blades from the wind was manipulated. Specifically, the blades were caused to stall in high winds. In low and moderate winds, the demanded generator torque and the resulting rotor speed were controlled and the wind turbine operated near maximum efficiency. Turbine models were developed and simulations of operation in turbulent winds were conducted. Results indicated that rotor speed and power output were well regulated. Preliminary investigations of system dynamics (E. Muljadi, K. Pierce, P. Migliore, A conservative control strategy for variable-speed stall-regulated wind turbines, AIAA-2000-31 A Collection of the 2000 ASME Wind Energy Symposium Technical Papers presented at the 38th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, January 2000). Showed that, compared to fixed-speed operation, variable-speed operation caused cyclic loading amplitude to be reduced for the turbine blades and low-speed shaft and slightly increased for the tower loads. This result suggests that implementation of the proposed control strategy will have a favorable impact on the turbine's fatigue life. The concept was implemented on a 275 kW wind turbine test bed (K. Pierce, P. Migliore, Maximizing the energy capture of fixed-pitch variable-speed wind turbines, AIAA-2000-0032 ASME Wind Energy Symposium Technical Papers presented at the 38th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, January 2000). Test data show that the wind turbine performance matches the predicted simulation results. The control concept was shown to operate the wind turbine near maximum efficiency in low-to-moderate wind speeds, while stalling the rotor in the high winds to regulate speed and output power.

MODELING OF LOW SHAFT SPEED BEARING FAULTS FOR CONDITION MONITORING

A general model of faulty rolling element bearing vibration signals is established. The efficacy of the envelope-autocorrelation technique for condition monitoring of such bearings leads us to derive the envelope-autocorrelation function of the model in the case of very low shaft speed. For application to the bearing condition monitoring, a simplified model is given. Finally the comparison of simulated data using the model within situdata confirms the satisfactory performance of the model and the effectiveness of envelope-autocorrelation as a fault detection technique for low shaft speed bearings.

Mass and Aerodynamic Imbalance of a Horizontal Axis Wind Turbine

It is very common among wind turbines of all designs that a spectral analysis of operating variables, such as yaw motion, torque, or electrical power output has a large concentration of variance at the rotational frequency of the low speed shaft. This spike in the spectrum of the operating variables has come to be known as the once per revolution response or 1P. In addition to this 1P spike, three-bladed wind turbine spectral signatures usually include the harmonic integers 2P and 3P. The two main sources of IP are mass imbalance in the rotor plane and various aerodynamic performance imbalances of the rotor blades. This paper is dedicated to determining the specific contribution of each of these sources to 1P as well as their effects on the 2P and 3P harmonics. Mass imbalance and aerodynamic imbalance issues are addressed separately so that the relative contribution of each can be quantified. The nonlinear differential equations describing the coupled azimuth and yaw motion with mass imbalance were analytically solved using a perturbation technique. A blade strip element technique was used to determine the effects of aerodynamic imbalance on 1P, 2P, and 3P fluctuations. It was found that 60 percent of the 1P low-speed shaft torque (LSST) fluctuation is due to mass imbalance and 40 percent due to aerodynamic imbalance. These results compare quite favorably with the NREL 15 kW Combined Experiment wind turbine data.

Wear of bearing liners at low speed rotation of shafts with contaminated oil

An experimental and theoretical investigation was carried out to investigate the relationship between wear of bearing liners and low shaft speed, contamination, oil temperature, bearing load and time. Experimentally, it was found that oil with no external debris added produced only slight polishing of the liners. When the oil was contaminated with 0.02 wt.% iron or quartz particles of a known distribution (less than 32 μm diameter), increased wear was detected. Also, comparison of the results of the clean and iron-contaminated tests with those for quartz-contaminated oil show that the character of the worn liner area has changed from evenly rubbed zones to equally divided polished stripes in the circumferential direction. During the tests, the bearing friction was measured. In no test did the friction rise drastically, as would have been the case if severe wear and scoring had appeared. In actual applications, though, this sometimes occurs. This may be explained by severe contamination in industrial lubricating systems, with larger particles and higher debris concentration. A clean lubricating system is thus crucial in order to avoid heavy wear at low speeds. The experimental results were then compared with those indicated theoretically for film thickness at low speed. The theoretical minimum film thickness at low speed was much smaller than the size of the debris. The results indicate that if So −1⩽340, B D ⩾0.73 and C⩾0.18 mm, no severe wear will occur.

Cascaded harmonic synchronous machine

In a number of applications of generating and motoring, a low shaft speed is required. The usual way of providing this low speed is either through large diameter multipole machines or through high-speed machines in conjunction with a gearbox. A novel machine has been designed which can interface the mains to a low-speed shaft without the necessity for a large number of poles. The principle of operation is based on a stationary DC magnetic field, a short-circuited rotor winding and a stator winding connected to the supply through a set of filters. The rotation of the rotor causes circulation of harmonic currents in the stator. The filters are designed to allow these circulations, but to divert the 50 Hz current into the mains. This effect provides a speed gearing, the level of which depends on the number of harmonics permitted to circulate. The principle can be applied to a single or 3-phase design. The operational characteristics are those of a synchronous machine possessing a distinct number of synchronous speeds.

