July/August 1995
Selecting a motor for your auto shredder is a choice that can determine the efficiency of your operation for years to come. With so many factors to consider, how do you ensure you’re making the best decision? Here’s what some experts have to say.
By Eileen Zagone
Eileen Zagone is Editorial Associate of Scrap Processing and Recycling.
With most scrap processing equipment, the motor that comes with a machine is the motor that comes with it—end of story.
But that’s not the case with automobile shredders, which can be outfitted with any of a handful of types of engines or motors, regardless of the shredder make or model. Shredder operators can even replace one kind of motor in their existing shredder with another very different type without any retrofitting of the shredder.
All this choice means shredder operators can install the type of motor that will best suit the particular needs of their operations. That's certainly an advantage, but there's a sticking point: How do you know which shredder motor is right for your situation? It's not an easy question to answer. Industry experts passionately disagree on the advantages and disadvantages of the different types of motors, though they do agree on one thing: Each shredder application presents different challenges and, therefore, choosing the best motor requires a case-by-case analysis of the kind and quantity of scrap to be shaded, energy availability, environmental concerns, financial strategy, and more at any particular shredder operation.
What's out There?
Of course, before you can assess these factors against the various types of-shredder motors available, you’ll need to know what kinds of motors are now out there and, more importantly, their distinguishing features and functions.
At the most basic level, auto shredder motors can be divided into two categories: internal combustion engines and electric motors. Both types can be further broken down based on a more specific description of the type of energy they employ. The internal combustion engine type, for example, encompasses those that use natural gas and those that run on diesel fuel. On the electric side, there are motors that use either alternating current (AC) or direct current (DC). And within the category of AC motors are two more classifications: squirrel-cage motors and wound-rotor motors, which, because of technical design differences, use the same type of energy in a slightly different way.
There weren't always so many choices available. Original shredders, introduced in the early 1960s, used squirrel-cage AC motors gleaned from retired World War IIMerchant Marine vessels and diesel engines from submarines, notes Keith Elkins, plant –manager for Sims-LMC Recyclers (Redwood City, Calif.). Wound-rotor AC motors and natural gas-powered engines came along next, he says, and DC motors entered the shredder market about six years ago, gaining entree with the development of silicon control rectification drives--technology that allows the alternating current supplied by electric companies to be converted to direct current.
While all the motor types are technologically compatible with any of today's auto shredders, energy availability at your shredder's locale could essentially rule out a particular motor type or types, especially those that run on electricity. Thus, a wise early step to making your motor decisions is to consult your electric company to determine whether or not the utility is equipped to supply enough electricity to your facility to support a shredder.
Ted Lipman, president of Denbo Scrap Materials Inc. (Pulaski, Tenn.), recalls that when his company installed its shredder in 1972, the local electric company informed the firm that it went with an electric motor, a new substation on its property an extensive electrical wiring that would be necessary –at the scrap processors expense. This, combined with what he describes as the “exorbitant” cost of electricity at the time, made an electric-powered shredder an economically unattractive option, he says. “The petroleum shortages in the early 1970s also made the cost of diesel high, so natural gas was the most appealing of all the options," Lipman adds.
L. Gordon Iron & Metal Co. (Statesville, N.C.) found a similar financial obstacle to electric power when it purchased its auto shredder in 1975, but according to President Saul M. Gordon, decided to go with diesel rather than natural gas as its internal combustion engine option because of insufficient natural gas availability.
And depending on your operation's location, you could find a totally different situation in terms of energy availability.
Narrowing the Choices
If sufficient electricity is available, an electric motor is likely your best bet, shredder manufacturers say, pointing out that such motors are widely regarded as more production-efficient than internal combustion engines. Electric motors are also said to require less maintenance than their gas- or diesel-powered cousins since they don’t need oil and filter changes, and they don’t need to be warmed up before use like engines do. In addition, environmental regulations have made internal combustion engines—particularly diesel-powered ones—a less attractive option, many industry experts note.
Some shredder operators that opted for internal combustion engines early on say that they are less satisfied with their choice today and are now looking into a switch to an electric motor.
One company that has already made this transition is Steiner-Liff Iron & Metal CO (Nashville, Tenn.), which replaced its natural gas engine with an electric shredder motor. “Our reason for making the change was increased efficiency through greater throughput and greater horsepower, as well as better environmental performance” notes Adam Liff, the firm’s president.
Also working in favor of electric motors today is the growing ability of scrap recyclers to negotiate reduced electricity rates in exchange for running their energy-hungry shredders during off peak hours or agreeing to shut down operations on short notice from the electric company to accommodate extra-heavy electricity demand in the area.
So let’s say you’ve decided to narrow your search to an electric motor. The next thing you should know is that even this half of the field has been thinned from a practical standpoint, with squirrel-cage motors falling out of favor in recent years due to what many see as their less-than-ideal efficiency compared with DC and wound-rotor motors. “They are just not designed to handle the overloading associated with shredding” says Elkins.
At issue is that most squirrel-cage motors work too hard to get up to speed, and ounce they do, are subject to inconsistent speeds, according to Jim Scwartz, vice president of engineering for Texas Shredder Inc. (San Antonio). He explains his point with an analogy, likening the two types of AC motors to two types of AC motors to two cars accelerating to 50 miles an hour from a standstill: Squirrel-cage motors operate much like a car whose gas pedal is instantly pushed to the floor, he says, while wound-rotor motors with liquid rheostat perform more like a car that slowly gains speed as gradually increasing pressure is applied to the gas pedal.
