machine building & automation - Industrial Technology Magazine

DRIVES, MOTORS & CONTROLSThe cost ofdowntime is asimple equationWhen a Hull port started experiencing problems with obsoletedrives, AC motors and resistors in two of its aging cranes, thesituation needed addressing immediately. Peter Scott explainshow the problem was solvedThe UK's leading ports group, AssociatedBritish Ports (ABP) owns and operates 21ports around the UK and handlesapproximately a quarter of the country'sseaborne trade. So it’s no wonder that in such abuzzing environment, downtime is never an option.With two cranes in need of urgent maintenance, theport’s management was expecting a substantial bill forresolving the issue.The equation looked like this: a new drive for thecrane, priced at over £20,000, plus a new motor, anadditional £20,000, plus installation and engineeringworth up to £60,000 and the additional cost of threeweeks downtime. Were this not enough to furrow thebrow of even the most resilient finance director, thetotal bill of £100,000 would be doubled, as therewere two cranes on site in need of maintenance.Unfortunately, as the cranes were over 20 yearsold, many of the parts needed had become obsoleteand were extremely hard to source. This is when ABPHull asked maintenance specialist CP Automation tocome up with a viable solution to their troublesomeequation, one that would not disrupt activity and keepmaintenance expenditures down.Permanent solution“We knew that the hoist drives on the two older craneswere coming to the end of their life,” explains KeithPayne, electrical supervisor at ABP Hull. “In the lastfew years we’ve tried to repair and update them to thebest of our capabilities, so that we could keep themgoing. But despite all of our predictive maintenance,there came a point when we encountered too manytechnical problems so we had to find a permanentsolution.”Having previously worked on a similarimplementation abroad, the CP Automation fieldmaintenance team took on the challenge of fixing theHull cranes. The solution to the product obsolescenceproblem came in the form of motor controllers for slipring motors from Sprint Electric. Sprint Electric’s JL/Xrange of slip ring motor controllers covers outputcurrents from 100 to 1850A in either two or fourquadrant mode.A slip ring motor is a type of induction motor witha wound rotor connected to external rotor resistors viaa slip ring. The motor’s speed/torque characteristic ismodified by resistors connected to the rotor slip ringscontrolled by JL/X relay output drivers. Out of the fourresistor control outputs provided, only one or twowould be used for applications such as slew, yaw orX/Y travel. Hoist applications however might use threeor four of these outputs. For hoist applications,lowering at speeds above synchronous speed isdetected and, as a result, rotor resistance is minimisedfor maximum regeneration into the supply. Anadditional digital output is provided for this purpose.Provision is made for selection of analogue orprogressive joystick switch speed demand, with tworaising and two lowering speeds. Analogue outputsrepresenting speed, torque and power are supplied inthe standard configuration.With the Sprint drives fitted, the cranes were givena new lease of life. CP Automation’s solutions used anew controller and retained the existing motor, whichled to a significant saving on the overall project costs.For the first crane, the total costs were significantlyless than the initial budget price of changing both driveand motor, a saving of more than 60%. The secondcrane costs were further reduced due to theprogramming and design engineering done for the firstcrane was implemented on the second crane refit. Thetotal bill for both cranes came in at a fraction of ABP’santicipated price.The crane refit in Hull is a perfect example ofmaintenance expertise combined with speedy andcommitted customer service. What’s more, the CPAutomation team simplified an otherwise financiallydaunting equation: two cranes functioning at optimumperformance, additional spares and support and only afew days of downtimewww.cpaltd.netPeter Scott is service manager at CP Automation32INDUSTRIAL TECHNOLOGY • April 2012