Not all drives are the same. There are different options that are better for some applications than others. Here’s a handy guide to help you decide. #Hint processing
The latest generation of plastics processing machines are increasingly incorporating a range of all-electric drive systems, such as variable frequency drives and servo drives, into their designs to improve energy efficiency and operational flexibility.
New hybrid drive concepts for plastics processing machines, such as Rexroth’s Sytronix variable speed pump drive system, combine intelligent control electronics and variable electric drives with powerful, energy-efficient hydraulic pumps.
In some injection molding applications, such as the blow molding machine shown here, electric servo drives are used with variable displacement pumps to accurately and dynamically control pump speed with performance comparable to other advanced servo valve control technologies. All devices are comparable.
The increase in component cost associated with the use of a variable frequency drive can be offset by lower energy costs: when comparing the power consumption of a 125cc axial piston pump, cm, driven by a high-performance engine with a constant speed of 100 hp.
The cost of using variable speed drive technology has become more economical in recent years, falling by 75% compared to 1995 and 2010, making the combination of electric variable speed drives with hydraulic systems an energy efficient and high performance alternative suitable for many applications.
In today’s plastics processing equipment, press power can take many forms and methods. Traditionally, most machines are hydraulic driven, where a hydraulic pump driven by a fixed (constant speed) electric motor drives most of the machine’s processes.
Over time, incremental improvements in efficiency, accuracy, and speed have been introduced. In the 1990s, a new technology for injection molding machines emerged: an all-electric motor that uses electric servo motors to drive gears, racks, and ball screws to control most of the machine’s functions.
The all-electric motor is characterized by low noise, high operating speed and the absence of some hydraulic problems, especially problems related to leakage in fluid handling equipment and process equipment. With all the benefits of all-electric motors, there are limitations such as machine load capacity and increased maintenance requirements for physical components due to wear on ball screws, belts and gears.
New drive technologies New drive technologies for large-capacity injection molding machines are now available that offer higher speeds, significantly improved energy efficiency and greater reliability. These new technologies are based on existing electrical drive systems found in all machines and can be grouped into three main types:
1. Variable frequency drive (VFD) with V/f technology 2. VFD drive uses vector drive technology 3. Electric servo drive
These advanced drive technologies, such as servo drives, include various electronically controlled components and are more expensive to install than simple fixed speed electric motors driving hydraulic pumps. As a processor, it is useful to evaluate the benefits that can be realized with these technologies in terms of energy efficiency, operational flexibility, and overall system control and efficiency.
Frequency converter: The frequency converter generates output power to drive a traditional AC induction motor. Using the V/f technique, it is possible to change the output frequency by changing the engine speed accordingly. Due to the electrical characteristics of the motor, the output voltage also changes in proportion to the frequency, hence the name V/f. This ratio helps keep the engine torque within its useful operating range.
These drives are relatively low cost, but can only provide about 10% motor speed accuracy under load. They have been used to power extruders for many years. Speed accuracy is more consistent with constant loads, and load changes can be improved by adding a tachometer to measure speed. However, the dynamic performance (the ability to respond to and control changes in pressure and speed) is low and suitable mainly for slowly changing processes. VFD drives using V/f technology have low speed limits and can only be used at 400-500 rpm.
Vector drive technology: In the 1980s, a new VFD technology emerged called vector drive technology. A microcontroller can control the drive output to accurately control the torque of a standard AC induction motor—even at zero rpm when the motor is at rest. This technology makes it possible to control motors on the principle of a servo drive with little additional cost.
The high inertia of asynchronous motors is a factor that limits the dynamic performance of drives. Sensorless vector VFDs provide high speed accuracy up to 200-300 rpm. Adding an encoder sensor to the motor allows the vector drive to operate at zero RPM and allows the motor shaft to be positioned electronically with high accuracy.
While both types of VFDs are capable of a wide range of applications, there are some technical design considerations. The driven asynchronous motor must be designed for an “inverter load” providing internal grounding of the shaft. This prevents high frequency currents in the VFD from being generated on the motor shaft and frame, which could damage the motor bearings.
In addition, low speed, high torque operation can cause destructive engine heat unless auxiliary air or water cooling of the engine is used. High speeds also have limitations, typically limiting top speed to two and a half to three times the engine’s rated RPM.
Servo drive technology: Electric servo motors are permanent magnet AC motors designed with minimal inertia and peak torque of two to three times the steady state torque, allowing for highly dynamic speed changes. High speed ranges, maximum torque at zero speed and extremely high dynamic performance are the hallmarks of servo motors.
This technology requires more sophisticated drive electronics, but this is offset by the ability to precisely control the speed, torque, and position of the servomotor. As with VFD motors, low or no speed operation combined with high torque may require additional air or water cooling. Servo motors have limited power compared to AC induction motors, so applications requiring higher power may require multiple servo motors.
