J.J. Hahne

Predicted versus actual performance of the model scale compulsator system

Performance testing of the model-scale CPA was completed at the University of Texas Center for Electromechanics. A major part of the project was the development of design and simulation codes that would accurately represent the performance of pulsed alternators. This paper discusses the components of the system and its operational sequence. Details of the performance simulation model are presented along with test data. The test result is compared to the predicted data.

Dynamic Load and Storage Integration

Modern technology combined with the desire to minimize the size and weight of a ships power system are leading to renewed interest in more electric or all-electric ships. An important characteristic of the emerging ship power system is an increasing level of load variability, with some future pulsed loads requiring peak power in excess of the available steady-state power. This inevitably leads to the need for some additional energy storage beyond that inherent in the fuel. With the current and evolving technology, it appears that storage will be in the form of batteries, rotating machines, and capacitors. All of these are in use on ships today and all have enjoyed significant technological improvements over the last decade. Moreover, all are expected to be further enhanced by todays materials research. A key benefit of storage is that, when it can be justified for a given load, it can have additional beneficial uses such as ride-through capability to restart a gas turbine if there is an unanticipated power loss; alternatively, storage can be used to stabilize the power grid when switching large loads. Knowing when to stage gas turbine utilization versus energy storage is a key subject in this article. The clear need for storage has raised the opportunity to design a comprehensive storage system, sometimes called an energy magazine, that can combine intermittent generation as well as any or all of the other storage technologies to provide a smaller, lighter and better performing system than would individual storage solutions for each potential application.

Wear of conductors in railguns: metallurgical aspects

The wear debris generated by an aluminum alloy armature sliding on copper rails was studied experimentally. The experimental parameters were set to maximize armature velocity. Preliminary rail and armature wear observations were documented as part of a database for solid armature development. A variety of debris collection and characterization approaches were used. Both in-bore debris and muzzle-exit debris were collected for size and chemical analysis. Debris constituents were copper-rich, and the compound Cu/sub 9/Al/sub 4/ was observed in the splat-quenched muzzle debris. The approach and method are detailed to indicate the potential of wear studies in defining the activity at the sliding interface. Some implications of rail surface roughening for zero wear measurements are also discussed, and the adoption of wear mechanism maps is proposed. >

High-speed compulsator stator thermal management

The compulsator stator armature winding includes multiple conductor layers. Each conductor layer is made of insulation wraps and transposed aluminum litz wire bundles with epoxy potting compound between the wires and the wire bundles. For a typical pulse-duty application, based on a room-temperature electrical loss calculation, the temperature rise in the aluminum winding is approximately 30/spl deg/C per shot. Adequate active cooling must be provided such that the stator insulation can survive thermally under a certain pulse repetition rate. As a result of the low equivalent thermal conductivities of the stator winding in the transverse direction, without compromising the overall structural integrity of the winding, the coolant passages have to be allocated as close as possible to the heat sources. For a given electric gun firing mission, four different cooling configurations have been investigated and analyzed by using water ethylene glycol mixture as an active liquid coolant. In this paper, thermal analysis cooling parameters, transient stator winding temperature distributions, and hot-spot temperature histories are presented, discussed, and compared.

Prediction of Windage losses of an Enclosed High Speed Composite Rotor in Low Air Pressure Environments

The frictional windage losses associated with non-ventilated airflows in the air gaps between the rotor and stator of a high speed rotating machine can greatly influence the rotor outer and stator inner surface temperatures. The characteristics of the radial and axial air-gap flows have been of general interest in many engineering applications. A rotating air gap flow is very complex, and in general, can be categorized as a continuum flow, slip flow, and free molecule flow, depending on the ratio of its mean free path to the air gap dimension. For a continuum flow between concentric rotating cylinders, secondary flow of rows of circumferential Taylor vortices in the air gap due to centrifugal flow instability of a curved flow at relatively high rotating speeds will typically be formed. As the air pressure in the air gap drops significantly, rarefied gas flow, departure from continuum flow, occurs when the mean free path becomes relatively large compared to the air gap dimension. This paper has developed and summarized an analytical approach to predict high speed windage losses (rotor tip velocities up to 900 m/s) at low rotor cavity air pressures (0.1 torr to 10 torr). The predicted transient windage losses at various air pressures and high rotor speeds are compared with measured windage losses generated in continuum and slip flow regimes. The agreements between the predicted and measured windage losses are relatively well.Copyright

Investigation of windage splits in an enclosed test fixture having a high-speed composite rotor in low air pressure environments

