John A. Mallick

Design, Construction, and Testing of an Inductive Pulsed-Power Supply for a Small Railgun

Advances in high-power-density batteries have rekindled interest in using inductive store as a pulse compression system. Although these batteries are considered very power dense, they lack over an order of magnitude of power density to drive a deployable electric gun. However, one can add an inductive circuit to a battery bank to make a hybrid system that has a much higher power density than batteries alone. A battery-inductor hybrid pulsed-power supply boasts several advantages over pulsed alternators, as inductors are static and relatively easy to cool. Inductors are potentially more energy dense than capacitors, making a battery-inductor hybrid pulsed-power supply an attractive alternative to capacitor-based pulsed-power supplies. The opening switch has been a major obstacle in previous inductive store projects, but in simulation, a new circuit topology-the Slow Transfer of Energy Through Capacitive Hybrid (STRETCH) meat grinder-greatly attenuates the problem. This paper discusses the design, construction, and testing of a small-scale STRETCH meat grinder system, which was successfully used to power a miniature railgun

A Review of the Integral-Horsepower Switched Reluctance Drive

An evaluation of the capabilities of the switched reluctance (SR) motor drive, particularly in small integral-horsepower sizes, is presented, and some of its special features are discussed. The simplicity of the construction of the rotor together with certain advantages in the power circuit, such as unipolar operation and the independence of the phases, are described along with some of the important performance parameters, which are compared with those of typical induction motor drives. It is shown that the ruggedness and simplicity of the SR drive are accompanied by a performance profile that matches that of modern induction motor drives in torque per unit volume, efficiency, converter voltampere requirements, and other parameters. A comparison of three SR motors, including one low-inertia design and one with two stator teeth per pole, shows torque/inertia ratios several times greater than those for induction motor drives.

Modeling of Solid Rotor Turbogenerators Part I: Theory and Techniques

This paper discusses techniques for modeling turbogenerators based upon measured machine properties. Of specific concern is the two port nature of the d- axis and the consequent requirement upon proper identification of the field winding properties. Techniques for deriving models from frequency response measurements are presented.

Stretch Meat Grinder: A Novel Circuit Topology for Reducing Opening Switch Voltage Stress

The slow transfer of energy through capacitive hybrid (STRETCH) meat grinder is an inductive- capacitive current multiplication circuit that reduces switching requirements and achieves a high degree of current multiplication while possessing an energy density approaching that of a purely inductive system. Initially, the STRETCH meat grinder operates like a single-stage meat grinder; it increases the current through an inductor by switching out a coupled inductor. However, during switching in generic meat grinder circuits, leakage flux caused by imperfect coupling and the sudden change in current induces a voltage across the opening switch well beyond what modern solid-state switches can handle. The STRETCH meat grinder mitigates these problems by using a capacitor to recapture the energy in the leakage flux and to slow down the turnoff of current in one of the inductors. The energy from the leakage flux is then used to reverse the current on the turned-off inductor, thereby further increasing the current multiplication. A system comprising several STRETCH meat grinders in parallel can develop currents in the mega-ampere range without exceeding the capabilities of solid-state switches. Such a system could be used to power a railgun.

Active-passive gradient shielding for MRI acoustic noise reduction

An important source of MRI acoustic noise—magnet cryostat warm‐bore vibrations caused by eddy‐current‐induced forces—can be mitigated by a passive metal shield mounted on the outside of a vibration‐isolated, vacuum‐enclosed shielded gradient set. Finite‐element (FE) calculations for a z‐gradient indicate that a 2‐mm‐thick Cu layer wrapped on the gradient assembly can decrease mechanical power deposition in the warm bore and reduce warm‐bore acoustic noise production by about 25 dB. Eliminating the conducting warm bore and other magnet parts as significant acoustic noise sources could lead to the development of truly quiet, fully functioning MRI systems with noise levels below 70 dB. Magn Reson Med 53:1013–1017, 2005.

A comparison study of low-profile power magnetics for high-frequency, high-density switching converters

The electrical and mechanical issues critical to the design of low-profile power magnetics are discussed. The disk and tube-type designs are evaluated over the frequency range of 0.5-10 MHz by an optimization process to maximize the efficiency for a given foot-print and height. The tube-type transformer is shown to have a higher power density at a lower height than the disk-type transformer, operating at the same efficiency at 1 MHz. The tube-type design allows the fabrication of transformers with heights as low as 50 mils.

Three Dimensional Transient Analysis of Superconducting Generators

Techniques for the transient analysis of a superconducting alternator are described. These techniques are based upon a three dimensional analysis. This transient simulation technique is felt to be a significant improvement over previous techniques which are based upon two-dimensional or quasi two-dimensional analyses. Typical simulation results are presented.

The Design and Testing of a Large-Caliber Railgun

A large-caliber railgun was developed to demonstrate the supersonic launch of 120 mm projectiles. A trade study that evaluated over 70 different railgun configurations and geometries resulted in the selection of a high-inductance-gradient, multi-turn configuration as the best overall choice. Subscale tests were performed on both the railgun and launch package configurations. A full-scale laboratory system was installed that included a dedicated high-current, large-caliber breech and gunline. A full-scale launcher was successfully tested to beyond the design conditions.

Use of SABER circuit simulation software for the modeling of compensated pulsed alternators driving a railgun load

A railgun launcher requires very high current pulsed power over the period of a few milliseconds. For laboratory systems, capacitor banks have traditionally been used to provide this energy, but field applications require a system with significantly higher energy storage density. Rotating machines that store the energy in the form of rotational kinetic energy and can quickly convert that energy to high current electrical energy have been designed and built. These low-impedance, multiphase, multipole synchronous generators are referred to as pulsed alternators. The AC output of the pulsed alternator is rectified to provide DC to power the railgun. The design of the rectifier set and control circuitry is very dependent on the alternator characteristics; to facilitate the design and evaluation of the overall pulsed power system, a modeling tool which accurately represents the performance of the pulsed alternator while allowing easy changes to the external circuitry and controls is needed. This paper describes a manner in which these pulsed alternators can be accurately modeled. The simulation platform SABER was chosen because of the robust modeling engine, the ease of integrating mechanical components, and the large library of existing models for a wide range of electrical components.

Modification and Testing of a Battery-Inductor Repetitive Pulsed Power Supply for a Small Railgun

Pulsed power supplies for electromagnetic launch have unique requirements, including very low impedance, relatively long pulse discharge times, and high stored energy. The three primary types of pulsed power systems for electromagnetic launch have been capacitive, inductive, and rotating machine. This paper describes recent results in the design, construction, and testing of a pulsed power system based on an inductor that is charged by batteries.