Using Energy Efficiently Can Boost Profitability
As energy costs increase, making efficient use of kilowatts is more important than ever. European firms have long known this, as energy costs are much higher in Europe than in North America. Although some retrofitting of older equipment may be helpful, newer equipment is designed and built to use less energy than machines put in service only 5 years ago.
Motors, drives, controls and machine design improvements each can save operating cost, reducing overhead. Some of these advancements are reviewed in this article.
Electric Feedback in All Drives
Power source regeneration uses the concept that motors act as generators whenever they decelerate, which is about as often as they accelerate. Turret indexing motors on Index and Traub machines, for example, are also used to drive rotating tools, and therefore, decelerate frequently. With modern machine tools, it is possible to recover power from both the axis and spindle motors.
Regenerative drives on advanced machines convert the motion energy of all braking operations on spindles and axis drives directly into electrical energy and feed it back to the supply network. This reduces energy consumption on a sustainable basis and reduces the heat emission in the proximity of the machine.
Drives capable of power source regeneration can redirect it and return it to the power supply through intelligent power modules. The result, it has been estimated, is that power-source regeneration can cut net energy use by as much as 40 percent.
Power source regeneration yields the best returns for applications with high speeds and frequent changes in acceleration. Multitasking machines with multiple tool changes in a cycle and only 1 second between cuts means the spindle motor has to decelerate from 8,000 rpm to zero almost instantly. As the motor decelerates or brakes, a lot of energy is released. Capturing it and returning it to the power supply is a great example of efficiency.
Use of Synchronous Motors
Properly sized motors and drives also add savings. Alternating-current (AC) motors feature a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting magnetic field that drives it, avoiding energy-wasting slip under most operating conditions. They operate synchronously with line frequency. An induction motor, on the other hand, slips in order to produce torque.
Synchronous motors are a highly efficient means of converting AC energy to work. They are typically used in positioning machine axes where high precision and constant speed is required.
Most European builders are prepared to provide power consumption details because of the required energy tags (CE) mandated in Europe.
Clever mechanical designs are also appearing, which reduce the weight of the axis components in the machine tool, thus requiring less energy to move them. The turret slides of an Index automatic lathe move in the X and Z directions on a single slide, plate-type slide system. This system, unique to Index, permits rapids as much as 60 m/min and accelerations as much as 1 G while maintaining rigidity. Using this plate-type guideway for the turret slides also means that turrets glide directly on the machine bed, for high stiffness and dampening, longer tool life and better surface finish while reducing energy consumption.
Saving Through Shorter Cycle Times
Because machines use energy when they are switched on and even more while they are cutting, the two most cost-effective options for reducing consumption are to turn the machine off and to machine less. An idle machine can use several hundred watts simply at rest.
Properly sized motors and servos relative to the mechanical weight and structure of the machine can also save energy, potentially reducing peak-power requirements. Plus, they respond quicker, and therefore, can help reduce cycle times.
Compared with a less well-designed machine system, it has been estimated that a machine designed with this type of efficiency in mind can save 1 to 2 kW an hour per machine, per shift.
If a machine has some scheduled stops during the shift or at the end of an unmanned job, the controller should be able to turn off affected parts of the machine—as any PC does. Turning off motors, such as those running fans, pumps and chip conveyors, can save a lot of energy, and while the spindle is not turning, it may also be powered down. Adjusting the cycle of the chip conveyor can contribute a meaningful energy savings, particularly when combined with turning it off completely at the end of production. The automatic shutdown function of machines today automatically shuts down all high energy consuming units after a user-defined time elapses following an unplanned production interruption (standby mode).
Centrally Controlled Machine Cooling System
Older direct-current (DC) motors and some large variable frequency drives (VFDs) include resistors on top of the control cabinet to dissipate the heat directly into the shop. Smaller VFDs, however, include bleeder resistors inside the drive housing, which means the heat is retained inside the cabinet.
In these cases, a larger air conditioner is installed on the cabinet to protect the electronics inside. But this arrangement results in an energy efficiency loss, plus the electricity to cool the cabinet and/or the shop costs money.
The cooling concept from Index Corp. cools the spindles, hydraulics and control cabinet constantly and the captured heat can be fed to another useful application via a “water interface,” for example, for service water heating or as process heat for other manufacturing steps.
The cold water interface provides the ability to dissipate heat in a climate-neutral manner if the machine waste heat stored in the cooling medium cannot otherwise be used. The cooling unit can, with the aid of the water interface, be used outside the shop as well as centrally across several machines. This offers considerable potential energy savings for shop cooling/climate control, and/or increased efficiency, as a result of centralized heat disposal.
Targeted Heat Dissipation
All high-loss heat sources of advanced machines can be cooled directly with different cooling media via multiple fluid circuits. In addition to the cooling circuits for the main spindle, counter spindle and milling spindles, the hydraulic system and control cabinet also have a separate cooling circuit. The lost heat energy is absorbed directly in the fluid and conducted from the machine to a central location.
Offline Simulation Techniques
Program simulation offers a major opportunity to optimize a machine’s operation, and therefore reduce wasteful, energy-using motion even before the machine’s power switch is turned on. Machining programs can be created, checked and optimized on a PC before they are loaded into the control with 3D simulation. Optimized cycles permit field-oriented applications, tested and reliable operation, maximum flexibility, shortest possible machining times and optimum machine usage.
As machines become more complicated, simulation becomes more helpful in saving setup time—potentially 50 to 75 percent—because users do not have to tool and run the machine to complete a setup. Machine time is very expensive and if users can save 50 percent on setup time, they can get that much more production out of a 24 hour day, avoiding machine idle time and maximizing productive time.
Cost savings are where you find them, and these are only some of the ways that machine builders are responding to the demand for more energy efficient machine tools. The quest is on, and it’s delivering tangible results.
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