The Power To Move
Faster, smarter and with greater flexibility, power transmission devices are ready to boost material handling throughput.
by Leslie Langnau, senior technology editor
If you’re looking for ways to raise productivity, cut costs and gain greater flexibility from your material handling equipment, you’re in luck. It just so happens that the latest power transmission devices, particularly motors and drives, offer such features. And your fellow material handling managers and engineers are taking advantage. According to a recent report from the ARC Advisory Group, more warehouses and distribution centers are installing motion control and automation systems. Many are using motion technologies to improve conveyor systems, augment packers with robotics and replace many manual setups with precision motion control systems.
One of the most frequently applied solutions is installing AC motors and drives. The main reason is flexibility. But a secondary reason is lower maintenance.
Part of the flexibility comes from the wider speed ranges these motor and drive systems offer. “The broader ranges give you the ability to increase your speed for those periodic bursts you need at start-up and overcoming system inertia,” said Brian Stephens, product manager, Demag Cranes and Components. “This ability increases throughput. Plus, you won’t have to pay for a larger-size motor to obtain a feature you won’t need all the time.”
Those greater speed ranges also let a system handle the increased load variability more common now, when loads change from ounces to a hundred pounds. Plus, turning up the speed a notch or two is an excellent way to hike throughput as package size shrinks. With the increase in smaller items going down conveyors, instead of cases or palletloads, higher throughput is crucial.
Multiple speed ranges are also useful in travel applications, like those involving cranes and hoists. AC motors offer greater control of acceleration and deceleration here. Such control helps avoid wheel slippage, which can occur during too high a rate of acceleration. “With an AC system, you get enough control of acceleration to overcome inertia but still keep it under the allowable wheel slip rate,” added Stephens.
Greater control of acceleration and deceleration and the ability to boost “power” when needed help in other types of material handling equipment, too. “We did an application where we increased the torque significantly, but we did not increase the package size of the motor itself,” said Rod Dorschner, senior product marketing engineer, Rockwell Automation. “Instead, we increased the acceleration and deceleration times of a stacker to get higher throughput.
“In another application we had,” continued Dorschner, “a picker picked pallets off of a rather flimsy machine; flimsy because of its aluminum frame. But, by changing the acceleration and deceleration rates of the motor, and increasing the resolution of the position control, we increased throughput, working within that system’s stability rather than against it.”
AC systems can greatly improve start-and-stop requirements, too. In many cases, these requirements can be “softer” for smoother conveyor rides, resulting in less product damage. “An AC drive is inherently a soft-start device,” said Dennis Fitzgerald, vice president, Yaskawa. “With any sort of variable speed capability, you automatically have soft start built in.”
Companion drives and their motors can start or stop conveyor lines in as little as 0.1 second. But, due to design changes to the motors, frequent stops and starts won’t damage them.
Because of AC systems ability to handle higher starting torque, engineers can usually specify smaller load-handling motors for many applications; a nice cost saver. It used to be a rule to buy a motor for the maximum torque needed by an application, even if that torque was needed for only a minute or two.
“The required horsepower to power a moving crane, for example, is not that much,” said Stephens. “But motors were traditionally oversized to provide enough starting torque to overcome initial inertia. AC motors can put out a lot of startup torque for a brief time to brake a crane free of inertia. It needs less torque, though, to keep the system moving. And determining what’s needed for constant motion is now the key variable when sizing a motor. Lots of motors can handle high starting torque without damage to the motor. Such a feature saves money as well as deadweight.”
“People would often oversize motors to avoid the heat problem,” agreed Dorschner. “Now, though, you rarely need to oversize a system.”
More accurate motor sizing lets engineers apply the saved dollars to another part of the application. Plus, with today’s improved temperature monitoring features, engineers can operate motors closer to their maximum capabilities with no damage.
You’ll notice that drives are physically shrinking as well, saving precious cabinet space, or space near other equipment. Designers have been able to reduce the size of the electronics considerably, thanks in large part to microprocessors. Gradually, the power supply portions are also shrinking, as developers find better ways to produce and filter incoming power.
