Applying adjustable speed AC drives to cooling tower fans

An economic evaluation was performed to determine the potential annual savings achieved by using adjustable-speed AC drives on cooling tower fans. It was found that using any adjustable-speed drive technology to track the wet-bulb temperature throughout the year will result in an annual energy savings of 83% when compared with running the fans at full speed all the time. Pulsewidth modulation was found to be the optimum solution for efficiency, harmonic distortion feedback, and power-factor considerations. The digital drive also offered the control options necessary to successfully operate with two fans in parallel on one drive. These options included the capability for bypass starters, automatic transfer, and selectable fault condition responses. In addition to the energy savings offered by the digital drives, being able to adjust the speed of the fans would result in smoother plant operations due to constant water temperatures. The adjustable-speed drives also enable the fans to be soft started