# Overflow Energy Loss Recovery System Based on Hydraulic Motor-Electric Generator
## 1 Introduction  
Hydraulic transmission has low energy efficiency. Taking the hydraulic excavator as an example, its energy utilization rate is only about 20%, and the efficiency of the hydraulic system is about 30% [1]. Therefore, the research on energy saving technology of the hydraulic system is of great significance [2]. The energy consumption of a hydraulic system mainly includes throttling loss and overflow loss [3]. Between of them, the overflow loss is mainly caused by the relief valve, which is used to limit the maximum working pressure of the system or to achieve constant pressure control. In the traditional system, the outlet of relief valve is connected to the oil tank. Since the pressure of the oil tank is approximately zero, the pressure drop at the inlet and outlet of the relief valve is the inlet pressure of the relief valve, while the pressure at the oil inlet is the target adjustment pressure of the user, which is set by the user and cannot be changed. The higher the overflow pressure level is, the greater the pressure loss at the valve outlet [4]. With the increasing pressure level of hydraulic system, the problem of overflow loss will be more prominent. At present, a lot of research has been carried out to reduce the overflow loss at home and abroad. However, most of these studies are from the overall system aspect, reducing the flow through the relief valve by the flow matching between the pump and the load, such as volume control technology [5; 6; 7], variable frequency technology [8; 9], and so on. However, there is still overflow in this technology. Aiming at the problem of overflow loss, [PERSON] et al. proposed an energy regeneration system through an accumulator to recover the energy of overflow loss. The accumulator was connected to the outlet of the relief valve to reduce the pressure difference between the inlet and outlet of the relief valve, reduce the overflow loss, and recover the overflow energy to the accumulator [10; 11]. However, in this method, the accumulator cannot becontrolled actively. Although the energy can be recovered, the energy recovery rate is limited. Besides, the parameter matching requirements of the accumulator are complicated. On the basis of the research of [PERSON] et al., this paper further proposes an energy regeneration system through a hydraulic motor and electric generator for overflow loss. A lot of research has been conducted on the energy regeneration system and control methods [12,13]. At present, energy recovery technology is mostly for negative load conditions. According to the different energy storage, energy regeneration system can be divided into mechanical type, hydraulic type and electrical type. Among of them, mechanical type uses a flywheel, spring or seed as energy storage device [14, 15, 16]. However, recovery time of the mechanical type is short, and energy storage capacity is limited. The hydraulic regeneration system uses an accumulator as an energy storage device [17,18]. However, because the accumulator cannot be controlled actively, the corresponding control valve block is needed in the system. As a result, the overall energy recovery rate is limited and the control is complex. The electric energy regeneration system is a combination of a hydraulic motor and generator. The generator is driven by a hydraulic motor to generate electricity. [PERSON] et al. studied the electrical energy recovery for a negative load of construction machinery, including boom lowering and rotary braking. However, the system adopts volume control. Compared with the original valve control, its damping coefficient is reduced and the overall controllability is poor [19, 20, 21, 22]. Moreover, the technology was aimed at the negative load condition and could not be directly applied to the overflow energy recovery system of the relief valve.