The University of Texas at Austin Center for Electromechanics (UT-CEM) has designed and conducted a series of composite rotor spin tests to measure the windage losses and temperature distributions of a test setup at high rotor speeds and low air pressures. The intent of the windage tests is to validate the windage loss predictions and investigate how the air-gap windage is distributed between the rotor and stator. The findings of the spin tests will then be used to perform windage-related thermal design and analysis of a high speed electrical machine. The radial air-gap flows under the test conditions, a low rotor cavity air pressure of 1 torr and high rotor surface velocities of 333 m/s and 614 m/s, were in a laminar flow regime. Transient rotor and stator finite element thermal analyses, using the measured windage losses and predicted laminar-flow windage splits, have been carried out to analyze the rotor and stator temperature distributions. This paper shows the detailed thermal analysis and compares the predictions with the measurements. The predicted and measured transient rotor and stator temperatures are in good agreements.

Fabrication and testing of a 30 mm and 90 mm laminated, high L' railgun designed and built at CEM-UT

With the emphasis on moving railgun technology from a laboratory based setting out into the field, a new lighter weight and stiff railgun structure was designed at the Center for Electromechanics at The University of Texas at Austin (CEM-UT). This innovative design takes advantage of stainless steel laminations with a composite overwrap to provide radial and longitudinal support for the rail/insulator package. The goal of this design effort was to build a 90 mm bore gun with a 9 MJ muzzle energy and provide comparable stiffness to the hydraulically prestressed 90 mm railgun at CEM-UT. The fabrication phase involved building a 90 mm/spl times/7 m long railgun in conjunction with a 30 mm/spl times/3 m subscale version for performance testing up to full design loads. The 30 mm subscale launcher has been successfully tested at CEM-UT up to its full current rating of 1.0 MA. This paper discusses the problems encountered during the final assembly of the 90 mm skid gun and how these difficulties were overcome. The paper also discusses the successful testing to full load of a 30 mm one-third scale version of the 90 mm gun. The paper concludes with a brief discussion on an additional 30 mm laminated gun which was redesigned based on experience with the previous laminated gun assemblies. >

Design and testing of a high-speed spin test for evaluating pulse alternator windage loss effects

Advanced pulse alternator designs require rotor surface speeds in excess of 1000 m/s. High tip speeds and an operating environment consisting of partial vacuum result in frictional windage losses and subsequent heating of the rotor and stator surfaces. Analytical models for cylindrical rotor windage loss exist. However, solving the combined fluid dynamics and thermal conduction problem for this specific operating regime requires significant code development. A series of spin tests with incremental levels of complexity have been designed and tested and are presented in this paper. The tests are intended to validate windage loss and heating codes used in the pulse alternator rotor design.

High strain insulation systems for compulsator rotors

As the power density requirement for new compulsator (CPA) designs increases, designers are driven to use more composites to reduce mass, spin the rotors faster to store more energy, and operate the machine at higher voltages to increase machine power output. In any particular compulsator design, the rotor windings are subjected to high strain levels as the rotor is spun and experiences radial growth. A critical component in the rotor winding design is the high voltage insulation. As the rotor is spun, the induced strains are applied to the insulation system on the coil conductors. This implies that over the operating life of a compulsator, the coil structure and the high voltage insulation must remain structurally intact, while undergoing repeated cyclic loading. This paper presents the design and testing of a compulsator rotor winding that has been recently fabricated at the Center for Electromechanics at The University of Texas at Austin. The paper focuses on the testing done both at room and elevated temperature to evaluate the winding structure and high voltage insulation system under both tensile and transverse strain conditions. Data presented suggests a factor of safety of at least five for strain to failure values and high voltage insulation good for at least twice line voltage after testing to strain failure.

Operating experience with the 90 mm railgun at CEM-UT

A 90 mm*10 m-long railgun has been operated at the Center for Electromechanics at the University of Texas at Austin (CEM-UT) with great success, and several improvements were recently added. The testing has involved various types of projectile packages, including split armature designs for midbody drive projectiles as well as base-push packages. An 8.1 MJ experiment was performed, as well as many lower energy tests, as part of an ongoing armature/projectile development program. One of the most important features added to the system was the development of thermal opening switches for smoother commutation into the gun and reduced acceleration loads on delicate projectile packages. Other new features include a copper-vapor laser for enhanced high-speed film data and a target X-ray system for capturing projectile data just before and after impacting the target. Both the copper-vapor laser and the target rays add to the excellent data collection and shot evaluation system. >