Retrofit Drives Production
Judd Wire Inc., Turner Falls, Mass., made its mark on the wire-and-cable industry by setting the standard for electron beam irradiation of wire. Now, it is among the first to retrofit all its production lines—and build new ones—using AC motors and motor-drive technology.
The ultimate goal for Judd’s manufacturing facility in San Marcos, Calif., is the conversion of all machines powered with traditional line shafts, pulleys and encoder gearboxes to electrical drive/motor control. Such a retrofit not only eliminates the mechanical equipment and labor-intensive maintenance required to keep it operational. Motor controllers in tandem with the motors also provide extremely precise control.
“Instead of using line shafts with gears, belts and equipment following the line shaft, we now just plug in two fiber-optic wires between the drives, three wires to each motor, and that’s it,” says maintenance electrician Tracy Wehrung.
The most severe electrical demands in wire production are made by the capstan (winder) and unwinder, which are often used to regulate the draw speed of any extrusion process—wire, jacket or cable. A recent retrofit of a key profiling line included replacement of the existing DC closed-loop electrical control with a 5-hp motor drive on the capstan, and a 40-hp drive on the extruder.
Built into the drives is an open-loop direct-torque control feature that enables them to calculate the torque and flux of a motor 40,000 times per second. This responsiveness to motor load not only makes the drives virtually tripless, but the absence of any required encoder for feedback from motor to drive also reduces capital costs for the controller, compared to pulse-width modulated drives.
“Because this open-loop control of torque is so precise, the drives can adapt to and handle changes in load, over-voltages and even short circuits immediately,” says Rob Brown, who works with Wehrung’s building production lines. “High torque at low speed is critical to processes that include multiple wire, braiding and jacketing of cable.”
On a cabling line, built from scratch to wrap string and Mylar around large-diameter cabling, Wehrung and Brown created a master/follower electrical control arrangement, with the drives and motors on the line’s planetary and tape feeds following the lead of the capstan master drive. The entire line, in turn, is controlled via a programmable logic controller. “Once these drives are out of the box, installed and running, we never have problems,” said Wehrung.
All 30 drives—ranging from 0.5 to 125 hp—at the plant operate their motors without encoders. Encoders, those fragile glass disks that historically have been hard-wired to motors and provide feedback to controllers regarding exact operating speeds are a weak link in hostile processing environments, notes Wehrung. They can also create electrical pulse and noise that interfere with production.
In addition to increasing throughput, elimination of these encoders has saved enormous maintenance time and labor costs. “A broken or failing encoder shuts down the process—and the entire line,” says Brown. “And it can be tricky to troubleshoot, correctly identify and fix the problem. Meantime, there’s pressure to get back up and on line. Sensorless operation eliminates all of these drawbacks in one fell swoop.”
The wire and cable producers have gained additional operating advantages from AC drive and motor retrofits.
“For one line, we bought a large reel take-up and put it into operation on a small reel,” says Wehrung, “so we had to over-speed the motor so that it could run at the normal line speed, until we could get another take-up built.”
Flexibility to over-speed the drive without damaging the motor saved a lot, he notes. And it kept the line in production.