Cement, Energy and Environment

I AC drives from Rockwell Automation have the torque capability and precise speed control to handle Ash Grove's spotting Process, typically a medium voltage drive application. control system, Ash Grove invested in an unusual, yet effective, solution - installing a technologically advanced, low voltage variable frequency AC drive to its existing 2300 hp, medium voltage AC motors. Ash Grove not only updated its drive technology, it modernized its approach to maintaining capital investments. This resulted in an initial savings of US$ 250 000 and 90% uptime. "Not too long ago, manufacturers had a run-it-until– it-breaks mentality. But now we have the tools to protect capital investments like our motors." Wright says. Tailored solution At Ash Grove, motors power everything from the massive, revolving, rock-filled kiln to the grinding of up to 4500 tpd of cement. Workers at Ash Grove had trouble when time came to service the three ball mill each month. For technicians to enter the mill for serv1c1ng, they used an antiquated 60 hp generator motor to rotate the mill inch-by– inch until it reached an exact position. The process of manually positioning equipment, called "spotting," became difficult because technicians had no effective way to accurately apply torque to the medium voltage motor directly from the power system - the technique used to slowly rotate the mill. "Along with the problems we had moving the bulky mill to a precise position, the cogging , or abrupt starting and stopping of the motor, can cause mechanical and electrical damage to equipment," explains Wright. The issue interfered with the company's goal of minimizing downtime while maximizing production . "Each hour we shut down operations to perform routine maintenance or resolve a fault translates to 300 t of cement that could have been produced," says Wright. Fed up with the frequency and expense of the problem, he shared his frustrations with his sales contact at Rock well Automation, Ash Grove's automation supplier for the rest of the facility. "Traditionally, this situation called for a new spotting controller and gear motor or a medium voltage drive," Wright says. In this case, Ash Grove replaced the generators that powered the mill spotting with preconfigured, 480 V, 450 hp, AC variable frequency drives from Rockwell Automation. The Allen-Bradley AC drives power three existing 4000 V, 2300 hp AC motors exclusively during the spotting process to efficiently rotate the ball mill and bring it to a controlled start and stop. To run the mill at full speed, Ash Grove uses a synchronous motor starter, which does not have any speed control. "With over 30 years of experience in the cement industry, I consider myself a DC devotee," Wright acknowledges. "I never believed AC technology could produce 1 00% torque at zero speed until Rockwell Automation developed an AC motors control solution for a high-torque application." Configured to replace a generator motor application, the low voltage Allen-Bradley AC drive has the torque control capability to operate at the 6 hz required by the ball m1ll for 22 >-