In physics, the law of conservation says that energy never “dies,” it simply changes form. This truism is a pretty good generalization for developments in energy efficiency in material handling applications. From simple changes in lighting and heating to moving to a new voltage level, material handlers are finding innovative ways to lower the cost of energy — often by simply changing “form.”
One of the more significant developments is the changeover from AC devices to DC devices in many material handling applications.
The direct connection
You can blame the trend of lowering voltage levels from 120V AC to 24V DC on e-commerce. But this trend is really a result of a convergence of factors, which include e-commerce and the subsequent shift to moving lighter loads. The other factors are:
• A demand for safer, more reliable and cost-effective systems;
• A need for easier integration to other material handling components and devices that already use lower DC voltages;
• A demand for faster response times from system controls;
• Technology advances that solve problems of excessive DC voltage drop over long cable runs.
Developments in power transistor ampere ratings are part of the technology changes. These developments have led to an increase in the variety of 24V DC-controlled devices, such as adjustable frequency drives, solid-state reduced-voltage starters and servo drives as well as contactors, control relays and sensors.
“24V DC is a much more friendly control voltage than 120V AC, especially for devices incorporating microprocessors,” says Neil O’Shea, product manager, Cutler-Hammer, Eaton Corp.
Usually the U.S. is ahead of the curve on new technology, but in this case, European companies and countries took the lead. “When European manufacturers started selling products here, many of those products already operated on DC voltage. It’s now catching on in the U.S,” says Greg Matthews, DC products manager, Interroll. “Everything has been AC powered in the U.S., which is somewhat dangerous because of the shock hazard. But with 24V DC, you eliminate that.”
A safer alternative
The numbers alone bear out the fact that 24V is a lot safer to work with than 120V. Despite the cautions and information available about proper grounding, accidents still happen, often — it must be said — due to poor or improper grounding. According to the National Institute of Occupational Safety, more than 5,300 deaths were caused by electrical accidents. This figure accounted for seven percent of all workplace fatalities in 2001. One problem is ignorance about electricity, but another is the use of shortcuts that substitute for safe electrical procedures. Such permissiveness makes people susceptible to unacceptable risk. Here’s why:
“The notion that a human body in parallel with a solidly grounded electrical control circuit is protected because current will take only the path of least resistance is a complete fallacy,” says John C. Thompson and David B. Durocher of Cutler-Hammer, in a paper published by IEEE titled 24V DC Control — An Emerging Alternative to Legacy 120V AC Control Applications in North America. “There is always some resistance to ground. Electricity does take low-resistance paths, including the one with least resistance, but it also takes every other path available to it.”
Many don’t know what voltage levels are dangerous. The authors go on to say, “Touch voltage from an energized object is about 75 percent of the rated voltage. So, even a 120V AC line-to-case fault from a control power circuit has a touch voltage potential of 90V. Death from electrocution can occur from as little as 50 mA in only a few seconds. To reduce potential current flow below 50 mA, industry standards suggest that potential touch voltage conditions never exceed 50V AC or DC.”
Even OSHA suggests 24V DC where feasible. Its regulations allow personnel to work on live control panels without special equipment as long as they are below 50V.
One of the side benefits of the use of low DC voltage is that it eliminates the need for lockout/tagout procedures on those processes that use the lower voltage. “A lot of times with lockout/tagout, especially in plants with the material handling going all over the place, people will often lock out the wrong area by accident,” says O’Shea.
In the U.S., though, work on 24V DC systems can be performed hot in unclassified areas. This is not true with 120V AC, where employees must follow tagout and lockout procedures. But using 24V DC virtually eliminates potential safety hazards.
Another advantage of switching to the lower DC voltage is that it speeds motor actuation. How fast you can run your line is partly tied to how fast devices can respond to commands or data from controls. The reason for this can be understood by looking at the sine wave that graphically describes alternating current. If you graph a sine wave, you see that it starts at a 0 or neutral point, rises up to a positive point, then crosses 0, then goes down to a negative point on the graph and back up to 0.
When you start a motor using AC power, you can engage the sine wave at any point on that graphical representation. If everything is timed perfectly, and the motor is started at the exact right point on the sine wave, the motor will start in 2 milliseconds. But, if timing is not perfect, as is the usual case, it can take anywhere from 3 to 8 milliseconds more to start the motor. “This is not a lot of time,” says O’Shea, “but when you have multiple motors starting, the time buildup may be significant.”
Add the motor delays to other device delays in a material handling application. Take bottling, for example. DC sensors take so many milliseconds to register a read. Then those data travel to an output card, taking another few milliseconds. Then a PLC processes that information — another few milliseconds. Then commands go to a motor, more milliseconds. When engineers add all these delays, the total number is significant.
