Pump-Controlled (Volumetric Speed Control) Systems
Efficient, precise hydraulic control solutions for industrial applications
Introduction to Volumetric Speed Control
Volumetric speed control systems operate through various variable pumps combined with related variable control valves to regulate speed. A key advantage of these systems is their minimal energy loss and high efficiency. The hydraulic pump with motor configuration allows for multiple complex control functions such as constant pressure, constant flow, and combined p+q+P (where P represents power) control.
Although their response speed is relatively slow, these systems meet the requirements of most industrial applications. Volumetric speed control works by continuously adjusting the input and output flow rates of hydraulic pumps or hydraulic motors, ensuring the system flow matches the load flow requirements of the actuators. Particularly in systems where the hydraulic pump flow is regulated, overflow energy loss is effectively avoided, reducing heat generation significantly. The hydraulic pump with motor arrangement is central to this efficiency.
Depending on specific speed control requirements, different configurations can be selected: fixed-displacement pump with variable motor, variable pump with fixed motor, or variable pump with variable motor. Each configuration offers distinct advantages, but all contribute to significantly improved system efficiency. Additionally, this speed control method provides high precision and excellent dynamic performance, though it comes with a relatively higher cost and more complex structure compared to other systems. The hydraulic pump with motor combination remains the heart of these systems, ensuring optimal performance across various industrial applications.
Key Advantages
- High energy efficiency with minimal losses
- Multiple control capabilities (constant pressure, flow, power)
- Reduced heat generation compared to other systems
- Precise control and good dynamic performance
- Flexible configuration options to meet specific application needs
Volumetric Speed Control Configurations
1. Variable Pump - Fixed Motor Volumetric Speed Control
The variable pump-fixed motor volumetric speed control circuit adjusts speed by changing the displacement (Vp) of the variable pump. The motor output torque (T) and circuit operating pressure are determined by the load torque. When the load torque is constant, the motor torque remains constant, earning this circuit the name "constant torque speed control circuit." The hydraulic pump with motor setup here ensures consistent torque delivery regardless of speed variations.
In this circuit, the operating pressure of the hydraulic pump essentially equals the load pressure, and the pump's output flow matches the system's required flow. This eliminates nearly all operational overflow, resulting in high working efficiency. The variable pump's displacement can be adjusted to very small values, providing a wide speed control range. The hydraulic pump with motor combination here is particularly well-suited for constant torque applications.
Typical applications include hydraulic systems for small internal combustion locomotives, construction machinery, marine winches, and similar equipment where maintaining consistent torque across speed ranges is critical. The hydraulic pump with motor configuration ensures that these applications can operate efficiently under varying load conditions while preserving torque output.
Key Parameters:
- T: Motor output torque
- n: Motor output speed
- P: Motor output power
- Vp: Pump displacement
Figure 1-9: Variable Pump - Fixed Motor Characteristics
Performance characteristics showing torque, speed and power relationships
2. Fixed Pump - Variable Motor Volumetric Speed Control
The fixed pump-variable motor volumetric speed control circuit adjusts the hydraulic motor's output speed (n) by changing the displacement (Vm) of the variable hydraulic motor. In this configuration, the hydraulic pump delivers a constant flow rate, and the motor speed is inversely proportional to its displacement. The hydraulic pump with motor arrangement here prioritizes consistent flow delivery while allowing speed adjustment through motor displacement changes.
The motor's output torque (T) is directly proportional to its displacement (Vm). When the load torque is constant, both the circuit's operating pressure and the motor's output power remain unchanged regardless of speed variations, classifying this as a "constant power speed control circuit." The hydraulic pump with motor combination maintains power output consistency across different operating speeds.
A consideration with this configuration is that as speed increases, internal frictional resistance grows while output torque decreases. If torque becomes too low, the system may fail to drive the load, resulting in a relatively narrow speed control range. Additionally, this setup cannot achieve motor reversal. The hydraulic pump with motor configuration here finds its primary applications in hydraulic systems requiring constant power, such as winding devices in paper and textile machinery.
