Machine Efficiency = Energy Efficiency
Waste not on your machine, want not on your bottom line
By James R. Koelsch
As energy costs continue to soar, all those little energy-saving tricks that everyone once ignored are gaining a measure of popularity. These clever uses of modern motion control and smart machining practice can do much more than just conserve a little energy. They also can generate efficiencies that energize the bottom line.
Look at the small three-axis, high-speed Robodrill VMC built by Fanuc Ltd. (Japan). The latest motion-control technologies allow it to consume 55% less power and complete jobs 68% faster than similar machines that don't have them. Retrofitting the machine with the latest motion-control technologies cut power consumption by 55% and shortened cycle times by 68%. "It can cut three times as many parts in a day, and use half the energy to do it," says Paul Webster, CNC product manager at GE Fanuc Intelligent Platforms Inc. (Charlottesville, VA).
These savings come from a variety of sources, but the biggest portion comes from power-source regeneration, a technique that exploits the fact that motors act as generators whenever they decelerate. "The amount of time that servomotors spend decelerating is almost as much as they spend running and accelerating," notes Webster. Even some spindles, especially those that change tools often, spend a lot of time decelerating. So machine tools fitted with the right technology can recover power from both the axis and spindle motors.
Unfortunately, conventional drives do nothing with this energy. They simply turn it into heat by means of a resistor and dissipate it into the atmosphere. Rather than wasting the energy, drives capable of power-source regeneration redirect it and send it back to the power supply line using intelligent power modules. The machines, then, contribute to a kind of minigrid within the factory. The result is that power-source generation can reduce a machine's net energy consumption as much as 40%.
Using this technique, machine tools emulate the Toyota Prius and other hybrid gas-electric cars that recharge their batteries as they go downhill or come to a stop. Actually, the truth is that hybrid cars are emulating machine tools because high-speed machines and large automotive factories have been using this technology for nearly two decades to manage their energy costs.
The only times that this technique fails to recover power from the spindles are those instances where someone hits the emergency-stop button. The reason for the loss is that most spindles use induction motors, which require electricity to induce the electromagnetic field. Once the energy is gone, the field disappears and, with it, the means for generating power. The opposite is true for servomotors, however, because they get their electromagnetic fields from permanent magnets, rather than from induction.
Although power-source regeneration has been around for a while now, it has garnered little attention here in North America. One reason is that some drive manufacturers, such as GE Fanuc, have been quietly putting the necessary electronics in some of its drives as a matter of course. "Most retrofitters aren't even aware that they are installing it," says Webster. "When they replace an old system, even a 20-year-old Fanuc system, they will find that energy consumption on the machine will decrease by a lot."
Cost and little payback are other reasons for the slow proliferation among a large class of "bread-and-butter" machines. Consider a three-axis machining center that puts 10,000 W into its spindle during a 2-min cut. "If it takes 2 sec to bring it up to speed and to stop it, only 2 sec worth could be recovered," says Kurt Zierhut, director of electrical engineering at Haas Automation Inc. (Oxnard, CA). "It's only 2 sec out of 120, or 1.6%. If you tell a customer that it's only going to cost him $5000 to save that 1.6%, he won't buy your product."
Maintenance costs only add to the difficulty. By the time you account for maintenance on the electronics, there might not be a payback for some machines. "It's not a one-time outlay," says Zierhut. "For a machine tool that has a life of five to ten years, there will be five years of minor maintenance and ten years of major maintenance. Most people wouldn't recover enough to pay for it over that time."
For other machines, however, the calculus is different. First, the cost of installing a regenerating power module is not always quite that high. They cost between $1500 and $5000, according to Karl Rapp, engineering manager at Bosch Rexroth Corp. (Hoffman Estates, IL). "And if it is costing you $5000 more than power supplies with bleeder resistors, then you are talking about a drive system that already costs $10,000 or more," he notes.
