Using an AC motor as a brake for web unwind applications

For any machinery that is unwinding or holding back any strand or web, 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 functioning, as well as the air line and air supply to them, go the route of the dinosaur," says Bob Sarnelli, product manager, ABB, Inc., Automation Technologies, Web Tension Products.

The electronic control replacing them, 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–a fact proven by several installations of this next-generation, integrated control.

Traditional air-actuated brakes on unwinders function in a very similar way to the brakes on the front wheelsof a car: 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, such as a labeler, form/fill/seal machine or printing press. More specifically, the brake is needed to keep unwinder tension constant, to avoid variations in print registration, wrinkling of laminated materials, uneven glue application, film stretching, or wire drawing and cabling, to name just the most critical functions.

The most modern techniques within these configurations use tension loadcell 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 loadcells 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 (4 to 20 milliamps) to proportional air pressure–an I-to-P (current-to-pressure, or I/P) converter. 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.

This has been the method used for decades, but air brakes and their air-supply system have many application shortcomings and invariably become problematic as they age.

The motor solution for small-horsepower
Since the basic operation of the brake is to apply holdback force, or torque, why not use a motor in a mode that requires torque to turn it? The analogy, says Sarnelli, is the alternator in your car, which produces electrical voltage and current. But to get this energy, the car must supply torque, or turning power, to the input shaft of the alternator.

Motors can be excited (powered) to act as a generator/alternator and will then have a resistance when the shaft is rotated. If the motor is excited to be a generator, it will act as a brake and will require torque to turn the input shaft. As the excitation current is increased, more torque is required to turn the shaft and more braking results.

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. And, 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 (VFD) technology has made, the AC drive/motor solution now provides performance and control superior to the traditional DC installations," notes Chuck Hollis, manager of several drives lines at ABB, Inc., Automation Technologies, Drives. AC motors in the small-horsepower ranges are widely available and extremely low-cost–a benefit to processors ready to retrofit or install unwinder controls.

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

Installation: a case in point
A recent installation illustrates the performance and cost advantages of using an AC motor-brake system in a plastic film application. IR Industries, a Brewster, NY, converter of printed tapes, had a two-roll unwinder with two, 10-year-old mechanical air brakes and a host of repeating problems: high pad consumption; pulsating braking, due to worn disks; poor tension control at the lower-tension values, especially nearing the core diameter (the point at which the lowest torque is required); and periodic maintenance of the air supply and the I/P regulator.

Product quality and production were suffering. IR Industries initially decided to replace the mechanical brakes with a new, more modern mechanical brake. But since the installation already included an ABB closed-loop tension-control system, ABB recommended a retrofit with a motor brake system, Sarnelli explains. ABB installed a 5-hp AC four-pole motor, a 4:1 gear reducer, and an ABB ACS 800 Direct Torque Control (DTC) drive, which has built-in flux braking. Because DTC calculates the state (torque and flux) of a motor 40,000 times/sec, no tachometer feedback was required, and the drive operated the motor in torque mode rather than in speed mode. The brake torque required is the web tension times the roll radius. And, by sending a 4- to 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. Since the motor/drive torque-response time is less than 5 milliseconds, 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 also can be sped up, to provide enough paper, film or foil to feed into the rollers. This kind of performance, along with zero maintenance and an extremely wide tension range, continues to make the plant very satisfied with its choice of a motor over a mechanical brake, according to IR Industries.

Extremely stable tension control also improves the quality of products. Such control enables processors and converters of all kinds of products to maintain and meet very tight tolerances–from the beginning to the end of the roll, points out Hollis. The company's integrated web-speed and tension-control solution–including motor, drive and loadcell–applies easily to printing, coating, film and foil.

Motor braking: future made present
The optimal operating benefits that motor braking offers, and their elimination of the constant mechanical problems associated with mechanical brakes, usher in a new era of unwinder control, according to Sarnelli.

The market is 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 packaging field that use unwinders. Remember, says Sarnelli, not long ago, machines utilized relay logic rather than programmable logic controllers, line shafts rather than direct-driven rolls, and mechanical dancers rather than electronic loadcells for tension control. "Motor brakes are a natural part of this evolution to more precise and electronic control," he concludes.

More information is available:

AC motors/drives: ABB Automation Technologies, Drives, 262-780-3873. www.abb-drives.com. Circle No. 212.