Comparative analysis of direct-drive and gearbox-coupled electro-hydraulic energy converters

Combinations of electric and hydraulic machines, also known as e-pumps or electro-hydraulic energy converters, are essential for the electrification of mobile working machinery. Currently, these machines are typically combined by axial stacking, and the electric machine directly drives the hydraulic machine. Alternatively, the hydraulic machine can be radially integrated within the core of the electric machine, or a gearbox in combination with a downsized electric machine can be used. However, to the authors’ knowledge, no systematic comparison of these different concepts has been published. This paper uses analytical methods to determine the dimensions of the active parts of hydraulic machines, electric machines, and gearboxes in order to compare different design concepts based on volume, aspect ratio, total mass, copper mass, magnet mass, electromagnetic efficiency, and inertia. Axially stacked concepts can yield the highest compactness. However, they achieve this compactness at low aspect ratios, with their lengths being several times greater than their outer diameters. For balanced aspect ratios, where the outer diameter and total length of the machine are similar, the radially integrated, direct-driven concept is most compact. • Summary of research on electro-hydraulic energy converters (also known as e-pumps). • Analytical sizing of hydraulic machines, electric machines and gearboxes. • First systematic comparison of integration concepts incl. radial integration or gearboxes. • Comparison includes volume, aspect ratio, material masses, efficiency, and inertia. • Optimal concept depends on application needs and target aspect ratios.