Response of power cables to fast transient loads
The most accepted standards for ampacity of power cables consider only steady-state loads. In "Cable Sizing for Fast Transient Loads," R.E. Henry, the transient temperature response of cables is addressed when installed in tray and conduits and undergoing fast transient heating. Herein, the fast transient work is extended by presenting how power cables respond to short heavy loads. Fast heating is usually found with variable frequency drives that deliver varying amounts of occasional non-periodic high torque, or with rectifiers that deliver occasional heavy currents to equipment, like melting of scrap in an arc furnace. Transient temperature rises are calculated for four sizes of cables subjected to spikes of kiloampere currents of short durations. These peak temperature rises are tabulated in a summary. With the rms value of a transient current profile and the NEC ampacity tables, this tabulation provides the means for a rapid cable size comparison, and cable size selection. The linearity and proportional responses of these peaks rises show that they are independent of how the cables are installed. Hence, this method of sizing feeders for fast transient loads applies to all installations of cables covered in the NEC ampacity tables.
Cable sizing for fast transient loads
The most recognized standards for sizing US manufactured cables are ampacity tables published in the National Electrical Code. These tables are only for steady-state loads. This paper addresses the transient temperature response of cables undergoing fast heavy loading. Exponential functions are analytically derived that express responses to step changes in cable currents. Formulas for coefficients and time constants are derived. Cable manufacturers' data and other sources for physical properties are used to calculate the coefficients and time constants for three copper conductor sizes. Steady-state end temperatures are based on National Electrical Code ampacity tables. The derived formulas are limited to cables in open tray or free air, and in conduit in air. The methods to develop data are detailed for other conductor sizes and aluminum conductors. Also presented are the means to size power cables based on calculated transient temperatures. These calculated temperatures are compared with cable temperature ratings to select a cable size. An example is included that illustrates sizing feeders for five 2000 hp crusher-conveyors.
Investigation of scatter radiation of non-circular X-ray scan trajectories
Modern interventional X-ray devices are highly maneuverable and allow complex 3D imaging trajectories. New scan trajectories enable not only optimized object sampling but also introduce new approaches for dynamic 3D imaging. In this paper, we investigated the scatter radiation of two different scan trajectories. The scatter radiation was sampled by a rail setup with dosimeters at defined positions. After data analysis, contour maps were generated which show for four exemplary heights the scatter radiation distribution. The alternative scan trajectory showed smaller dose exposures at heights correlated to upper torso and head areas of a model physician. Keywords—Image acquisition, alternative scan trajectory, circular tomosynthesis, scatter radiation distribution