Few things are as disposable as paper towels. But the next time you nonchalantly toss one away, keep in mind that there’s a lot more to it than meets the eye. Paper towels are manufactured through a finely tuned system of electrical, electronic and mechanical components. Coordinating all those components and keeping them running productively 24 hours a day is challenging.
Rob, an electrical engineer at a US paper mill, knows these challenges well. He is responsible for electrical and electronic issues related to two large tissue machines that generate several tons of paper towels an hour. Each machine is run by coordinated drive systems. With about 12 drives and motors plus auxiliary equipment that adds another 100 motors or so to each machine. The motors range from 3700 W to a 4500 kW behemoth.
“I troubleshoot everything from VFDs and motors to PLCs and instrumentation. We have a lot of valves, a lot of instrument sensors, and a lot of coordinated drive systems,” Rob says. On any given day he gets troubleshooting calls ranging from nuisance issues to potentially catastrophic failures.
Starting with the basics
Motor and drive problems are the most common calls. “Normally, your first action is to do an insulation check on the motor and wiring to make sure it is not shorting to ground somewhere,” Rob explains. “If the insulation check is reading low, that’s usually a dead giveaway that it is time to change out the motor.”
Sometimes, however, the problems are a bit more complicated. “If you have a sporadic fault or the motor is tripping out because it is pulling too many amps, or it does not have enough resistance, you have to investigate further,” he adds.
Because the machines run 24 hours a day, troubleshooting is done with the machines running if possible. “If it is a smaller component, most times you can isolate it and just fix that one area while the machines are running,” Rob notes.
The environment in a pulp and paper mill is wet, steamy and dusty. Technicians have to climb ladders to reach some of the equipment, so tools need to be easy to carry, rugged, and above all, reliable. Rob has found that Fluke tools perform well in that environment.
Responding to sensitive VFDs
Rob is almost never without his Fluke 87V DMM, whether he is doing first-line troubleshooting or delving down into more complex problems. A case in point: one night a VFD-controlled motor kept tripping. Technicians were concerned that they would have to replace the motor. That would have taken about eight hours, plus another two hours to restart the machine. “Using my trusty Fluke 87V, I was able to determine that the motor was fine, but the real issue was a 5 volt instrument supply module,” Rob recalls. “Older VFDs have a very low tolerance for these modules. The 87V was reading 4,77 volts on the supply module. The lowest possible voltage this drive can handle from the supply is 4,8 volts.” The solution was to replace the communication power supply board, which cost about R5000 and took just 15 minutes. “Had it not been for the accuracy of the Fluke meter, we probably would have changed the motor and still been stumbling around trying to find the problem. It’s a real testament to the accuracy of these meters,” he adds.
In a similar situation, Rob was able to find a subtle problem in a motor lead with a Fluke Thermal imager. In this case, a VFD kept faulting out after technicians had upgraded the drive and motor. “We checked the wiring with the insulation tester and found a slight degradation, but it was still within the acceptable threshold,” Rob explains.
They changed the VFD, but the problem continued. They were just about to change the motor when Rob decided to bring out a Fluke infrared camera. “We scanned the motor leads with the camera and the centre phase lead lit right up,” Rob recalls. It turned out that technicians had left the old wiring when they upgraded the VFD and motor. “The insulation was still within our specification, but the new VFD was far more sensitive than the old one and it detected the slight insulation breakdown and kept kicking out,” he adds. The wiring and the problem were solved.
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