Power-source regeneration, moreover, tends to yield the greatest returns for applications requiring high speeds and frequent changes in acceleration. Consider a 20,000-rpm machining center cutting a part requiring 15 tool changes. Machines made for this kind of work usually change the tools in less than a second chip-to-chip. "The spindle motor has to decelerate from 20,000 rpm to zero in 400 msec," says Rapp. "You have to remove a lot of energy from the spindle rotor in a short time and do something with that peak energy."
Although he acknowledges that bleeder resistors can be the most cost-effective solution in some machines, appear at first glance. The reason is that they have to dump the heat someplace, and sometimes those places require air conditioning. Old direct-current (dc) motors and some large variable-frequency drives (VFDs) have the resistor arrays mounted on top of the control cabinet to dissipate the heat directly into the shop, and would not impose an extra burden in un-air-conditioned shops. Smaller VFDs, however, have the bleeder resistor inside the drive's housing, which means it sits inside the cabinet.
These latter instances require installing a larger air conditioner on the cabinet to protect the electronics inside. "Air conditioners also generate heat," notes Rapp. "Every time that you have an air conditioner involved, you have an efficiency loss. If the facility itself is air conditioned, then you pay for the electricity to cool the cabinet, and for the electricity to cool the building." Any cost-benefit analysis, therefore, must also consider these costs.
If the cost is still too high, perhaps a compromise solution will trim your energy bill. Instead of dissipating the energy through a resistor, this compromise directs it to one or more capacitors installed in the drive system, and stores the energy locally on the machine. These capacitors can cost around $500 or $1000, depending on their capacities. Their chief limitation, however, is that they can store only small to medium amounts of energy, roughly between 100 W-sec to 1000 W-sec, depending on the voltage variation. The savings are often several kilowatt-hours per shift.
Rapp reports that Bosch Rexroth installs a lot of these capacitors in applications with rapid, short-stroke movements. Examples that he offers are flying cut-off and roll-fed machines used in metalforming. "You are continuously accelerating and decelerating the servomotor at a high rate, perhaps at 600 cycles/min," he says. "Here we don't use power regeneration because a capacitor is typically big enough to store the energy for reuse during acceleration."
Specifying the right motors and drives can add to this savings. Sometimes builders specify motors that are slightly oversize to allow the machine to accelerate faster, thereby shortening cycle times. For example, giving the core a different shape and switching from a ferrite to a rare-earth magnet can give a smaller servomotor the same acceleration as a larger one.
"Ferrite magnet motors have high inertia and low energy density, so they don't accelerate very fast," explains Webster at GE Fanuc. "Rare-earth magnets lower the inertia and increase the power." So, builders and retrofitters might consider rare-earth magnets if the machine does not require the inertia provided by ferrite motors.
To boost the acceleration of spindles, Webster suggests looking into an induction motor with a dual winding. Normally, these motors are wound for either high speed or high torque, but those with two windings offer both. "When you want high speed and acceleration, you command the high winding," he says. "When you need torque, you can use the low winding." It's like having a gearbox that shifts much faster, but without the energy losses.
Because of the extra copper, though, the price premium is usually about 20%. "So, the energy savings might not be enough to pay for it," notes Webster. "But the production and energy savings certainly would. If you knock 20% off your cycle time, you can cut 20% more parts."
The latest generation of PWM drive amplifiers for servomotors contributes to energy savings by minimizing switching losses. Besides recommending these amplifiers, Webster urges retrofitters to match them to the motors, rather than simply picking the cheapest ones. The reason is that each controls-and-drives manufacturer has tuned the operating parameters for its systems.
Tuning is an important source of efficiency. "You can save quite a bit of energy by making sure that the servo system is sized and tuned properly," says Rapp at Bosch Rexroth. By a properly tuned servo system, he means one that not only minimizes the heat losses through the electronics, but also does not jerk the machine around. "Jerking the mechanics increases the peak-power requirements, and puts unnecessary energy into the machine structure," he explains.
