Variable speed electric drive options for electric ships

Variable speed electric propulsion motors are operated from a variable frequency drive (VFD), which supplies power to motors at a frequency appropriate to the desired speed. These VFD's are normally very heavy and large in size. A variety of designs are commercially available but they have characteristics unsuitable for ship applications. The objective of this study was to evaluate various options for shipboard applications and to recommend designs that meet navy performance, weight and size constraints. It is possible to dramatically reduce the size and weight of the VFD by optimizing the VFD and the motor as a system, utilizing an optimal distribution voltage, eliminating distribution frequency transformers, and utilizing the weight and size reductions available with liquid cooling. Three systems were studied in detail: cycloconverters, series connected low voltage inverters, and multi-level medium voltage inverters using 6 kV class IGBT's or similar switching devices. On the basis of this evaluation, a multi-level diode-clamped pulse width modulated (PWM) drive topology is recommended for navy ships. A three phase distribution voltage of 6 to 9 kV RMS minimizes the number of series semiconductors and control complexities of the drive.