Safe torque-off features in variable-speed motor drives let wrapping machines use simpler controls and smaller electrical cabinets.
Automatic stretch wrapper machines from Orion Packaging Systems, Alexandria, Minn., use ACS355 model variable-speed drives from ABB, Cary, N. C. Each machine hosts at least three drives. One rotates either a turntable holding a pallet full of material, or the wrapper mechanics around a stationary pallet. Another moves the lift carriage containing the film-delivery system up and down as it dispenses film. A third drive runs the multistretch mechanism that actually stretches and pays out the film as the pallet turns. Many Orion automatic-stretch wrappers also incorporate conveyors run by three additional ACS355 drives.
Safe torque-off is a built in and certified function that prevents unexpected start-ups. It essentially keeps a drive from delivering random rotational energy to motors. This reduces the need for separate safety components — such as a safety contactor to disconnect the drive — and lets machinery start up quickly after a safety stop. It also enables machine builders to fulfill the requirements of Machinery Directive 2006/42/EC.
For example, drives not equipped with torque-off functions must completely shut down in the event of an E-stop. But torque-off-equipped drives recover more quickly after an E-stop because they do not completely power down.
ACS355 drives are also bookshelf mountable, meaning they can mount in the panel side by side. This helps maintain the system UL rating and does not delimit the drive’s ability to dissipate heat.
The variance in load on the motors can be significant because of factors such as the wide range of pallet weights that the turntable must rotate. In their most core function, drives sense the amount of each load and generate enough motor torque and speed to handle it.