8/18/2011 9:37 AM
|By taking into account the physical components, application and processes, it is possible to |
not only reduce the energy consumption of the machines being used, but to also save money.
Nearly half of the electricity used in non-metal fabrication industries is consumed by motor-driven equipment, according to the U.S. Department of Energy (DOE). Chief among the equipment are woodworking machines, which use power-hungry motors for pumps, fans for dust collection, and in their spindle and servo drive systems.
To reduce energy consumption, the DOE recommends a “systems approach” — considering the physical components, application and processes to maximize the potential savings.
A Systems Approach
Energy Efficient Components: The motors for pumps, dust extraction systems, and spindle and servo drive systems are the most significant consumers of electricity in a woodworking machine. Using motors that only run when the machine is “in-cycle” will reduce energy usage. Replacing induction motors with permanent magnet synchronous motors with intelligent speed control or true servo systems also will bring greater efficiency.
State-of-the-art spindle and servo drive amplifiers use low-loss power devices that achieve efficiencies as high as 99%, and the product packaging is designed to dissipate any heat generated outside of the electrical cabinet. Spindle and servo motor speeds are often controlled using pulse width modulation (PWM) and higher PWM frequencies reduce motor iron-losses. High response vector (HRV) control closes the motor control loops at a very high rate, allowing faster and more precise motor acceleration and speed control.
Energy Recapture: The servo motors are continuously accelerating and decelerating as the axes change direction during machining. When they accelerate, they draw energy from the electrical system. When they decelerate, their kinetic energy is converted back into electrical energy, which is dissipated as heat in a resistive load in a process called dynamic braking.
Some DC-bus servo drives may share some of the excess energy with other motors on the machine. More sophisticated AC drive systems pump the energy back into the main electrical supply, reducing the net energy used.
Power Savings Mode: Today’s CNC systems include a programmable machine control (PMC) interface. When the machine is not in cycle, the PMC interface can turn off devices based on a schedule that considers how long the machine has sat idle, and the time it takes to restart each component.
Cycle Time Reduction: When a machine is in cycle, the total power consumed on a per part basis is largely dependent on the cycle time. Software can take into account the machine’s mechanical capabilities, then use acceleration/deceleration techniques to optimize the cutting feedrates in real-time. Smoothing algorithms convert small line segments back into complex curves that can be processed faster by the CNC and reduce machine shock. Combining these strategies potentially reduce cycle times by 50 percent or more, while maintaining the same part accuracy.
Power Monitoring: By displaying the electricity consumption dynamically and historically, part programming optimization and other process-related improvements can be evaluated more effectively.
The systems approach to reducing energy consumption ensures that all avenues are considered; there are success stories of saving 40-50% in machine tool applications. It also means using machine components designed for efficiency, drive systems that recuperate rather than burn excess energy, machine interface implementations that consider power saving modes and cycle time reduction strategies.
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