A cool solution for cooling tower fans!

Written by: Baldor UK Ltd | Published:

A new direct drive technology for controlling commercial building cooling tower fans greatly improves reliability by eliminating moving parts while also saving energy and running quieter. Based on a novel permanent magnet motor from Baldor, it packs the high torque required into such a compact space that it can easily be retrofitted into cooling towers - sitting underneath the fan in the space currently required for the gearbox element of conventional power transmission systems.

Air cooling towers are commonly used as part of the HVAC (heating, ventilating, and air conditioning) systems of medium to large-scale office and industrial buildings. Large fans pull air over a water soaked media to cool the water before it is returned to the chiller unit.

The most common technique for driving such fans has been a standard AC induction motor, connected to the fan via a drive shaft and disc coupling arrangement into a right angle gearbox - which reduces the motor's speed to the range required for fan circulation. Such arrangements are prone to a number of drawbacks: the gearbox runs at high speed and requires regular inspection and maintenance of the lubrication and seals; misalignments in the power transmission system can cause vibration, wear and noise; and the complex power transmission system introduces significant energy losses.

Baldor's unique new solution, the VS1 Cooling Tower Drive, provides a direct slow-speed drive for the fans combined with variable speed control that is optimised for the building cooling application. By eliminating the drive shaft and gearbox the solution minimises moving parts, greatly improving reliability and power efficiency - as well as reducing system bill-of-materials.

The unique interior permanent magnet synchronous motor design uses energy efficient technology that, in combination with the elimination of the gearbox and drive train transmission losses, results in a more efficient system compared to conventional fixed speed designs. Additional significant energy savings can be gained by being able to operate the fan at reduced speeds during non-peak load conditions. The direct drive also greatly reduces noise and eliminates the issue of cooling tower water becoming contaminated from leaky gearboxes.

The direct drive fan motor is controlled by a Baldor VS1 drive with a specially developed speed control algorithm. Variable speed control of fans can save an enormous amount of energy compared with older fixed speed solutions. Baldor's algorithm provides sensorless control of the permanent magnet motor that is optimised to manage the large inertia of the fan, with a low starting current requirement.

The motor is a synchronous type from Baldor's RPM family, based on permanent magnets. Thanks to dramatic improvements in the magnetic and thermal properties of permanent magnet materials in recent years this technology now represents a viable alternative to conventional AC induction motors and delivers a significant energy efficiency advantages - even compared with the latest premium-efficiency types of motor. Laminated frame technology is used in the design, eliminating the conventional cast iron outer frame of large AC motors and allowing more room for active (torque producing) magnetic material. This produces a highly torque-dense motor, allowing Baldor to also offer the technology to the cooling tower retrofit market, for installation in the same space currently used for the gearbox. The motor will provide continuous constant torque over its entire range from zero up to base speed, and employs optimum pole construction to maximise both efficiency and power factor.

A high degree of protection is an important factor for this application, as the operating environment is extremely humid. An epoxy compound insulation applied via vacuum pressure impregnation - a technique derived from a system originally developed for Navy applications - is used for the stator windings. The drive end of the motor is also protected by a metallic, non-contacting, no-wearing, Inpro labyrinth shaft seal in combination with a slinger cover to prevent the ingress of moisture and contamination into the motor.

Before launch, Baldor's new product was trialled and compared with a conventional cooling tower fan drive system, at a university building with identical twin cooling towers housing 5.5 metre (18 feet) fans. One tower was left as originally constructed. The other was retrofitted with the new Baldor permanent magnet motor and variable speed drive. Independently verified power measurements showed a decrease in input power of approximately 13% for the direct drive arrangement when running at full load - yielding very significant electricity cost savings.

"Direct drives represent a win-win technology for commercial building HVAC applications," says Baldor's Rich Schaefer. "It makes the drive system a lot simpler for manufacturers of cooling fans, and delivers a range of benefits to the end user in terms of higher reliability, greatly reduced maintenance, lower running costs and less noise."

The drive system is now available in nominal power ratings from 7 to 145 kW (9.5 - 192 HP), 150 to 500 RPM base speed, to drive cooling fans from 2.1 to 5.5 metres (7 - 18 feet) in diameter.

The motor flange footprint is designed to interchange with gear reducers that are widely used in today's global cooling tower market. Adding a foot mounting adapter plate is a simple means of fitting the motor in other gearbox footprints.

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