Direct-drive system suits larger cooling tower fans

First introduced in 2009, Baldor's direct drive system for cooling tower fans eliminates the gearbox and right-angled coupling of conventional drives to save energy, improve reliability, and reduce noise. The elimination of gearbox losses is aided by the use of a permanent-magnet motor that increases energy efficiency dramatically. This construction reduces the size of the motor substantially, enabling it to sit underneath the fan in the space currently required for the gearbox of a conventional power transmission system.

The new technology is already changing the way that cooling tower fans for commercial buildings are constructed. The latest products extend the power rating of the motors to deliver maximum torques of up to 13,500Nm. In total there are now 26 choices of rating available, spanning a drive power range from 7.5kW at 500rpm base speed to 187kW at 130rpm base speed, allowing optimum choices to be made for a very wide range of cooling applications from building HVAC (heating, ventilating, and air conditioning) systems to the large sizes of cooling tower fans used in many heavy-duty processing plants and power generating stations.

Advantages over conventional arrangements

The most common technique for driving HVAC fans has been a standard AC induction motor, connected to the fan via a drive shaft, disc coupling and right-angle gearbox. Such arrangements are prone to a number of drawbacks: the gearbox requires regular inspection and maintenance of the lubrication and seals; misalignment in the power transmission system can cause vibration, wear and noise; and energy losses in the power transmission system.

Baldor's alternative, 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 system minimises moving parts, greatly improving reliability and power efficiency - as well as reducing system bill-of-materials.

The interior permanent-magnet synchronous motor design (which is claimed to be unique) 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 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 leaking gearboxes.

Speed control

Fan motor control is achieved using a Baldor VS1 drive with a specially developed speed control algorithm that provides sensorless control of the permanent magnet motor - which is optimised to manage the large inertia of the fan, with a low starting current.

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 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, non-wearing, Inpro labyrinth shaft seal in combination with a slinger cover to prevent the ingress of moisture and contamination of the motor.

Proven energy saving

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 5.5m (18ft) fans. One tower was left as originally constructed, while 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 per cent for the direct-drive arrangement when running at full load - yielding very significant electricity cost savings.

The drive system is now available in nominal power ratings from 7.5 to 187kW (10-250 HP), 500-130rpm base speed, to drive cooling fans from typically 2.1-8m (7-26ft) in diameter.

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

Follow the link for more information about the VS1 Cooling Tower Drive.