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."

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.

An encoder is a device that gives feedback to a drive based on the rotation of the motor or of something it powers. The encoder translates the rotation into analog or digital data. Based on this data, a processor in the drive directs changes to the current being fed to the motor, turning it on or off and altering voltage (to control speed) or amperage (for torque). Many motors, such as servomotors, come with encoders built in. Other encoders are sold on an aftermarket basis, for attachment to the motor's shaft or the driveshaft of a conveyor or other equipment.

Encoders are a well-established technology, especially in servos, whose use in packaging machinery has greatly increased in recent years. Drives that use them, known generically as closed-loop drives, are about 10 times more precise in speed control than open-loop drives.

But encoders have their drawbacks. They add to the cost of the motor, and they're susceptible to accidents and environmental stress.

"The encoder can get knocked off the motor because it was hanging out in an aisle or it was sustaining damage from gases or dust particles--something that would impede its operation," says John Tisdale, a sales manager for ABB Industry Oy.

That's where no-encoder drives can help. No-encoder technology has spawned a new generation of AC drives that can greatly improve a motor's accuracy while maintaining the simple design of a threephase AC induction motor. These drives use an especially sophisticated algorithm in analyzing the current that the motor draws at any given point in its operation.

# posted by Medical Information : 1:21 AM