WEG's operational experience with two medium-sized wind turbines

The Wind Energy Group has been responsible for the design, construction, commissioning and operation of two medium-sized wind-turbine generators. The first of these on Burgar Hill, Orkney, was commissioned in September 1983, and has since operated synchronously with the grid for more than 9700 h (as of January 1987). The machine has a 20 m diameter, 2-bladed horizontal-axis rotor and is rated at 250 kW. It was designed as a test bed in that its rotor can be operated with a teeter hinge, or locked solid with the main shaft, and at fixed or varying rotational speed. The 200 sensors on this machine include strain gauges, crack-detection systems, displacement transducers and accelerometers. They are connected via fibre-optic cable to a computer-based data-handling system. The second machine was commissioned at its site near Ilfracombe, in Devon, during December 1984. It differs substantially from the Orkney machine in that it has been designed to meet the market requirement with respect to cost and specification. It has a 25 m diameter, 3-bladed horizontal-axis rotor of wood-epoxy construction, rigidly mounted to the low-speed shaft. Twenty of these machines were installed in Altamont Pass, California, in December 1986. The paper gives brief specifications of the two wind turbines and describes the operational experience gained. Experimental data is presented and, where appropriate, compared with theoretical predictions resulting from the use of specially developed computer codes.

Simulator Study of a Vertical Axis Wind Turbine Generator Connected to a Small Hydro Network

The paper describes a simulator study of a veritcal axis wind turbine generator connected to a small isolated hydro network. The connection to the network is done through a back-to-back de link, which is used as a static frequency changer. The penetration level of the wind turbine is in the range 13-33 percent of the rating of the ac network. The study includes the transient performance of the de link and the wind turbine drive during electrical braking and for system disturbances such as ac system faults. The study shows that the use of a stabilization signal in the de link helps to reduce large torsional vibrations in the wind turbine low speed shaft. These vibrations are observed after ac system faults. The study also shows that it is possible to obtain satisfactory operation of the overall system by adopting suitable control strategies. The study was performed as part of a Research Project (223G342) for the Canadian Electrical Association (CEA).

Simulator Study of a Vertical Axis Wind Turbine Generator Connected to a Small Hydro Network

The paper describes a simulator study of a veritcal axis wind turbine generator connected to a small isolated hydro network. The connection to the network is done through a back-to-back de link, which is used as a static frequency changer. The penetration level of the wind turbine is in the range 13-33 percent of the rating of the ac network. The study includes the transient performance of the de link and the wind turbine drive during electrical braking and for system disturbances such as ac system faults. The study shows that the use of a stabilization signal in the de link helps to reduce large torsional vibrations in the wind turbine low speed shaft. These vibrations are observed after ac system faults. The study also shows that it is possible to obtain satisfactory operation of the overall system by adopting suitable control strategies. The study was performed as part of a Research Project (223G342) for the Canadian Electrical Association (CEA).

Calculation of asbestographite-packed stuffing boxes of low-speed shafts and rods

Calculation of asbestographite-packed stuffing boxes of low-speed shafts and rods

Periodic ball behaviour in a deep groove ball-bearing under conditions of low radial load and low shaft speed

The actual ball behaviour during one cage revolution was investigated by using a new measuring method for three-dimensional ball motion. From the experimental results, periodic ball motion with slip was recognized and according to the differences in ball behaviour distinct peripheral regions of the bearing could be discerned.

Methods of Resynchronizing Wind Turbine Generators

The soft, low speed shaft which is common in the design of large wind turbine generators makes it impossible to reclose in time to prevent loss of synchronism following most electrical disturbances. In fact, reclosing is not suggested for the prototype units; instead, the initial recommended procedure is to stop the wind turbine generator, restart and then perform normal synchronization. This paper describes methods of resynchronization which markedly reduce the time to reestabLish normal operation following an electrical disturbance.

The circular form of the linear superconducting machine for marine propulsion

The superconducting linear synchronous machine (LSM) has been shown to be a highly efficient means of propulsion for application to advanced ground transport systems. We have shown that it would also appear to have significant advantages in marine engineering, e.g. for the propulsion of large commercial ships, tankers and military vessels, where the basic requirements of high torque at relatively low shaft speeds, together with the ease of reversibility, are readily met. A design utilising the circular form of the LSM as a ship's drive motor has been proposed and the details of a specific form of the LSM, namely the linear commutator machine (LCM) are discussed. The many advantages of this type of heteropolar machine are listed and compared with the superconducting homopolar motors which have been developed for ship propulsion. In particular, its flexibility in design, the built-in redundancy factors, hence reliability, together with other design details of the cryogenic magnets and the armature winding are described.

Performance of Gears Under High Loads and Low Velocities

To obtain information concerning the ability of conventional parallel-shaft gears to transmit motion smoothly at low velocities under high load, experiments were conducted on a single commercial gear mesh about the same size as the last stage proposed for a radio telescope drive. Two torque levels of 130,000 and 197,000 lb-in. were used. The largest amplitude of angular disturbance which was measured was less than 10 arc sec, while the average disturbance was about one arc sec at the low speed shaft. The smooth performance of the industrial-quality gear trains eliminated the expense of special designing. In similar applications where smooth motion is a requirement, components of good reliability and comparatively ready availability can be specified with confidence.