Browsing by Subject "field coil"
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Item An Assessment of Losses in the Field Coil of the Compulsator Under Dynamic Conditions(IEEE, 2003-01) Pratap, S.B.; Driga, M.D.Air core compensated pulsed alternators are being developed as compact power supplies for tactical electromagnetic gun systems. The field coil of the compensated pulsed alternator is a critical component that establishes the excitation magnetic field. Since the machine is air cored, the number of ampere-turns required from the field coil are significant. The rotating nature of the field coil requires that it be light so it can be supported under centrifugal loads. This implies that the current density in the field coil conductors is quite large. Charging the field coil too fast also results in transient losses due to proximity and skin effects. These must be accounted for in the design and simulation of these machines. During the discharge into the load, transient currents flow in the armature winding. These currents produce magnetic fields that interact with the field coils and produce additional losses. This calculation is complicated by the fact that there is relative motion between the conductors. This paper describes a two-dimensional numerical analysis that has been conducted to evaluate the losses in the field coil under these two dynamic conditions.Item Field coil insulation testing for pulsed power alternators(IEEE, 2007-01) Hearn, C.S.; Hahne, J.J.; Manifold, S.M.; Pish, S.P.Current pulse power alternator designs operate at high speeds and high current densities. The field coil insulation systems for pulse power alternators must provide sufficient stand-off voltage while limiting the amount of thermal resistance for actively cooled designs, and also withstand the strain excursions experienced at full operating speed. Repetitive cycling of the strain excursion overtime may induce cracks in the surrounding field coil matrix. The primary insulation surrounding the field coil conductors must be able to stop crack propagation that may develop in the surrounding matrix material and provide voltage hold-off. Thermally conductive thermoplastics are currently being investigated for the field coil matrix materials where decreased thermal resistance is necessary for actively cooled field coil designs. In order to evaluate conductor insulation and thermoplastic matrix filler quickly, small coupon motorettes were developed per previous designs. The motorettes simulate the hoop strain the conductors would experience due to rotor growth at high rotational speeds. Once the motorettes had been mechanically loaded, the coupon was hi-potted to verify insulation integrity under strain. This paper will discuss the design and testing of these motorettes to evaluate thermally conductive thermoplastics as filler material for the field coil matrixItem The High Voltage Homopolar Generator(IEEE, 1986-11) Price, J. H.; Gully, J. H.; Driga, M. D.A limitation of iron-core homopolar generators (HPG) is that the magnetic field strength and thus terminal voltage of the generator is dependent on the saturation limit of the material in the magnetic flux path. The Center for Electromechanics at The University of Texas at Austin (CEM-UT), in cooperation with GA Technologies, Inc. in San Diego, California, has designed and fabricated a 500 V, 500,000 A, 3.25 MJ, air-core pulsed homopolar generator. GA Technologies designed and constructed the 5 T, superconducting, solenoidal field coil. The stator subassembly, consisting of the rotor, bearings, stator, and output current conductors was designed and fabricated at CEM-UT. This experimental machine will be the first pulsed HPG with a superconducting field coil. Aspects of the machine design as well as the machine test program are discussed. Brushgear and bearing performance in high magnetic fields are also covered.Item Limitations on the Minimum Charging Time for the Field Coil of Air Core Compensated Pulsed Alternators(IEEE, 1991-01) Pratap, S. B.;Air core compensated pulsed alternators (compulsators) are being developed for a variety of field-based applications in relation with electromagnetic launchers. Since these applications are essentially field portable, minimizing system mass is of great importance. It is also desirable to use a room temperature field coil, since carrying cryogens (such as liquid nitrogen or liquid helium) onboard a vehicle has logistic problems. These requirements have led to the use of a self-excitation scheme using a room temperature field coil. A discussion is presented of the eddy currents induced in the shield of the passive compulsator and the circulating current induced in the compensating winding of a selective passive compulsator during the charge-up of the field coil. It is shown that reducing the charging time of the field coil greatly enhances the efficiency. In some cases the rate of charging may be the determining factor on whether the field coil is room temperature or cryogenic and whether it needs active cooling. There is, however, an optimum charging rate. Increasing the charging rate beyond this limit does not result in proportional benefitsItem Self Excitation of Iron Core Homopolar Generators(IEEE, 1986-11) Perkins, D.E.; Nalty, K.E.; Walls, W.A.In the interest of reducing homopolar generator (HPG) auxiliary requirements, a self-excited field coil for pulsed duty, iron-core HPG has been developed and tested at the Center for Electromechanics at The University of Texas at Austin (CEM-UT). In order to minimize rotor energy expended during excitation, a low-resistance, low-inductance coil was desired to allow field current to rise as rapidly as possible. A seven-turn field coil, having a nominal resistance of 500 µΩ was fabricated for the CEM-UT compact HPG system tester and subsequently tested. At 6,000 rpm, a field current rise time of 1 s was achieved and resulted in an average peak field density of 1.94 T. Only 300 kJ, about 13% of the 2.27 MJ stored in the rotor was required to fully excite the generator.Item Thermoplastic applications for pulse power alternators(IEEE, 2007-01) Hearn, C.S.; Hahne, J.J.; Liu, H-P.; Werst, M.D.The field coil is the primary component of the rotor assembly that provides the rotating magnetic field for the pulse power alternator. The design of the field coil is optimized so that it will produce the required magnetic field with minimum transient losses. The high currents required to produce the correct amp-turns, along with the mechanical loads due to high rotational speeds, present further design requirements for selection of field coil material, insulation, and surrounding material that completes the matrix of the field coil sub-assembly. With the addition of active cooling requirements in the field coil design, surrounding materials must be selected that retain electrically insulative properties and are thermally conductive to allow sufficient heat removal from the field coil. Thermoplastics are now being reviewed to replace traditional glass-epoxy potting compounds (thermosets) that have been used extensively in pulsed alternator designs. Fillers can be added to tailor properties of the thermoplastic, such as ceramics to increase thermal conductivity at the cost of an increase in density. Thermal analyses have been performed that show the benefits of using thermally conductive potting compounds. In addition, subscale field coil mockups (motorettes) have been encapsulated and tested to demonstrate encapsulation of current field coil geometries