Smoother flow

Crane control technology has not kept up with the needs of its users, which has resulted in an array of problems. In many cases, the installed base of DC electro-mechanical control is reaching the end of its economic life, typically 20 years. New technology is needed to address the ever-increasing concerns with rising energy costs and the need for improved performance and reliability.

That, at least, is the view of Eaton Corporation, a leading supplier of drives and control technology, which has produced 'a technologically advanced' replacement for the traditional DC electro-mechanical series motor drives used for crane control.

Eaton's Cutler-Hammer Respondor DDC drive has been in operation for about a year at the Canton, Ohio facility of The Timken Company, a US manufacturer of bearings, steel, and components.

Timken has a dual hoist crane that operates in a pickle line where 10 ton bundles of tubing are dipped into large acid tanks at approximately 35 lifts per hour. The DC Constant Potential (DCCP) control boards were almost 30 years old and worn out due to harsh environmental factors, primarily moisture and acid. As a result, the facility experienced mechanical failures with the DC controls about once a week, with each failure causing 10 or 15 minutes downtime. Timken wanted to eliminate this downtime, and also wanted a smoother, more energy efficient control on the dual hoist crane.

Eaton says that it acknowledged these issues when creating the Respondor DDC drive, a solid state DC to DC drive designed for crane applications. High-speed semiconductors are used in the microprocessor drive to maximises crane performance and reliability and minimise downtime, maintenance and energy costs.

"Unlike traditional DCCP systems, which use resistors to regulate motor current in switched stages, the Respondor DDC's use of semiconductor switches to control the speed and torque of the motor eliminates costly maintenance expenses associated with resistors and contactors," claims Duane Repp, product line manager at Eaton's Cutler-Hammer business unit.

Timken turned to Eaton for advances in crane control technology due to a long-standing relationship between the two companies, and coincidentally, Eaton was planning a Beta test for the Respondor DDC. The new drive was installed on 11 February 2002.

As with any new technology, there was some initial scepticism because "if the crane operators aren't happy, no one's happy," says Robert Borgman, Cutler-Hammer senior sales engineer. However, any concerns were overcome by the ease of start up and lack of complaints from the crane operators, adds Borgman.

The Respondor DDC is designed to withstand harsh mill environments. It has a stainless steel enclosure and sturdy packaging that minimises shock and vibration effects. It is retrofitted within the existing space where the DCCP control used to be. It is engineered as a 'drop in' replacement using the same connections. In Timken's case, the Respondor DDC was 200mm narrower and at 1.5m high it was 300mm shorter.

Timken wanted a control that would reduce wear on other system components. The Respondor DDC drive is microprocessor controlled, which regulates the operation of the semiconductors to provide the current necessary to move the load at a rate of speed commanded by the operator. In the traditional contactor and resistor type, the operator requests a speed point, which results in the stepping in and out a series of contactors and resistors. Each time a set of resistors is placed in the circuit, the motor sees an inrush of current, which contributes to mechanical wear. According to Eaton, the microprocessor of the Respondor DDC provides smooth acceleration and deceleration, reducing the inrush amounts of current, thus prolonging the life of the motor. The smooth acceleration also controls the impact of the speed changes to other mechanical components, reducing the cost of mechanical repairs.

At the Timken facility, the Respondor DDC was installed on one side of a dual hoist crane while the other side continued to operate on the DCCP technology. Plans are under way to compare the power consumption of the DDC and DCCP through actual field measurements. The measurements will compare the Respondor DDC and the DCCP performing numerous applications such as raising and lowering the hoist with no load, as well as half load and full load.

Eaton claims that in a separate study, raising a half load, the Respondor DDC used 340 amps less than the DCCP on initial start up, used slightly less energy during the final phase of hoisting, and maintained the load at approximately 15 amps less than the DCCP.

According to Reed Brown Jr, unit manager - finishing reliability at the Timken facility, the maintenance staff required minimal training and felt the Respondor DDC drive operated well. The operators at the Timken facility liked the new technology because it is microprocessor controlled rather than a system of contractors and resistors, providing for better control of the crane and the load. Timken has placed an order a Respondor DDC drive for another overhead crane and also for a non-crane application.