Unwinders used in manufacturing processes can benefit from ac motors, drives, and load cells used instead of mechanical-brake-and-air-supply systems.

For any processor unwinding paper, film, or foil, precise brake control – utilizing an ac motor to replace a traditional mechanical-brake-and-air-supply system – will provide a quantum leap in operating, maintenance, and cost benefits.

"All those mechanical considerations of keeping an air-powered brake, and the air line and air supply to them, functioning go the route of the dinosaur," says Bob Sarnelli, product manager at ABB Inc., Automation Technologies, Web Tension Products. "And, the electronic control replacing it, across the full range of horsepower applications, erases all those mechanical headaches, and provides superior unwinder control – for a cost savings that keeps paying and paying users back."

The ACS 800 direct torque control drive from ABB Inc. can be used for any processor unwinding film, foil, or paper. It uses an ac motor to replace the traditional air-actuated mechanical brake system.

Traditional air-actuated brakes on unwinders function in a very similar way to the brakes on front car wheels; both have a disk, calipers, and friction pads. The function of the brake is to control the tension of the unwind material into the process; more specifically, the brake needed to keep unwinder tension constant, to avoid variations in print registration, wrinkling of laminated materials, uneven glue application, and film stretching.

The most modern techniques within these configurations use tension load-cell systems to monitor the web tension, and a setpoint controller that feeds the difference between the desired unwind tension (setpoint) and the measured tension from the load cells to the air-brake control. Because both the tension system and the setpoint controller are electronic, a current-to-air-pressure valve is used that converts the electrical current signal to proportional air pressure. If the tension is too low, the brake's air supply is increased, which applies greater braking force (torque) and increases the tension – just like putting more pressure on the brake pedal of a car.

Small Horsepower Solution

Since the basic operation of the brake is to apply holdback force, why not use a motor in a mode that requires force to turn it?

ABB's integrated web speed and tension control solution – including motor (left), drive (center), and load cell (right)– applies easily to printing, coating, film, foil, flexographic, wire and cabling, and textile machines.

The idea of using a motor as a brake is not something new and has been used successfully in unwinding applications, but mostly in large applications that require high horsepower, such as paper manufacturing where the rolls are extremely large. In these applications, a dc motor, gearbox, and drive control are used to control the braking force. Since the motor is generating power, the energy must be dissipated in some way, such as returning the power back to the plant or heating resistor banks, which essentially drain off the energy. Returning the energy back to the plant yields payback, but it is generally used on motor brakes above 25 hp.

Historically, the biggest obstacle to using a motor as a brake on small-horsepower unwinder applications has been cost. "But with all the recent leaps variable frequency AC drive technology has made, the ac drive/motor solution now provides performance and control superior to the traditional dc installations," says Chuck Hollis, manager of several ABB drives lines at ABB Inc., Automation Technologies, Drives and Motors. "The ac motors in the small-horsepower ranges are widely available and extremely low cost."

Because ac drives and motors have continued to add performance features while reducing price per horsepower, ac drive/motor solutions have become "extremely competitive with mechanical brakes," according to Hollis, "and they are lower in price, when considering the lifetime costs of installing, operating and maintaining a mechanical brake-and-air-supply system."

Encoder-Less Motor Control

"The brake torque required is the web tension times the roll radius – and by sending a 4-20 mA reference from the setpoint controller to directly control the motor torque, a perfect tension can be maintained at any speed, including zero speed," Sarnelli says.

Since the motor/drive torque-response time is less than 5 msec, optimal tension can be maintained from zero speed through rapid acceleration, then to steady, constant-speed state, and down through rapid deceleration to a stop. To maintain extremely light tensions, the motor changes torque direction seamlessly, to help maintain tension even during fast acceleration.

For threading the machine, the motor speed also can be increased to provide enough paper, film, or foil to feed into the rollers. This kind of performance, along with zero maintenance and a wide tension range provides operating advantages over a mechanical brake. "Such control enables processors and converters to maintain and meet very tight tolerances – from the beginning to the end of the roll," says Hollis.

"We are beginning to see more and more demand for this advanced solution, and the competitive costs of small-horsepower, low-voltage ac motors and drives will continue to make this compelling for end users throughout the host of industries that use unwinders," Sarnelli says. He also points out that, not long ago, machines used relay logic rather than programmable logic controllers, line shafts rather than direct-driven rolls, and mechanical dancers rather than electronic load cells for tension control. "Motor brakes are a natural part of this evolution to more precise and electronic control," he says.