To help counteract this problem, Lindemann Recycling Equipment Inc. (Charlotte, N.C.) offers a “turbo coupler” attachment, which acts like a clutch to control the squirrel-cage motor’s starting speed and maintain motor peaks, according to Gunn Philips, the shredder manufacturer’s sales director. But while the company has had much success marketing the attachment in Europe , he says, it hasn’t really caught on in the United States .
The Electric Distinction
Put all of these comings and goings together, and today, while you still have a variety of motor types to choose from, you’re likely to hear shredder authorities debate the pros and cons of just two types: AC wound-rotor motors and DC motors.
The primary difference between these motors lies in how they deal with speed control. With a DC motor, you can adjust the machine’s speed, until you change it—something you might do to accommodate a load of a different variety of scrap. AC motors, on the other hand, cannot be set to different speeds but will accelerate on their own when necessary, such as when a particularly bulky piece of scrap is shredded.
This difference can affect a shredder’s electricity, proponents on both sides say, though they don’t agree on what those effects are.
On one side, advocates of DC shredder motors point out that these motors limit electricity demand charges thanks to their speed control abilities. This logic is built around the fact that many electric companies bill industrial customers based on the maximum amount of power used in a billing period—not the actual amount. Thus, if your shredder uses an average of 700 kilowatts most hours of operation, but pulls triple that power once during a billing period, you could be billed according to the 2,100-kilowatt peak. Because a DC motor eliminates such huge spikes, DC proponents say, electric bills can be better kept in check.
On the other side of the debate, AC proponents concede that because of the AC motor’s ability to rapidly increase speed and horsepower on its own, “spikes” in electricity demand are likely and can result un higher demand charges than with DC motors. But, they maintain, there’s a trade-off on efficiency.
As Scott Newell, president of Newell Industries Inc. (San Antonio), explains it, spikes in AC motor typically occur when a shredder encounters a particularly dense piece of scrap and powers up to keep the hammer striking force at a consistent rate. A DC motor in the same situation might “droop,” resulting in reduced speed and, thus, reduced striking force and longer shredding time for the particular load, he asserts. Elkins adds to this thought: “In my opinion, a good shredding operation is one that has a steady and even flow through the shredder, with no stopping. You need something that is continuous to get high tonnages. As far as electricity goes, I’m waiting to see if there is much of a difference in terms of overall cost between AC and DC motor cost inn tons per hour?”
Old or New? The Cost-Life Equation
Beyond electricity efficiency, the DC vs. AC motor debate turns to another apparent trade-off: initial cost vs. proven performance.
Virtually all of the DC shredder motors available today are reconditioned heavy-duty motors retired from pump applications such as oil fields, mines, and locomotives. Because they are used equipment, they generally carry a much lower price tag than AC motors, which are typically purchased new. This is especially true in light of today’s weak dollar against the German mark and the fact that Schorch motors, considered industry wide to be the hands-down favorite as far as AC wound-rotor motors go, are imported from Germany . In fact, according to some industry experts, a new AC motor can cost twice as much as a rebuilt DC motor.
As you might guess, there is much contention over whether those cheaper motors, because they are rebuilt, suffer in terms of performance and expected Life. Those who favor AC motors contend that wound-rotor motors have demonstrated their ability to work successfully in the shredding industry since its start, whereas DC motors are relatively new and unproven competitors. “Schorch-brand motors last 18 years on average,” claims Sabine Mueller, vice president of Mueller Engineering (New Berlin, Wis.), who notes that the comfort in knowing that a wound-rotor motor is likely to last a particular length of time may justify its additional expense when weighed against the unknown life span of a DC motor.
Another advantage of Schorch motors, she adds, is that they are specifically designed for shredding applications, while rebuilt DC motors come from other industrial applications.
On this point, even DC motor manufacturers admit they’re at a disadvantage, and emphasize that when buying a rebuilt DC engine, you must be sure that the bearings are heavy-duty enough to withstand the extensive vibration of shredding. They’re not willing to go along with any assertions that a rebuilt DC motor will need earlier replacement than a new AC motor, however. Paul Rembach, president of OMC Control Ltd. (Houston), for one, denies that DC motors produced by OMC are of any lesser quality or longevity than AC motors, explaining that OMC ’s reconditioned motors include all new parts except for the outer casting and shaft. Still, he notes, not all rebuilt DC motors necessarily boast this feature.
Sifting Through the Speculation
Though shredder experts can agree that the best choice for most shredder operators is a round-rotor AC motor or a DC motor, they can’t concur on which of these is most promising. Similarly, while they all say the future of shredder motors lies in the two types of electric motors, they’re split on which will be the most popular. Ralph “Bud” Murray, shredder production manager of Universal Engineering (Cedar Rapids, Iowa), figures that given the choice between a round-rotor and a DC motor, “it appears that the DC motor is the way to go most often these days.” Schwartz agrees, explaining that the high initial investment cost of the leading round-rotor motor makes a DC motor package a more attractive option.
But, retorts Newell, “ I think we are seeing people going to DC motors by mistake and for wrong reasons.” Elkins agrees, saying, “I’m waiting to see if people who installed DC motors are truly happy with the results.” Only time will tell if either type of motor will come to dominate the industry.
Given these kinds of varying opinions and the importance of the task of selecting the right motor, the best decision may be to find a shredding equipment consultant—most are electrical engineers—to help you weigh the factors affecting your operation. In fact, nearly all the shredder experts recommend that motor buyers consult an independent industry expert to best evaluate the motor question.
Selecting a motor for your auto shredder is a choice that can determine the efficiency of your operation for years to come. With so many factors to consider, how do you ensure you’re making the best decision? Here’s what some experts have to say.