DC voltage, on the other hand, operates on a constant cycle. When you start a motor, it always takes just 2 milliseconds to begin.
“24V DC lets equipment respond faster,” says O’Shea. “It makes the whole logic a lot quicker. And faster response means you’ll have greater throughput, enabling your systems to make decisions better and speed up your line.”
Another concern involves brownout conditions. “If you’re in a weak electrical grid,” adds O’Shea, “then you’ll typically get an 8- to 10-cycle ride-through. If you’re running 120V AC, though, you’re dropping out all your motors in three cycles. At 24V DC, fewer motors will drop out in brownout conditions.” Noise is less of an issue, too.
The technique used to control most 24V DC devices is pulse width modulation. It permits a high closing force, low steady state current in the starter and higher contact force. Plus, it offers 100 percent ride-through at 0 volts for 100 milliseconds and unlimited ride-through at 50 percent voltage.
An additional benefit of DC-powered devices is the physical size that’s possible. In general, these devices are about one-third smaller than traditional AC devices. The reason for the difference is due to the separation of the control and logic circuits, which reduces power requirements. Such a design also overcomes several obstacles that prevented the use of 24V DC control, such as power supply costs, transistor and hard contact ampere ratings as well as voltage drop on long cable runs.
This voltage level also puts less stress on the mechanical system. Starting torque is not as high as with 120V AC systems, which can give you more operating hours with belts, gear boxes, chain drives, motor bearings and motor shafts.
The driven load also experiences less stress because of the lower starting torque. Plus, ramp times are adjustable, enabling the system to provide enough torque to accelerate a load without shocking it into movement. The result is far fewer jolts on conveyors and, therefore, less product damage.
As far as energy efficiency, 24V DC systems use less. Explains Matthews, if a 110 AC motor is pulling 5 A, it’s running at 550 W of power (110 times 5). A 24 DC motor, on the other hand, will pull 1.5 A, resulting in 36 W of power used. The less energy drawn, the less money spent.
DC does have a limitation engineers must consider when looking to apply this technology. And that limitation is torque. Some material handling tasks require more torque than 24V DC can provide. “Some applications need the power of 120V AC to get the job done,” says Matthews. “24V DC doesn’t work everywhere.” Check with your supplier to be sure if your application will work with the lower voltage.
Some material handling equipment receives energy from batteries, not motors. Battery maintenance and expense have long been a drain on a budget. But one solution is emerging as a real find.
One of the main reasons vehicle batteries lose their charge is because of sulfation buildup. Sulfates are the natural byproduct of battery operation. They form on the battery plates, growing to the point where they block energy absorption during recharge. About 95 percent of a battery’s energy is stored within the plates. Thus, even a 5 percent to 10 percent sulfate buildup can stop a battery cold.
What if you could prevent this process, though? Better yet, what if you could reverse it? A development from PulseTech Products Corporation promises to do just that, reverse and even prevent sulfation buildup.
The technique uses a precisely controlled pulse waveform that acts on a small amount of the lead sulfate crystal deposit on a plate and reconditions the plate, turning it into an active electrolyte.
“We don’t allow the sulfation to crystallize over the plate openings,” says Scott Schilling, VP sales and marketing at PulseTech. “It doesn’t harden after it deposits.”
This technology can lengthen the life of the battery. “From both lab and scientific studies,” continues Schilling, “and field applications, it seems to be extremely consistent that you can extend the life of a battery by three to five times using this technology. A key factor is the condition of the battery at the age the technology is applied. A guy who’s getting five years might get only two times the life. A guy who’s getting two years might get five times the life. It’s dependent upon the application too, but it’s fairly consistent that it’s probably three times the cycle life of what they were accustomed to.”
The technology is embedded in a device that simply rides on top of the battery. It takes a little bit of energy from the battery and converts that to pulsing DC current. Then it will burst that energy back into the battery and begin the cleansing. The cleansing maximizes the battery’s ability to absorb energy. “The reason this is a real energy saver,” continues Schilling, “is because, ultimately, it reduces the internal resistance within the battery. If the internal resistance is reduced, the battery more easily and willingly accepts energy. If it more easily and willingly accepts energy, it does it faster with greater quality.
“The first positive change you’ll see is additional run time,” Schilling continues. “You see additional discharge capacity. The second thing you’ll see is that the batteries recharge faster with greater quality.”
Charging is inherently an inefficient process. But the more you reduce the internal resistance, the easier the process becomes. The recharge time drops because the pulsing technology reduces the amount of energy that turns to heat instead of being absorbed.
“If you’re talking about energy conservation,” adds Schilling, “if you, through this technology, reduce your recharge time say 30 percent every day, you can save about $48 per day, assuming you have 210 lift trucks and your electricity cost is 8.43 cents per KWhr.”
