Direct drives for cooling-tower fans save space
Baldor has developed a motor for driving the fans of cooling towers that can be positioned directly beneath the fan in the space required for the gearbox element of a conventional power transmission system with an AC induction motor outside the tower itself and connected to the gearbox by a horizontal drive shaft. The new system makes possible variable-speed control that is optimised for cooling buildings.
The permanent-magnet (PM) motors used are much smaller and lighter than standard cast-iron frame induction motors. For example, a 37 kW induction motor might weigh 1800 kg, have a height of a metre and a width of 70 cm. The equivalent finned permanent-magnet motor with a laminated frame weighs just 760 kg, is 56 cm high and 56 cm wide. These PM motors can be retrofitted to existing cooling towers.
To enable the speed of the fan to be varied, an application-specific sensorless control scheme was developed.
Experience in the USA has shown significant savings. The trial was for a university building with two identical cooling towers with 5.5 m-diameter fans. One tower was retrofitted with the PM motor and variable-speed drive. The power requirement at full load was reduced by 13%. Energy savings will be in excess of 50% when running at average load levels of around 75%.
