Scrap Metals Supplement--Driving Questions

During the 1990s, scrap yards could choose their shredder drives. The rule of thumb applied to drive selection was that alternating current (AC) wound-rotor drives were preferable for super-sized shredders (6,000 hp and above) while direct current (DC) drives were suitable for large and mid-size systems (5,000 hp and below). However, in 2004 most shredder manufacturers are only offering AC wound-rotor drives.

What happened to DC shredder drives? And while AC wound-rotor drives predominate in the start of this decade, are they the  AC "squirrel cage" and diesel drives dominated the early days of the auto shredder industry.

CURRENT EVENTS. AC "squirrel cage" and diesel drives dominated the early days of the auto shredder industry. Sabine Mueller of Mueller Engineering, New Berlin, Wis., says, "As shredders increased in size, AC wound-rotor drives were developed to offer more power." Mueller Engineering started installing and maintaining AC systems in the 1960s.

"During the ‘90s, DC drives were introduced, first as helpers for AC wound-rotor drives and then as independent drive systems," Jim Keck, president of Joliet Equipment of Joliet, Ill., says. "DC drives had some advantages, but the main one was price. Reconditioned DC motors, primarily from railroad locomotives, were readily available and inexpensive. By contrast, most AC wound-rotor drives were manufactured in Germany and mainly purchased new."

Over the last few years, the use of AC wound-rotor drives has grown to the point where they now dominate the market. But DC drives may be poised for a comeback.

One notable trend has led the auto shredder industry to move toward AC motor drives: consolidation.

Consolidation within the recycling industry has led to large organizations with several scrap yards. These organizations have resources to start up and to operate the biggest shredders they can get. Therefore, for the last few years, the number of super-sized shredders sold has been steadily increasing.

AC wound-rotor drives have the extra power needed in the 6,000-plus-hp range. Throughput is everything for large recycling organizations, particularly with yards in major metropolitan areas.

However, the industry is always evolving.

A scrap yard owner with two yards in rural areas is planning on opening a third yard with an auto shredder. He says, "A recent increase in the price of scrap steel has created an opportunity for smaller, regional scrap yards to break into the auto shredding market. We need a 4,000-hp shredder drive and are very price conscious. A DC drive with two reconditioned DC motors running in tandem may be the best solution for us."

In addition to consolidation among scrap yards, a second issue working against DC shredder drives has been misunderstandings related to drive maintenance.

MAINTENANCE MATTERS. AC wound-rotor drives with liquid rheostats (resistors for regulating a current by means of variable resistances) have primarily been installed and maintained by companies such as Mueller Engineering.

"A scrap yard typically has a maintenance contract with Mueller. We come out twice a year to do preventative maintenance," Mueller says. "People think AC drives and liquid rheostats are maintenance free, but they require just as much service as a DC System."

Joliet Equipment and the former OMC Controls were two providers of DC drives in the 1990s. Often, the companies were criticized for not making arrangements with scrap yards for maintenance.

Yards ran the DC drives until they broke down and then called for emergency service from Joliet or OMC.

In the absence of regular preventative maintenance and servicing, DC drives developed a reputation for being expensive to maintain.

DRIVE PROVIDERS. Shredder manufacturers typically sell a shredder drive as part of a complete shredder system, reselling and outsourcing the drive portion of the system.