Key Parameter:
- Vm: Motor displacement
Figure 1-10: Fixed Pump - Variable Motor Characteristics
Performance characteristics of constant power speed control circuit
3. Variable Pump - Variable Motor Volumetric Speed Control
The variable pump-variable motor volumetric speed control circuit combines the advantages of both previous configurations. During startup, the hydraulic motor displacement (Vm) is set to maximum, while the pump displacement (Vp) is gradually increased from minimum to maximum. This allows the motor speed to rise while increasing output power, achieving high torque at low speeds. The hydraulic pump with motor combination here offers the versatility of both variable elements working in harmony.
In this phase, with approximately constant load, the motor output torque remains constant regardless of speed changes, placing the system in a constant torque operating state. During normal operation, the pump displacement is maintained at maximum while the variable motor's displacement adjusts automatically based on load changes. When load increases, motor displacement increases; when load decreases, motor displacement decreases. This keeps system pressure relatively stable, resulting in minimal fluctuation in pump outlet pressure and placing the system in a constant power operating state. The hydraulic pump with motor configuration here optimizes performance across both operating states.
Neglecting system efficiency, the motor's output power equals the pump's input power, remaining essentially constant. The ability to adjust both hydraulic pump and motor displacements results in an expanded speed control range. In practical applications, the adjustment process typically reverses: during operation, as external load increases, the motor displacement is adjusted first (constant power speed control phase). When the motor reaches maximum displacement and still cannot adequately adapt to load changes, the pump displacement is then adjusted (constant torque speed control phase).
The speed control range of this circuit equals the product of the variable pump's speed range and the variable motor's speed range, resulting in an exceptionally wide range. This makes the hydraulic pump with motor configuration ideal for applications requiring both constant power and constant torque, such as hydraulic systems in port lifting and transportation machinery and mining excavation equipment.
Figure 1-11: Variable Pump - Variable Motor Characteristics
Combined characteristics showing both constant torque and power regions
Application Comparison of Configurations
Variable Pump - Fixed Motor
Key Characteristics:
- Constant torque output
- Wide speed range
- High efficiency
Typical Applications:
- Small internal combustion locomotives
- Construction machinery
- Marine winches
- Material handling equipment
The hydraulic pump with motor in this configuration excels in applications requiring consistent torque across varying speeds.
Fixed Pump - Variable Motor
Key Characteristics:
- Constant power output
- Narrower speed range
- Cannot reverse motor direction
Typical Applications:
- Paper machinery
- Textile machinery
- Winding devices
- Unwinding equipment
This hydraulic pump with motor setup is ideal for applications requiring consistent power across speed variations.
Variable Pump - Variable Motor
Key Characteristics:
- Combines constant torque and power
- Very wide speed range
- Highly versatile performance
Typical Applications:
- Port cranes
- Mining machinery
- Heavy construction equipment
- Industrial presses
The hydraulic pump with motor in this configuration provides maximum flexibility for complex industrial applications.
Efficiency Analysis
One of the primary advantages of volumetric speed control systems is their superior energy efficiency compared to other hydraulic control methods. By matching pump output directly to load requirements, these systems minimize energy losses that would otherwise occur through pressure relief valves or throttling.
The hydraulic pump with motor combination is crucial to this efficiency. In variable pump systems, energy waste through overflow is virtually eliminated because the pump output is adjusted to precisely match the system's needs. This not only reduces energy consumption but also minimizes heat generation, extending component life and reducing cooling requirements.
While initial costs are higher than less efficient systems, the hydraulic pump with motor configuration provides significant long-term savings through reduced energy consumption and lower maintenance costs. This makes volumetric speed control systems particularly cost-effective for applications with continuous or heavy-duty operation.
Conclusion
Volumetric speed control systems represent a sophisticated approach to hydraulic control, offering significant advantages in energy efficiency and performance versatility. The three primary configurations—variable pump with fixed motor, fixed pump with variable motor, and variable pump with variable motor—each provide distinct characteristics that make them suitable for different industrial applications.
The hydraulic pump with motor combination forms the core of these systems, enabling precise control over speed, torque, and power output as required by specific applications. While these systems involve higher initial investment and greater complexity, their efficiency advantages and performance capabilities make them the preferred choice for many industrial applications where precise control and energy efficiency are paramount.
As industrial processes continue to demand higher efficiency and more precise control, the role of volumetric speed control systems will only grow in importance. The hydraulic pump with motor configuration will remain central to these systems, providing the flexibility and performance needed to meet evolving industrial requirements.