It’s also an environmentally desirable solution. Fewer batteries need to be bought because they last longer. Several battery manufacturers are working with PulseTech on this technology. It’s available from some manufacturers now, such as Exide’s transportation division, with others offering it in the near future.
Pushing heat down
Facilities with lots of square footage often have difficulty controlling their internal environment. Heaters and air conditioners are typically mounted near the ceiling, where the heat and cooling stay. When it’s not feasible to move heaters and air conditioners to a better location, the solution can be to move the heated or cooled air instead. This is not as difficult as it might sound. The solution is big ceiling fans. These fans can have blades 10 to 20 feet long to move large amounts of air.
BJ’s Wholesale Club in Franklin, Massachusetts, found fans to be their solution. The 142,000-square-foot facility has 75 loading doors and serves stores in Maine, New Hampshire, Connecticut, Rhode Island and part of New York.
“Originally we bought ceiling fans for cooling and air circulation in summer,” says Dennis Berry, maintenance supervisor. “But we noticed a difference the first day we turned them on.”
The fans, from Big Ass Fans, are targeted specifically for areas where there’s more physical labor, such as pallet loading and some receiving areas. But the fans get the air moving throughout the whole building.
Other solutions, such as screens, presented security problems. “We do have big wall fans on the sides of the building,” continues Berry. “One pulls in air, the other expels it. The ceiling fans, however, help circulate the air throughout the building.”
From DC to AC
Electric lift truck sales have reached 55 percent of the overall market, says a report from Toyota Industrial Equipment. In raw numbers, that’s about 100,000 trucks a year. Part of the reason for the change from internal combustion to electric is to meet California requirements for “green” engines.
“It’s reduced about 80 percent of the pollutants that have gone into the air,” says Tom Rigby, fleet manager, Komatsu. “These green engines reduce more pollutants than other efficient engines, which already reduce pollutants by about 70 percent over gas engines. Plus, they let you use trucks in situations where the only solution is electric because of the confined spaces.”
But the industry is working with other motors as well. Series-wound are popular, but are high-energy consumers. Alternatives include separately excited and AC electric motors. There are benefits and drawbacks to both.
AC offers high travel speeds and good acceleration. But few users need to travel at 9 to 12 miles per hour through congested warehouses. A more important benefit of AC is the reduced size of the motor, enabling more efficient cab and lift truck design. Because these motors lack brushes, engineers don’t have to design access so that maintenance can reach all sides of the motor.
Maintenance may prove to be the ultimate criterion in motor selection. “We’re taking the approach of going after the separately excited motor,” says Lou Micheletto, warehouse products manager, Yale Materials Handling Corporation, “because, in our world, the technician’s ability is everything.” AC motors are complex and require a high level of training. “The technical support available in the marketplace hasn’t been elevated to do the maintenance work necessary to keep AC running,” continues Micheletto.
One of the issues is heat. Energy from DC lift truck batteries must be converted to AC. This process requires a cyclical converter, which gives off heat as part of the process byproduct. “The European facilities, where AC is popular, don’t have this problem,” adds Micheletto. “They have longer runs, and it’s not uncommon for an operator to take the truck outside on an apron and unload a trailer. Here in the States we don’t do that. We stay on a dock, or we go into an enclosed trailer. We travel shorter distances, so we don’t have the time to get rid of the heat. It’s like leaving your car in first gear and traveling all around the city. Sooner or later you’ll overheat the motor. We’re seeing this with the motor and the cyclical converter in AC. We think AC technology will be the trend, probably in two to four years. It’s just not ready today. The problems can be solved, probably with electronics, and it will happen.”
AC motors can offer slightly quicker acceleration than separately excited motors. European designers, however, have found ways through electronics to boost the acceleration of separately excited motors. These newer separately excited motors can offer many of the benefits of AC without many of the drawbacks.
Thus, the final choice of which motor for lift trucks is still a toss-up. MHM
For more information:
Cutler-Hammer, Eaton Corp., www.cutler-hammer.eaton.com, 800-809-2772
EG Energy Controls, www.egenergy.com, 902-661-2007
HVLS Fan Co., www.hvls.com, 859-233-1271
Interroll Corp., www.interroll.com, 800-830-9028
Komatsu Forklift USA Inc, 800-821-9365
Nissan Forklift Corp., www.nissanforklift.com, 815-568-0061
Power Standards Lab, www.powerstandards.com, 510-658-9600
PulseTech Products Corp., www.pulsetech.net, 800-580-7554
Rockwell Automation, www.rockwellautomation.com, 800-223-5354
Toyota Industrial Equipment, www.toyotaforklift.com
Van der Graaf Inc., www.vandergraaf.com, 905-793-8100
Yale Materials Handling Corp., www.yale.com