AC DC DRIVES

Plastics film extrusion speeds have been increasing over the last decade. Speeds of typical lines have increased by atleast 70-100%. For instance, 10 meter wide large BOPP film lines with a capacity of almost 30,000 Mta, are required to run at speeds of 600-800 Meter/Minute. The high running speeds exert more demand on winders.

The winders today are expected to provide very low tension and yet maintain uniform winding at such high speeds. This is possible because all the modern winders are AC driven and not operated by DC drive. The maintenance of the AC driven winders compared to DC drive is much low, making AC the preffered drive for winders. In the current scenario, practically all winders have made the switch to AC drive. Tremendous amount of development work has been carried on by machinery manufacturers in the last 10 years to make them more efficient and accurate.

The prime advantage of AC drives is not the speed, but low maintenance. DC drives have brushes that have to be replaced and can run at lower speeds only with external air cooling. AC drives have no brushes, don’t need cooling, are more precise and have greater speed range. More recently, AC servo drives have been used instead of AC vector types to give more accurate speed and position control. With DC drives, speed and torque are not linear, and at speeds below 10% there is no torque at all. On the other hand, AC motors—both vector and servo—have absolutely linear torque development from zero to maximum speed. That means that a DC drive has a usable speed range of about 80:1; and AC vector or servo drives have a 2000:1 range. New-generation winders can also maintain lower tensions at lower line speeds than ever before—an advantage with soft, thin webs like prestretched stretch film and with blown films. Blown film is inherently less even in gauge than cast, so it needs to be wound more loosely to make a flat roll. If stretchy films are wound too tightly, thin areas get thinner, and thick areas become ridges. With thin films, the gauge variation may be smaller, but a roll accumulates many more layers, so the total effect of the variation on roll quality may be greater. Blown film winds typically at 100 meters/minute and up to 200 meters/minute for thin HDPE blown film can wind at up to 300 meters/minute.

Older winders are being rebuilt with new drive motors, electronics and load cells at two-thirds the cost of a whole new winder. All the film processors have to look at improving the cost to remain competitive. High productivity is the way to go where new generation winders play an important role.

Content Courtesy :plastemart.com