Comparing DC, AC Wound Rotor Shredder Drives

During the 1990s, scrap yards could choose their shredder drives. The rule of thumb was DC for large and mid-size systems (5000 hp and below) and AC wound rotor for super shredders (6000 hp and above). In 2004, most shredder manufacturers are only offering AC wound rotor drives. What happened to DC shredder drives? Are AC wound rotor drives the right choice for everyone?

History

AC squirrel cage and diesel drives dominated the early days of the auto shredder industry. Sabine Mueller, of Mueller Engineering says. “As shredders increased in size AC wound rotor drives were developed to offer more power,” she remarks. Mueller started installing and maintaining AC systems in the ‘60s.”

Jim Keck, president of Joliet Equipment, notes, “During the ‘90s, DC drives were introduced, first as helpers for AC wound rotor drives and then as independent drive systems. 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.”

AC vs. DC

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. Why has the auto shredder Industry moved in this direction? What options are available now?

Industry Trends

Consolidation of the recycling industry has lead to large organizations with several scrap yards. These organizations have resources to start up and operate the biggest shredders they can get. For the last few years, the number of super shredders sold has been steadily increasing. AC wound rotor drives have the extra power needed in the 6000+ HP range. Throughput is everything for large recycling organization, 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 with an auto shredder. He says, “Recent increase in the price of steel has created an opportunity for smaller, regional scrap yards to break into the auto shredding market. We need a 4000 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.”

Maintenance

AC wound rotor drives with liquid rheostats have primarily been installed and maintained by Mueller Engineering. Sabine Mueller says. “A scrap yard typically has a maintenance contract with Mueller. We come out twice a year to do preventative maintenance. People think AC Drives and liquid rheostats are maintenance free but they require just as much service as a DC System.”

Joliet Equipment (JE) and OMC were two of the main providers of DC Drives in the 1990s. Often, they were criticized for not making arrangements with the scrap yards for maintenance. Yards ran the DC drives until they broke down and then called for emergency service. DC drives developed a reputation for being expensive to maintain.

Shredder Drive Providers

Shredder Manufacturers typically sell a shredder drive as part of a shredder system, but they resell and outsource this part of the system. For AC wound rotor drives they buy new motors from Germany and hire Mueller to install and maintain the liquid rheostat. This has proven to be a reliable solution for manufacturers to offer their customers.

Joliet Equipment still sells and refurbishes motors but no longer provides the controls for DC Shredder Drives, and OMC has gone out of business. With this vacuum in providers of DC Shredder Drives you can hardly blame Shredder manufacturers for preferring to promote AC Drives.

For the past few years, Quad Plus, Inc. has been attempting to fill the void left by these manufacturers, providing service for yards with DC drives and selling new DC shredder drive systems. Chris Tooley, president of Quad Plus says, “Though we are not well known in the recycling world, Quad Plus is important because we are giving scrap yards an alternative to AC systems.”

Features Comparison:

AC Wound Rotor

Advantages

o        Best choice for 120” super shredders

o        Can run effectively above rated loads

o        Lowest initial cost, 6000 hp and above

Disadvantages

o        Liquid rheostat maintenance and cooling water requirements

o        Least efficient power consumption

o        High inrush (starting) and peak current

o        Motor brush maintenance

DC Drive

Advantages

o        Lowest initial cost, 5000 hp and below

o        Short lead times for new systems

o        More torque at lower speed

o        Complete speed control

o        Jog forward and reverse

o        Brake to stop

Disadvantages

o        Control electronics maintenance

o        Motor brush maintenance

Scrap Yards have a Choice

If you are buying an auto shredder you need to do your homework. AC wound rotor drives are probably the best choice for super shredders, but DC Drives are a cost-effective alternative for systems of 5000 hp and below. Joseph Crosetto

The author is is a Sales Engineer for Quad Plus, Inc. Quad Plus, Inc. is an  engineering and system integration company specializing in DC Shredder Drives and Automation. Contact them at 877-870-7823 or www.QuadPlusInc.com.