Variable Frequency Drives For More Efficient Manufacturing Operations

ABB Inc., New Berlin, Wisconsin

Energy savings is one of the key ingredients in reducing costs for any manufacturing operation. A simple, but very effective, way to save energy is by making fans run more efficiently. Almost every manufacturing application uses fans, and variable frequency drives (VFDs) provide the adjustable motor speed that can actually reduce energy consumption in fans. This concept is demonstrated in the following example where VFDs are used in tandem with industrial robots for paint spray booth applications.

The process of using robots to apply paint in a paint spray booth is fast, safe, highly accurate, and produces little waste. The use of paint spray booths also poses two major challenges: variable booth pressure and changing fan speeds. Most spray paint booths either need to keep a slightly positive pressure inside the booth to reduce the possibility of outside dirt or contaminants, or they need to maintain a slightly negative pressure to prevent paint vapors from escaping and contaminating other work areas. Because automated manufacturing requires that products enter and exit the chamber on a regular basis, air pressure within the chamber is reduced when the chamber doors open.

In a typical positive-pressure example — a manual product spray booth — 5,000 CFM (cubic feet per minute) is supplied, 4,500 CFM is exhausted, and 500 CFM is lost through small leaks and serves to keep the booth at a slightly positive pressure. Total airflow of the system is 5,000 CFM; however, there is no control over the pressure in the chamber and, as a result, the quality of the spray action may be reduced when the doors are open.

In a VFD-controlled system (see figure), the VFD increases fan speed, and thus the supply air CFM, to compensate for the loss of air through the open doors. The automatic paint spray booth improves product quality, by automatically adjusting for losses in chamber pressure. The control system typically monitors a pressure differential. One pressure sensor is located within the paint booth, and another sensor is located externally. These transducers input a differential pressure to an external set point controller or the VFD's "on board" PID controller.

In addition to booth pressure control, a variable-flow system allows for automatic compensation for filter loading. As the paint booth filters become dirty (loaded), the pressure drop across the filters increases. To keep a constant flow or pressure downstream of increasingly loaded filters, the VFD system simply increases the speed of the fan to compensate for the pressure drop across the filter.

Compared to outlet damper control or other methods of flow control, variable speed drive control saves a substantial amount of electrical energy. As energy costs continue to rise, customers need to find ways to stabilize costs. The combination of variable motor speed, industrial robots, and energy management in one package results in a highly cost-effective industrial system.

Variable Frequency Drives increase fan speed in order to compensate for the loss of air through open doors, enabling the automatic paint spray booth to adjust to a loss in chamber pressure.

This work was done by Ken Graber for ABB Inc. For more information on drives, contact Chuck Hollis at (262) 785-3505 or chuck.hollis@us.abb.com. Visit ABB online at www.abb-drives.com.