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Big fan drives offer much simplified power trains

11 February 2011

Baldor has extended the power ratings of its novel direct-drive system for cooling tower fans in response to demand from power generation and large processing plant operators. This novel approach to large fan control avoids additional mechanical components in the drive train – including gearboxes and couplings - to deliver energy-efficient and less maintenance-intensive variable speed fan control

First introduced in 2009, Baldor's direct drive system has delivered a more efficient method for controlling cooling tower fans, eliminating the gearboxes and right-angled couplings of conventional drives in order to save energy, improve reliability, and ensure quieter running. The elimination of gearbox losses is aided by the use of a novel permanent magnet based motor that has a very positive impact on energy efficiency. This construction reduces the size of the motor substantially, allowing it to sit beneath the fan in the space currently required for the gearbox element 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 product additions now extend the power rating of the motors to deliver maximum torque of up to 13,500Nm. In total, Baldor now offers 26 choices of ratings, spanning a drive power range from 7.5kW at 500rpm base speed, to 187kW at 130rpm base speed, which extend the range of applications from building heating, ventilating, and air conditioning (HVAC) systems to the larger sizes of cooling tower fan deployed in many heavy-duty processing plants and power generating stations. The range provides sufficient capacity to drive cooling fans from typically 2.1m to 8m in diameter.

The most common method adopted for driving cooling tower fans is to use a standard ac induction motor, connected to the fan via a drive shaft and disc coupling arrangement into a right angle gearbox, the latter reducing the motor's speed to meet the air flow specifications. Such arrangements are prone to a number of drawbacks. The gearbox runs at high speed and requires regular inspection and maintenance, including frequent inspection of lubricant levels and seal condition. Misalignments in the power transmission system can cause vibration, wear and noise, while the complex nature of the power transmission system introduces significant energy losses.

Baldor's VS1 cooling tower drive, on the other hand, provides a direct slow-speed drive for the fan combined with variable speed control that is optimised for the building cooling application. There is no need for a drive shaft and gearbox so there are far fewer moving parts, less drive train transmission losses and a generally much quieter operation. There is also zero risk of leaking gearbox oil contaminating the air flow. Add to this, the energy-efficient permanent magnet synchronous motor, and you have a far more efficient system compared with conventional fixed speed configurations. Moreover, additional energy savings can be gained by operating the fan at reduced speeds during non-peak load conditions.

Thanks to improvements in the magnetic and thermal properties of permanent magnet materials in recent years, permanent magnet synchronous motors now offer a viable alternative to conventional ac induction motors while delivering significant energy efficiency advantages - even when compared with the latest premium-efficiency motor types.

Laminated frame technology avoids the conventional cast iron outer frame of large ac motors and allows more room for active (torque producing) magnetic material. The result is a highly torque-dense motor that can be retrofitted into the space currently occupied by the gearbox. Furthermore, the motor provides continuous constant torque over its entire range, from zero to base speed, and its optimum pole construction maximises both efficiency and power factor.

The motor itself is controlled by a Baldor VS1 drive running a fan speed control algorithm, which requires no sensor feedback. This control strategy is optimised to manage the large inertia of the fan, with a low starting current requirement.

Environmental protection
A high degree of protection is an important factor in 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 other forms of contamination.

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 offers a simple method of fitting the motor into other gearbox footprints.

Before launch, the new product was tested and compared with a conventional cooling tower fan drive system. The location was a university building with identical twin cooling towers housing 5.5m 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% for the direct drive arrangement when running at full load - yielding very significant electricity cost savings.

Baldor motor products that are held in stock by the company in the USA (which, incidentally, are also available on quick turnaround in Europe) are fully described in a 600-page catalogue, the latest edition of which covers literally thousands of Baldor Reliance motors and virtually every application area for rotary motion.

The single- and three-phase motors span the power range 0.18 to 1,200kW and cover an exceptionally broad spectrum of application types - including general purpose, premium efficiency, severe duty and explosion proof, washdown, dc, vector and brake - together with a large number of application-specific types such as pump, HVAC and farm duty. Motors in both IEC and NEMA frames are listed, and EISA-compliant motors are available to all automation builders serving the North American market.

Stocked motors are easily customised, and the catalogue details around 100 common modifications that can be rapidly applied before shipment. The publication is available in printed form on request, or it can be downloaded free-of-charge from the company’s website by going to the download section and clicking on ‘501 stock product catalog’
 


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