This wasted energy can be significant enough to show on your electric bill. A proper tuning alone can shave 1–2 kW-hr per machine per shift from the bill. It may not sound like much by itself, but it can add up to significant money in facilities that run a lot of machines.
"In the past, it was difficult to convince people to put the extra effort into generating efficiency because energy was really cheap," says Rapp. "People said I just save a couple of bucks a day. If I pay for 20 more hours of engineering at $100/hr, it will take me years to get my money back." Now that energy is more expensive, Rapp finds that more manufacturing companies are willing to pay the price.
Consequently, many machine builders optimize their servo systems as a matter of course, and amortize the cost of doing so over the entire line. "It depends on the builder, though," he says. His advice is to ask about power consumption. In general, European builders will probably be better prepared to give you details because they have to compute them for the energy labels that are required in Europe.
No matter what technology you install and how well you tune it, there is a very real limit to how much energy you can save. Cutting metal demands large amounts of energy. "It's a matter of physics," says Zierhut at Haas. "It takes power to remove metal." And anyone who wants to know how much can look in Machinery's Handbook for the table that correlates material removal rates with power.
Zierhut urges machining facilities that want to conserve energy to look at the broader and perhaps more significant way in which time, money, and power are related. Because machines consume energy when they are on, and even more while they are cutting, the two most cost-effective options are to turn the machine off and to machine less.
Following this logic, Zierhut's first piece of advice is to turn the machine off when it's idle. "An idle machine in our product line is going to use between 100 and 300W just sitting there," he says. "The more idle time, the more wasted power."
With the wide range of cheap-to-expensive loading automation available today, there really is no excuse for machines to be idle during breaks or at any other time during a shift. If, however, the machine does stop during the day or at the end of an untended job, the controller should do what personal desktop computers do. It should just turn the different parts of the machine off.
As on a PC, some CNCs also have a setup page that permit users to specify for each peripheral device an amount of idle time after which it will shut off the device. The first thing that comes to mind for most people is the CNC's screen, but turning it off might save 30–40W at most. More significant is turning off the various motors, such as those running fans, pumps, and chip conveyors. When the spindle is not turning, you can turn many of those motors off.
Some CNCs also allow adjusting the duty cycle of the chip conveyor. "Most of the time, it's going to turn faster than chips are produced," notes Zierhut. "So you can request a 50-50 duty cycle to turn it on for a minute and off for a minute." This technique can contribute a measurable amount of energy savings, especially when combined with turning it off altogether when a program stops.
If you don't know about these settings, it's probably because most machine tool builders will not turn them on for you. You usually need to do it yourself. The reason is that turning some functions off arbitrarily ahead of time can be dangerous. Consider the problems that turning off the hydraulic pump on some lathes can cause. The chuck depends on hydraulic pressure to keep it closed. "If the machine has a very long, heavy part chucked on only one side, the check valve will relax sometime between five minutes and five hours later," notes Zierhut. "A heavy part could fall out."
His other piece of advice for reducing energy consumption is to machine less—that is, to machine more near-net shapes. He points to the waste generated in the machining of wall sections for airplanes, the classic antithesis of near-net-shape production. A high-speed machine may whittle a 500-lb (227-kg) slab of aluminum down to a 60-lb (27-kg) shell. "This is a tremendous amount of waste, in terms of metal, time, and power," says Zierhut. "If the machining of that part could be started at close to the right shape, you would spend a lot less time and power removing the material."
Although he doubts that energy conservation alone will convince manufacturers to cut more near-net shapes, he thinks that the combination of soaring metal prices and rising energy costs will. "In today's world, the cost of metals is going up 30–40% a year," says Zierhut. It goes to show that efficiency pays dividends in more than one way.
This article was first published in the September 2008 edition of Manufacturing Engineering magazine.
Published Date : 9/1/2008