John Casey teaches electromechanical technology at Fox Valley Technical College in Appleton, Wis. He worked as a machine engineer/designer for close to 20 years at Metso Paper and Pierce Manufacturing.Just as with humans, electric drives can work against each other or share a burden unequally. Sometimes this produces good results, however. Some machines require differential torque and speed between separate machine axes, and industrial electronic drives can provide better coordination and control than purely mechanical systems.
In many web-processing machines, drives work toward opposite ends. A drive at the unwind end of the machine pulls the web backward, while a drive at the windup end pulls the web forward. The purpose is to maintain appropriate web tension for the process. It is typical for the windup drive to operate in speed-control mode, allowing for adjustment of the machine speed, and for the braking unwind drive to operate in torque-control mode to maintain proper tension.
For example, at steady speed, a braking torque of 9,000 lb-ft applied to a 120 in. diameter, 300 in.-wide unwind roll will result in tension of 6 lbs per linear inch (pli). The situation is complicated by the fact that the system also must maintain proper tension during acceleration, deceleration and at zero speed. Insufficient tension can result in wrinkled product, and excess tension can cause the web to break.
[pullquote]For webs with low elasticity, the dynamic torque response of the unwind drive is critical. Historically, mechanical tension feedback devices and dc motors controlled winding machines. Tension feedback from electronic load cells and development of advanced ac drive technologies such as direct torque control have greatly improved these processes. Another improvement has been regenerative energy recovery from the braking unwind drive.
The web-processing industry also provides an example of drives that pull in the same direction but share the load unequally. In a two-drum winder, paper rolls are wound up by rotating them in the valley between two driven drums. The rear drum operates in speed-control mode, and serves as the master speed reference for the machine. While winding up a paper roll, the front drum operates in torque-control mode with a setting that will deliver a guaranteed higher percentage of torque than the rear drum. This torque differential produces tight rolls with wound-in tension.
A mechanical automotive differential allows two wheels to turn at different speeds while applying equal torque to the wheels. If one wheel loses traction, that wheel will spin with very low torque applied, limiting the torque that can be delivered to the other wheel to that same value. This is a case in which uneven load sharing is desirable, so that higher torque can be applied to the wheel that maintains traction. An electric motor with dual rotors could supply the required differential torque.
Electric machines with two or more rotors can be supplied from separate electronic drives or a single drive. If a single drive is used with two induction rotors, and the output frequency from the drive produces a synchronous speed faster than both rotor speeds, then both rotors will be driven as motors. If the synchronous speed of the supply is controlled to be slower than both rotor speeds, then both rotors will be braked, and energy will be regenerated. If the synchronous speed of the supply is controlled to be between the rotor speeds, then the faster rotor functions as a generator and the slower rotor functions as a motor.
An electric machine with non-coaxial rotors can provide differential mixing action for chemical-blending applications. A common housing assembly fixes two rotor axes at an angle to each other, which provides for blade separation and outward thrust components, while maintaining a compact motor head assembly. The electronic control can be programmed for mixing cycles that include speed changes and periods of either co-rotating or counter-rotating blades. Opposing forces are also helpful for material stretching or separation processes, especially after slitting fiber or sheet metal. Rollers that are slightly angled to each other will pull material primarily in the same direction, while also pulling outward in opposite directions. Electric drives provide take-up at the desired speed.