The smaller sizes also help contribute to reduced maintenance. “One of the other reasons people are turning to AC technology is because it requires less maintenance,” said Fitzgerald. “At about 30 horsepower and below the devices are almost throwaways.” In addition, the mean-time-between-failure rates are up to 10 to 20 years.
Another benefit of AC systems is their regenerative braking feature. In some applications, mechanical brakes can be eliminated. This feature can also make reverse operation of traverse conveyors simpler.
Spreading the word
The need for more information about what is happening during the process ranks high as a reason many are turning to AC power transmission systems. AC drives can connect to industrial buses and send data on the operating status of whatever material handling system they’re controlling.
“Our customers are seeing a requirement for more information, especially from their upper management,” said Fitzgerald. “Management wants to know what’s going on. What’s down, what’s operating, what the throughput is hour by hour, what can go faster. The biggest hurdle, however, with this information, is that much of it is still compiled and sent up manually.”
In addition, some AC systems can replace programmable logic controllers or personal computers in certain control applications. Several AC drives offer I/O points within their hardware.
Because of all these features, drives are increasingly being used as a replacement for smart contactors. Such an exchange is commonly done with a 10 horsepower motor and drive. “The price of these systems is comparable to that of contactors,” continued Fitzgerald. “Plus, you gain additional features, such as communication.”
Emerging in some material handling applications, such as those that run 24/7, are vector control motors and drives. “A vector control is an AC drive with feedback to indicate where it is at high speeds,” said Fitzgerald. “Also gaining in popularity are servo systems. These systems provide the benefit of positioning control. The trend is to know a lot more about where product is and what is happening on the line. Hence, the move to such power transmission systems. Some of these systems even interface to some ERP systems.”
Open-loop vector drives are finding a way into material handling applications. An example would be equipment that can operate with speed that’s within one-half percent of specified speed. The benefit is the elimination of encoders and resolvers for feedback.
On four wheels
AC systems are also finding their way into lift trucks. Compared to DC systems, AC offers several advantages:
• Travel speeds to 12.5 mph and lift speeds to 108 fpm;
• Up to 30 percent less energy consumption than conventional electric lift trucks due to efficient controllers and regenerative braking;
• Totally enclosed motors, gears and multiple disc brakes that are maintenance free.
In addition, AC drive systems often offer more battery life than DC systems. For material handling operators, this lets them get more work done with the same battery using conventional charging methods — a two-way savings.
NEMA, CEE and even the federal government are talking about the importance of saving energy. Because motors are one of the biggest users of electrical energy, it would make sense to use the most efficient designs possible. However, the material handling industry does not appear to be in a hurry to turn to the more energy-efficient motors.
Part of the reason for this is the intense focus on productivity, among other criteria such as cost and space utilization. “There are so many factors material handling managers and engineers are concerned about at the moment, that energy efficiency is a bit low on the list,” said Dorschner.
“In addition,” added Stephens, “many applications where you have a one-half or one horsepower motor that sits idle a good deal of the time, there’s little cost justification for a high-efficiency motor.”
Organizations such as NEMA and the Consortium for Energy Efficiency (CEE) are pushing the use of more efficient motors nonetheless. NEMA recently launched its Premium Efficient Electric Motor program. The organization has trademarked its Premium designation. Thus, only products that meet requirements stated in a memorandum of understanding between the motor manufacturer and the organization may use the designation, NEMA Premium.
This designation sets standards for temperature, torque, inrush current, power factor and overall design parameters. These motors will not require new starters or new wiring in the systems they’re installed in. Users can maintain existing compliance to electric codes and safety regulations.
Plus, this standard designation covers more types and sizes than did EPACT requirements. Motors covered include 1 to 200 horsepower definite and special-purpose motors, medium voltage as well as motors to 500 horsepower. Design A and B motors are also included.
