Analogy Between Hydraulic Transformers and Variable-Speed Pumps

Abstract

The increasing focus on energy efficient systems and energy recuperation functionalities calls for multi actuator hydraulic systems to be tightly integrated with regards to energy distribution. Such systems need to allow power to pass back and forth between loads and the supply, while also enabling the ability to store energy. Here two approaches are obvious; namely the usage of hydraulic transformers interconnected via common pressure rails containing accumulators, and variable-speed pumps interconnected via a common electric dc-bus containing capacitors and/or batteries, both having the potential for energy storage and power sharing. A main question is when to apply which of these technologies, when considering specific requirements to actuator dynamics and control features. This paper presents an investigation into the analogy between hydraulic transformers and variable-speed pumps. The investigation takes offset in model based analyses of these technologies regarding their actuation dynamics related to torque generation, input-to-output pressure dynamics with and without a cylinder load. Finally a numerical study is presented, verifying and comparing their transient characteristics and energy losses. It is found that, disregarding the conventional speed control loop in variable-speed pumps, these are in fact equivalent with hydraulic transformers regarding secondary control abilities, dynamics and energy recovery, wheres hydraulic transformers generally must be expected to be more efficient than variable-speed pumps.

Electro-Hydraulic Variable-Speed Drive Networks—Idea, Perspectives, and Energy Saving Potentials

“…The relation between the core loss P core and the rotor speed may be approximated by the proportionality P core ∼ ω 3/2 m [60]. In addition, the core loss magnitude may be described by some scalar ε c of the copper loss at nominal conditions, i.e., P core,nom = ε c P cu,nom .…”

“…Using the pressure dynamics Equation (57), and constructing the combined flow decoupling and load and sum pressure control schemes from principles similar to those of the VDN-PP and VDN-L, the shaft speed references appear as Equations ( 58)- (60):…”