System Considerations for VSDs in Industrial Machinery

Despite ominous predictions in the 1980s and '90s, the complete replacement of hydraulics with electrical drives did not occur, and fluid power still plays an important role in modern drive technology.

When large forces and high torques are required, hydraulic drives are irreplaceable because of their enormous power density. During the last 30 years, the field of traditional hydraulics has evolved into electro-hydraulics, the integration of electronics and closed-loop controls. This integration has become an industrial standard. Combining strength and intelligence, modern fluid power drives are perfect examples of highly integrated mechatronic devices.

Advanced Solutions

Ongoing competition with electrical drives and ever-increasing performance and energy efficiency requirements have led the hydraulic drive industry to more advanced system solutions.

Electrohydraulic drives typically use resistive or volumetric power control. In resistive control, a valve—usually a proportional device—controls the system by throttling 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 has excellent dynamic characteristics but poor energy efficiency. Heat dissipation from throttling the hydraulic fluid in the control valve can cause significant energy costs associated with cooling the hydraulic equipment, fluid reservoir and equipment spaces.

A number of broad manufacturing trends are influencing the increased interest in and potential value of pump variable speed drives (VSDs).

Rising costs of energy and increased awareness of environment issues has resulted in new trends in drive systems. During the last two decades, improvements in electrohydraulic efficiency were a main goal of many companies and research institutes. Given advances in drive technology and closed-loop control, efficient volumetric control using speed variable pump systems is replacing the resistive throttling method.

Reducing throttling losses lowers the amount of heat transferred to the oil and oil reservoir. Lower heat generation allows for a smaller capacity cooling system and lower parasitic power needed to maintain optimum oil temperature.

Using variable frequency drives (VFDs) with hydraulic power units can also lower average noise emissions, as the pump speed is reduced during partial load operations, such as pressure holding or idling. The average sound level can be lowered by as much as 10-20 dB(A), improving environmental working conditions.

Growth of VSDs

The combination of advanced electric drive technology and hydraulics opens a new chapter in electrohydraulic drive systems. Machine builders can benefit from both the traditional hydraulic characteristics of robustness and power density and the advantages of electric drives: inherent drive intelligence and ease of integration with factory automation systems.

Image 1. A refit and customized hydraulic system (Images courtesy of Bosch Rexroth Corporation)

This changing landscape of hydraulic drives pushes fluid power specialists to quickly adapt to using VSDs. For many engineers, this is not easy. Optimum use of VSDs often requires additional considerations and modifications to a system design.

Fluid power engineers must understand the distinctive features of VFDs and the requirements of the production process to define the critical points for the electric and hydraulic elements. The goal of the system is to meet the requirements of dynamics and accuracy while maintaining overall energy efficiency. When applying a VFD-driven pump, using a conventional induction motor or a permanent magnet servo motor (PMM), dynamic behavior will differ from a system using a constant speed drive and throttling control valve.

System Design Options

Systems using VSDs can use a number of different design solutions. The selection of the best drive and circuit design depends on a number of factors, including the type of hydraulic circuit, the required response times and accuracy and the power required.

Dynamic performance and power requirements determine the choice of the motor. For applications requiring the fastest response times and highest accuracies, a PMM—used extensively in plastic injection molding machines—may be the best selection. These drive systems offer extremely high performance and have high productivity rates.

Because of 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 60 kilowatts (kW). Units with output power greater than 60 kW may require multiple PMM-pump groups.

Standard asynchronous induction motors driven with VFDs can be used in higher power applications, where direct control of high dynamic axes is not required. Using standard induction motors with VFDs, operating in a sensorless vector control mode (no separate motor feedback device required), results in a cost-effective system. The designer, however, should be aware of the limitations of direct control using these drives, including long system response times that result from the high inertia of the induction motor.

VSDs using standard induction motors with VFDs are used in the woodworking industry, on press applications, in plastics machinery, in heavy industry applications and for machining tool applications for which 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 hydraulic power units are often sized based on corner power, which is calculated from the maximum pump pressure and flow. Duty cycle is commonly not taken into account.

This results in the installation of excess motor horsepower. If partial load comprises a significant portion of the machine cycle, the excess motor power can be significant.

When sizing PMMs for VSDs, users should follow methods commonly used in electromechanical actuator drives, including 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.

The dynamic torque requirements for accelerating and decelerating the motor's rotor and pump inertia also should be added. These calculated values are used to select the proper size drive and motor. Specialized tools, which can analyze p/Q profiles and motor load factors, can be used for drive sizing and system optimization. For a more complex analysis, a numerical simulation can be used to model the system.

Dynamic simulations provide more insight into system dynamics, such as pressurization, and interactions between the drive and the hydraulic system. These simulations also help with analysis of the performance of closed-loop controls. These dynamic simulations can be carried out using software that includes drive and hydraulics model libraries.

Image 2. A core shooter at a foundry

Figure 1. Using variable displacement pumps integrated with VFDs allows more efficient operation during partial load conditions, when compared with a standard variable displacement pump running at constant speed.

Figure 2. In recent years, the cost of using variable speed drive technology has become more economical, making it effective to combine variable speed electric drives with hydraulics.

Pump Selection Criteria

When selecting a pump for variable speed operation, several important factors must be considered:

• Is the pump construction suitable for variable speed operation?
• What are the minimum and maximum allowed revolutions per minute for the pump?
• Can the pump be used bi-directionally, and, if so, are there any pressure and speed limitations?
• Is the pump suitable for start and stop operation?
• What is the maximum operating pressure allowed for the pump, and does pressure affect the maximum allowable pump speed?
• What fluid will be used in the system, and does it limit the pump's pressure and speed, based on viscosity and pump lubrication?
• What is the mechanical and volumetric efficiency of the pump at the design operating points?
• What is the pump acoustic noise level based on expected speed, pressure and displacement?
• What will the pressure and flow pulsations be over the range of operating speeds?
• Is the hydraulic circuit open or closed?

Improper pump selection or operation outside allowed conditions can result in premature pump failure or suboptimal control performance. Pumps used with VSDs 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. Because of 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, 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 to lower 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 operating conditions. High response variable displacement pumps can have reaction times an order of magnitude faster than a VFD-driven induction motor. Combining this type of pump with a VFD-controlled induction motor results in greater efficiency and much higher system response than does variable speed alone. This is of greater value when used with higher horsepower drive motors.

VSDs are expected to replace an increasing number of standard motors in hydraulic power units in the future. This change has already been adopted by many plastic machinery manufacturers and is quickly expanding in press applications. Understanding the principles of VSDs and the interaction between hydraulics and electric drives will be key to future fluid power engineering.