About Constant Power Variable Pump And Constant Pressure Variable Pump

The functions of constant power pumps and constant pressure pumps differ significantly: constant power pumps ensure the motor does not exceed its power rating, providing high flow at low pressure and low flow at high pressure; constant pressure pumps can maintain pressure with zero flow.

1) Constant Pressure Pumps
Constant pressure pumps are typically used in hydraulic systems that require low-pressure rapid advance at the beginning, followed by slow approach, and finally stopping with pressure holding, such as in hydraulic presses. In such cases, the pressure set by the constant pressure pump is the pressure required for maintaining the system. This approach adheres to the principle that "the pressure in a hydraulic system is determined by the load and limited by the relief valve." Alternatively, it can be understood as "the system pressure is determined by the load and limited by the constant pressure pump." For instance, in a press, the reaction force of the workpiece can be large, and the exact pressure applied is adjusted by the constant pressure pump.

2) Constant Power Pumps
Constant power pumps are mainly used in machinery like construction equipment powered by a single engine to maximize its power utilization. In these systems, the pump delivers high flow at low pressure and low flow at high pressure. While this appears similar to constant pressure pumps, the two differ fundamentally. In constant power pumps, power remains constant as pressure and flow change. Conversely, a constant pressure pump operates as a fixed displacement pump with maximum flow until the preset pressure is reached. Once in constant pressure mode, it adjusts flow as needed to maintain a steady pressure.

3) Comparison of Variable Pumps

· Constant pressure variable pumps start varying their displacement only after reaching the preset pressure, with flow decreasing sharply.

· Constant power variable pumps vary their displacement across almost the entire pressure range to maintain consistent power output. Within the pump’s set power range, pressure increases while flow initially remains at maximum. Once the pressure exceeds the set point, flow decreases to maintain constant power output. However, actual power usage is not constant below the rated power.

· Electrically controlled variable pumps have their displacement curves determined by electronic controls, independent of actual pressure. Regardless of the pump type, matching the motor and pump power is critical.

4) Energy Efficiency and Applications
Constant pressure pumps excel in energy efficiency by maintaining pressure while adjusting flow. Constant power pumps are better suited for applications requiring variable speed based on load changes.

5) General Applications
In fixed industrial hydraulic systems, constant power pumps are rarely used. They are more commonly applied in mobile machinery (e.g., construction equipment) powered by engines to maximize power utilization. However, exceptions exist, as hydraulic systems vary widely.

6) Tailored Solutions
For systems with significant variations in both pressure and flow during cyclic or random operations, customized solutions are often employed.

For varying flow demands, options include:

1. Combining multiple fixed displacement pumps in parallel.

2. Using variable frequency drives with fixed displacement pumps.