Motors & Drives Information

A new type of AC motor drive has the potential to provide packaging machinery users more precise control for little extra expense.

No-encoder drives use advanced methods of analyzing current and other factors to allow for extra-tight control of speed and torque. They offer the low cost and relatively easy maintenance of AC motors, avoiding the extra expense and fragility of encoder mechanisms.

These motors are not yet used extensively by packaging machinery manufacturers. But as manufacturers strive for more precision and interconnectivity, no-encoder drives are emerging as an alternative and/or supplement to servos, steppers, standard three-phase AC motors and other devices.

"We model the motor itself and from that, based on what the currents are doing, we can tell what speed it's running," says Glenn Frazier, a product manager for Rockwell Automation's drive business. "It's not quite as precise as with an encoder, but it's pretty good."

Speed or torque?

Controlling the speed and torque of an AC motor depends on several factors. In a sense, speed or torque control is an either/or proposition. A drive can keep the motor's speed at a given rate and vary the torque to stay at that speed; or the torque can stay constant, and the motor will speed up or slow down depending on how much there is to pull. This dichotomy is due to the way AC motors respond to electricity. Speed is a function of the intensity of electric current (expressed as volts), while torque depends on the amount of current (amps).

Both speed and torque control have their place in many kinds of packaging machinery. For instance, in form-fill-seal equipment, speed control is necessary to match the film feed to the packaging flow, says Steve Bergholt, chief engineer for Triangle Package Machinery Co. Torque control is needed to hold and feed the film with just the right amount of tension. A typical form-fill-seal machine will have motors with both torque-based and speed-based drives.

Achieving more precise control has always been a goal of AC motor manufacturers. (In general, AC motors are preferred for industrial machinery. DC motors offer more precise control, especially at low horsepower, but their design makes them harder to maintain.) One increasingly common way to increase speed control for AC motors is through the use of encoders.

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For the marketer, who sees the carton as an influential point of entry for the product packaged inside, image is everything.

But the carton is more than just a pretty box that holds product and distills information. As convenience drives consumers' purchasing decisions, cartons designed for functionality are attractive assets. The abundance of packaged kits for health care products or easy-to-prepare meals supports this growing trend.

Because there is no such thing as a simple carton, there is no such thing as a simple cartoning machine. In fact, because cartoners include many moving parts they are riddled with complexity. With carton machinery, the challenge, men, is making sure all of the machine's components are kept in sync. Without precise synchronization, cartoning equipment will jam and a jammed cartoner will disrupt the packaging line, causing time-consuming and expensive downtime.

Electrified cartoners

During the last five years there has been a shift from using mechanical line shaft driven cartoning machines to adopting machines that are electronically line shaft driven. When a cartoner's components are electronically line shaft driven the machine can be made 'stiffer,' producing a more closely coupled, tightly linked system.

According to Dan Throne, food and packaging industry manager for the electric drives and controls division of Bosch Rexroth Corp., changing from a mechanical line shaft to an electronic line shaft shows immediate cartoning equipment benefits including:

1. A reduction in the amount of mechanical components in the machine. Machines are no longer restricted to mechanical gears and cams that link components together.

2. With electronic line shafting, cartoning equipment is programmable and every axis that has a servo motor can be changed on the fly, which means fast changes for package sizes, gluing applications, product placement, filling and conveyor speeds. By using digital servo drives, a controller and a human machine interface (HMI), changes can be made at the touch of a button.

3. Electronic line shafting is simply a more sanitary design. There no longer has to be grease and lube slinging all over the machine and into crevices and gears. Consequently, cartoning equipment can easily be washed down and is more sanitary for food, beverage, pharmaceutical and cosmetic packaging.

The heart of the machine

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