Rexroth hydraulic pumps – energy efficiency comparison of four pump control modes

1. Quantitative pump + relief valve system

This is the most basic and low-cost hydraulic pump system, because the displacement of the pump is fixed, and the output flow is fixed under the condition that the speed of the prime mover is unchanged; A portion of the flow drives the load, and the excess flow is returned to the tank through the relief valve, which is always in the overflow state. As a result, the dosing pump system will output full flow and maintain the maximum pressure set, which is a high-pressure full-flow state.

The energy efficiency diagram is as follows, the product of the flow rate and the pressure difference is the power, the useful power is the yellow part, and its energy is used to drive the load to do work, which is what we need. The energy loss is divided into two parts, one part is the overflow loss, and the excess high-pressure oil is returned to the oil tank through the overflow valve and converted into heat; The other part is the throttling loss, the greater the pressure difference on the throttle valve, the greater the energy loss.

Although the energy efficiency of the fixed pump + relief valve system is poor, it is also used in many occasions because of its low cost. Especially when the pressure of multiple loads is relatively close and the flow demand is also relatively close, the energy loss can also be well controlled when the displacement of the pump and the pressure setting of the relief valve can be well matched.

2. Constant pressure variable pump system

The system with constant pressure variable pump will output full flow when the system pressure is less than the set PC pressure value (maximum pressure/compensation pressure); When the system pressure reaches the set PC value, the pump is distorted to reduce the flow rate and keep the system pressure at the PC set value. Under normal circumstances, the constant pressure variable pump system is in high pressure and small displacement when standby, and its ideal working interval is in the section of the P-Q curve downward (PC pressure setting range), in this section, the displacement of the pump will be automatically adjusted according to the needs of the system, so as to output only the flow required by the load, no excess flow waste, but the pressure is maintained at the PC set value.

Compared with the fixed displacement pump system, because the constant pressure variable system only outputs the flow required by the load, there is no overflow loss, only throttling loss, which is more energy-efficient.

The constant pressure variable system maintains high pressure throughout the piping system during standby, and is suitable for certain applications that require high response. Because of its highest pressure control, it is also commonly used for pressure cut-off protection in hydraulic systems.

3. Load-sensitive variable pump system

With a load-sensitive variable pump system, the pump output flow will automatically adjust according to the load needs, and only output the flow required by the load, so that there is no excess flow waste. The pressure of the load is fed back to the pump through the load sensing valve, and the output pressure of the pump is only slightly larger than the maximum load pressure, and the large degree is determined by the differential pressure valve on the pump, which is generally 20-30bar.

The pressure difference between the output pressure and the load pressure in a load-sensitive system remains constant and only a little higher (20-30 bar) than in a constant pressure pumping system. There are no overflow losses, throttling losses are smaller, and thus more energy efficient.

In construction machinery, load sensing systems are usually used, which has good energy-saving effect and fuel economy can be guaranteed.

Fourth, the electric proportional displacement variable pump system

The electric proportional displacement pump actively controls the displacement of the variable pump through electrical signals, thereby regulating the flow output of the pump. If the electrical signal remains constant, it can be seen as a fixed pump mode. Therefore, for system designers, it is necessary to actively adjust the control current to adjust the flow rate according to the actual working conditions and the flow demand of the load, so as to have better energy efficiency performance.

Although each pump control method has its own characteristics, it is not only used independently. The same pump can be integrated in a variety of ways, such as load sensing + constant voltage variable, electric proportional displacement + constant voltage variable, electric proportional displacement + load sensitive, etc., so designers need to make the best match according to the actual needs of the system to achieve the best system performance.

Rexroth hydraulic pumps – energy efficiency comparison of four pump control modes

Rexroth hydraulic pumps – energy efficiency comparison of four pump control modes

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