A consortium of motor industry manufacturers and service centers, trade associations, and electric utilities and government agencies have developed a nationwide campaign that will encourage the use of better motor planning and management to conserve energy. The Motor Decisions Matter campaign (www.motorsmatter.org) encourages businesses to develop a motor plan. This plan will address common motor decisions before equipment failure to ensure motor availability and lower downtime and energy costs. This consortium is working with both NEMA and CEE. MHM
Stopping 60 Tons of Moving Power — Safely
When a 60-ton automatic guided vehicle bearing a steel coil comes at you, you want to know that it will stop safely. Thus, selection of the right brake system is crucial.
For Mentor AGV Inc., Bedford Heights, Ohio, the choice was an electric brake design made by the Stearns Division of Rexnord Corp. Mentor AGV is a major manufacturer of large automatic guided vehicles (AGVs). These vehicles are available in a range of sizes, with fork, ram or piggyback carrying configurations for various applications, including coil handling.
The Model 331 electric brakes fit within the driven hub of the AGVs and hold these vehicles in a parked position when they are not operating. The compact brakes, which are mounted to the back end of the drive motor, also provide emergency stopping.
In addition, the brakes offer a proving switch. This switch is important to proper AGV operation as it signals when the brake is released. Said Matthew J. Curry, mechanical engineering manager at Mentor AGV Inc., “Often, there’s no operator to notice if the vehicle tries to move before the brake is retracted, so we need a signal from the brake to tell the processor it is ready. Plus, the switch can also function as a wear indicator, if the pad gets too thin to pull in properly.”
All of Mentor’s laser-guided AGVs are powered by a standard steer-drive package that varies in size but not in basic design. Curry said, “We have 3 horsepower, 6 horsepower and 9 horsepower drives, for vehicles from 20 to 60 tons. Basically, the steer-drive package is a wheel that contains a planetary gearbox driven by a DC motor and the brake mounted on the back. By mixing and matching these with gear ratios from 32:1 up to 100:1, we can slow the vehicle down and get the torque we need, even for heavy loads.” The largest AGVs use four of the drive wheels, so the vehicle also has four brakes. Smaller vehicles generally use only one drive wheel and brake.
Travel speeds are typically between 120 and 200 feet per minute, or the pace of a medium to fast walk. In some applications, laser bumpers that will replace mechanical bumpers will scan ahead and allow future travel speeds of up to 300 feet per minute, according to Curry.
These armature-actuated, spring-applied disc brakes decelerate or hold inertial loads when voltage is disconnected from the brake coil. The brake design is direct-acting with only two moving parts. When electrical power is applied, an electromagnetic force in the brake’s magnetic body pulls the armature, which overcomes the spring action. This allows the friction disc to rotate freely. When electrical power is interrupted, the electromagnetic force is removed, and the pressure spring mechanically forces the armature plate to clamp the friction disc between itself and the pressure plate. This develops the torque needed to stop or hold the load.
Previously, Mentor AGV used a brake made in Germany but found it difficult to get the necessary technical support. The Stearns brakes are U.S.-made bolt- on systems. Service has been excellent, Curry stated.
One of the first applications to use AGVs with these brakes was the Alcan Aluminum plant in Oswego, New York. These AGVs are specialized, three-wheel piggyback-type carriers that use two idler wheels and one driven steering wheel. Powered by 48-volt electric motors, they move at 120 fpm when loaded. The electromagnetic brake stops them within one-half inch of their destination.
The brake serves two functions. During normal operation, motor controls slow the vehicle to a stop. Here, the brake holds the unit in its parked position.
When the emergency stop switch is activated or the vehicle processor determines there is a problem, all power to the drive system is interrupted, and the brake applies automatically to stop the vehicle. Curry noted, “We must have a spring-applied brake to meet safety requirements.”
Most vehicles use a brake with a 110-pound-foot rating. However, the Alcan system was designed to handle a coil that was 50 inches in diameter and 10 inches wide. Because the coil is carried on its edge, the stopping force of a larger brake could cause the coil to tip if applied in an emergency. A smaller brake provides a softer stop.
Another feature of the brake is easy maintenance. The AGV is designed with a door that opens to provide access, with a hand crank that allows the motorized wheel to be oriented to face the door. Mounting the brake on the high-speed end of the motor not only aids accessibility but also allows the use of a brake with a lower torque rating.