The changing landscape of hydraulic drives is leading many fluid power specialists to quickly adapt to using variable speed pump drives. Optimum utilization of these drives requires, in many cases, additional system design considerations.
Despite ominous predictions in the ‘80s and ‘90s, the complete replacement of hydraulics with electrical drives did not occur — fluid power still plays an important role in modern drive technology. Moreover, it is impossible to imagine many fields of industry working without hydraulic drives.
The enormous power density makes hydraulic drives irreplaceable when large forces and high torques are required. Over the last 30 years “traditional” hydraulics has evolved into electrohydraulics, where the integration of electronics and closed-loop controls has become an industrial standard.
For years it was common to depict hydraulics as “strong but stupid.” Today this reputation no longer applies; hydraulics are not only strong, but also intelligent. Modern fluid power drives are perfect examples of highly integrated, mechatronic devices.
Drive for More Advanced Solutions
Ongoing competition with electrical drives and ever increasing requirements of performance and energy efficiency have led the hydraulic drive industry to new and more advanced system solutions.
The power control of electrohydraulic drives typically uses resistive or volumetric control.
In resistive control, the system is controlled by a valve, usually a proportional device, to throttle oil flow delivered from a power supply source. In volumetric control, flow is controlled by adjusting the displacement of a pump or by directly changing the drive speed of a fixed or variable displacement pump. Resistive control possesses excellent dynamic characteristics; however it exhibits poor energy efficiency. Significant power losses occur due to dissipation in the control valve. This is, in effect — hydraulic resistance.
There are a number of broad manufacturing trends that are influencing the increased interest in and potential value of variable speed pump drives:
Growth of Variable Speed Pump Drives
The combination of advanced electric drive technology and hydraulics opens a new chapter in electrohydraulic drive systems. Machine builders can now benefit from the traditional hydraulic characteristics of robustness and power density and additionally benefit from the well-known advantages of electric drives, i.e. — inherent drive “intelligence” and ease-of-integration with factory automation systems.
This changing landscape of hydraulic drives has pushed fluid power specialists to quickly adapt to using variable speed pump drives. But for many engineers, this may not be an easy step. Optimum utilization of variable speed pump drives requires, in many cases, additional system design considerations and modifications to a hydraulic system design.
Fluid power engineers need to understand the distinctive features of VFDs and the requirements of the production process being driven. This will define the critical points for the electric and hydraulic elements. The goal of the system is to meet the requirements of dynamics and accuracy, but also to maintain focus on overall energy efficiency. When applying a VFD-driven pump, using a conventional induction motor or a permanent magnet servo motor (PMM), the dynamic behavior will differ from a traditional system using a constant speed pump drive and throttling control valve.
System Design Options
Systems using variable speed pump drives can use a number of different design solutions. The selection of the design solution depends on a number of factors. The type of hydraulic circuit, the required response times and accuracy, the power required and other factors all come into play when deciding the best drive and circuit design.
Dynamic performance and power requirements determine the choice of the motor. For applications requiring the fastest response times and highest accuracies, a PMM may be the best selection. Today this technology is used extensively in plastic injection molding machines. These drive systems offer extremely high performance and have high productivity rates.
Due to their high power density and low drive inertia, PMMs have the highest acceleration capabilities. These high dynamics allow complex machine control tasks, such as direct force, speed and cylinder position control to be realized without proportional valves. The main limitation when using PMMs is maximum continuous output power — typically less than 60kW. Power units with higher output power, greater than 60kW, may require multiple PMMpump groups.
Standard asynchronous induction motors driven with VFDs can be used in higher power applications, or where direct control of high dynamic axes is not required. Using standard induction motors with VFDs, operating in a sensor- less vector control mode (no separate motor feedback device required), results in a cost-effective system. However, the system designer should be aware of the limitations of direct control using these drives.
The system response times can be long as a result of the high inertia of the induction motor. Variable speed pump drives using standard induction motors with VFDs are used today in the woodworking industry, on press applications, in plastics machinery, heavy industry applications and machining tool applications where the control task is typically regulating system pressure or flow.
Selecting Motor Size
The motor size and drive should be selected based on the pressure-flow-time cycle (p/Q profile) for the application machine. In practice, motors on standard HPUs are often sized on “corner power,” calculated from the maximum pump pressure and flow, and duty cycle is commonly not taken into account. This results in the installation of excess motor HP. If partial load comprises a significant portion of the machine cycle, the excess motor power can be significant.
Sizing PMMs for variable speed pump drives should follow methods commonly used in electromechanical actuator drives; calculating the root mean square (RMS) value based on the load torque and average drive speed. Once a pump size is determined, the motor torque and speed are calculated from the required pump pressure and flow. Additionally, the dynamic torque requirements for accelerating and decelerating the motor’s rotor and pump inertia needs to be added.
These calculated values are used to select the proper size drive and motor. Drive sizing and system optimization can be done using specialized tools, such as Bosch Rexroth’s SytronixSize, which allows analyzing p/Q profiles and motor load factors. For a more complex analysis, a numerical simulation can be used to model the system.
Dynamic simulations help by providing more insight to the system dynamics, such as pressurization dynamics, and to investigate interactions between the drive and the hydraulic system. Analysis of the performance of closedloop controls can be done as well. These dynamic simulations can be carried out using software that includes drive- and hydraulics-model libraries. Simulation tools, such as Bosch Rexroth’s Simster 3, ITI SimulationX or MATLAB/Simulink Simscape can be used for these requirements.
Pump Selection Criteria
When selecting the pump for variable speed operation, several important factors must be considered:
Improper pump selection or operation beyond allowed conditions can result in premature pump failure or suboptimal control performance. Pumps used with speed variable drives are typically internal gear or piston pumps. In some cases, properly selected vane pumps may also be used.
Internal gear pumps are characterized by high-efficiency, low-flow ripple, low-mechanical inertia and high-pressure capability. Due to these characteristics they are widely used with PMMs on injection molding machines. The limitations are fixed displacement and limits on the minimum drive speed during pressure holding operation.
Axial piston pumps, of either fixed or variable displacement design, have the desired property of high efficiency at low speed, making them ideal for pressure holding operation. Using a variable displacement pump allows the possibility of reducing the motor torque during pressure holding in the machine cycle. This can result in higher system efficiency since the motor and pump can operate under more desirable conditions. High-response variable displacement pumps, such as Rexroth’s DFEn 5000, can have reaction times an order of magnitude faster than a VFD- driven induction motor. Combining this pump with a VFDcontrolled, induction motor, results in greater efficiency and much higher system response than a sole variable speed alone; it is of greater value when used with higher HP-drive motors.
It is expected that in the future speedvariable pump drives will replace an increasing number of standard motors in hydraulic power units. This change is already utilized by many plastic machinery OEMs, and is quickly expanding in press applications. Understanding the principles of variable speed drives and the interaction between hydraulics and electric drives will be key to future fluid power engineering.
Article reprinted courtesy of the Bosch-Rexroth Corporation.