Drive options for various applications Simple VFD and vector VFD are well used in extrusion. Servo motors are also standard on all-electric injection molding machines. Hydraulic machines, including the vast majority of molding machines, are increasingly using these drive technologies to power their hydraulic systems.
Injection machines using fixed speed electric motors have evolved from simple hydraulic drives using fixed displacement pumps to more modern and efficient variable displacement pumps. Variable displacement pumps can be more energy efficient, but energy losses can be significant when servo and proportional valves are used to provide highly accurate pressure and flow control.
A new and more efficient approach is to use variable speed motors and drives to accurately and dynamically control pump speed. By adjusting the speed of the pump, precise control of pressure and flow can be achieved.
Servo driven pumps compete in performance with all but the top servo valve machines. Pressure and flow control responses in the 10ms range are easily achieved, resulting in high precision and high dynamic response.
Direct control of flow and pressure by the pump minimizes energy losses resulting in energy savings of 10-40%. It should be noted that molding machines with servo drives and special pumps are more expensive than traditional hydraulic presses, but much cheaper than similar all-electric machines.
Presses with lower performance requirements can use inexpensive vector drives and variable frequency drive pumps to operate over a wide range of speeds. This drive technology also has the potential to save energy, but has limitations in terms of driving dynamics. Due to the common use of high inertia induction motors, acceleration and deceleration speeds are limited.
For VFD vector drive technology, slower machines and products that require less precision may be suitable. Pressure control also becomes slower and less accurate due to limited drive power, and more lossy hydraulic components may be required to improve pressure control accuracy.
VFDs with Smart Hydraulic Pumps To improve machine performance when using VFD drives, adding smart hydraulic pumps can reduce negative motor drive dynamics. One of the products is a “smart” piston pump that communicates directly with the electronics of the VFD.
This technology allows electronic control of pump flow and pressure by varying the mechanical displacement of an axial piston hydraulic pump optimized for variable speed operation.
For steady state operation, the pump sets the speed of the VFD drive to meet the required demand. For dynamic operation, a fast-acting pump displacement controller implements speed and/or pressure changes while a relatively slow-acting actuator catches up with demand. This combination provides system response times of tens of milliseconds, even with drive acceleration times of hundreds of milliseconds.
The benefit of this technology is that it can be applied to retrofit and retrofit machines to extend service life and achieve targeted efficiency gains that would otherwise require replacement of servo-driven machines. While the cost includes a variable frequency drive, possible motor replacement, and specialized pump and control system modifications, this can be a cost-effective solution to retrofit a machine with a good return on investment.
Other Applications Any machine requiring fast and precise movement that may have overlapping cycles can benefit from modern drive technology. An example is a blow molding machine. Workpiece and mold movements often overlap and must be precisely controlled at high speeds. Hydraulic systems typically use fluid storage accumulators for high flow, while pumps cover medium demand.
These and other “hydraulic accumulator” systems can use newer drive technologies to save energy. For example, with a variable frequency drive driving a hydraulic pump, the motor drive speed may vary according to the machine’s average flow demand. Reducing pump speed to flow demand and minimizing RPM during periods of inactivity reduces power consumption and noise levels.
Electrical Aspects All the drive technologies discussed here create varying degrees of electrical interference in a plant’s electrical network. The generation of high frequency, high current, and high voltage signals is known as harmonic distortion, which can adversely affect a plant’s electrical system. Effects range from electronic interference to failure of electrical equipment. These issues should be taken into account when choosing a supplier’s equipment, and they become even more important in plants that use these technologies to upgrade or retrofit existing machines, as well as in plants that use this technology to add more machines.
New drive technologies provide long-term savings. These drive technologies save energy, which is attractive in many applications. The cost of drive technology must be weighed against the expected energy savings. Incentives for utilities and the high cost of utilities (often by geography) are driving these changes in technology. These technologies have become dominant in the market for new injection molding machines. The benefits of energy savings plus high performance are comparable to those of fully electric motors and are suitable for the largest machines.
Despite the added expense, the ROI and total cost of ownership make a strong case for evaluating these technologies, whether in new machines or retrofit/refurbishment applications. Processors can benefit from lower power consumption and lower noise levels, as well as improve machine performance.
Neil Gigliotti has 32 years of experience in the hydropower industry, 24 of which he has worked in the plastics machinery market, applying hydraulic and electrical products, and developing motion control and drive systems. He is currently Group Manager for the Plastics Equipment Group at Bosch Rexroth Corporation in Bethlehem, Pennsylvania. Contact: (800) 739-7684, neal.gigliotti@boschrexroth-us.com, boschrexroth-us.com.
Paul Stavrou has 39 years of hydropower experience in the marketing, design and development of electro-hydraulic components and systems. He is currently System Application Manager at Bosch Rexroth. Contact person: paul.stavrou@boschrexroth-us.com.
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Post time: Mar-31-2023