Cooling Tower Gains From Simplified Design

July 1, 2009
Motor and drive system reduces complexity, increases efficiency.

The story of Baldor’s new adjustable-speed, direct-drive cooling tower motor and drive system began in the Fort Worth, Texas, home of cooling tower manufacturer Tower Engineering, Inc. (TEI). TEI is a 27-year manufacturer of high-efficiency, field-erected water cooling towers. Company president Rod Applegate’s lifelong dream was to develop a “one moving part” mechanical drive system for use in water cooling towers.

After many years of work to fulfill that dream, Applegate learned of Baldor Electric Company’s similar work with permanent magnet rotor (PMR) technology for heavy industry applications.

Inside Cooling Tower
The motor portion of Baldor’s adjustable-speed, direct-drive cooling tower fan system replaces the conventional motor/driveshaft/gearbox combination. It fits in the space previously occupied by the right-angle gearbox, with the fan blade mounted directly to its vertical shaft.

In 2008, the technologies of permanent magnet rotors, finned laminated frames and specialized drives converged to produce the high power density at low rpm needed to fit the system into the available space. The two companies decided to pool their extensive experience with demanding applications in harsh environments for the cooling tower application.

The collaborative effort resulted in a drive system designed specifically for the rigors of water cooling tower use. This technology replaces the traditional NEMA-design motor, speed reducer and drive shaft assemblies with one moving part for benefits of green operation, improved safety, increased reliability and the highest operating efficiency available today for cooling tower use.

Baldor’s proof-of-concept PMR motor produces 50 hp at 208 rpm. Installed in mid-2008, it drives an 18-ft.-diameter fan in one of two identical cooling tower cells at Clemson University in South Carolina. It replaced a two-speed 326T-frame motor producing 50/12.5 hp at 1,765/885 rpm, which drove the fan at a ratio of 8.5:1 through a size 155 gearbox.

Clemson undertook the role of test bed with some trepidation. Originally built in 1986, its twin cells had given little trouble, but the university’s facility managers were unsure how much maintenance effort the units had received. Concerns about losing maintenance personnel and their expertise, along with the promise of improved efficiency, led them to take on the tests.

Measurements before the retrofit indicated the same current draw – 47 A – on both cells at full speed. After the retrofit, at full power the new drive system drew 11.8% lower kW than the unmodified cell. At an average price of $0.08/kWh, Baldor calculates annual energy cost savings of $900 to $3,200 depending on the duty cycle and proportion of high and low speeds.

The new system also is quieter (74.4 dBA versus 82.3 dBA) and exhibits lower vibration. “Baldor and Tower Engineering may be onto something with reduced maintenance and energy savings,” says Mike Parker, Clemson capital projects manager. “The new motor provides both those benefits.”

Test details were presented at the 2009 Cooling Technology Institute Annual Conference in February. The paper can be found at

Reliability concerns in the fan application center around the axial and radial loads exerted by the large fan blade, and exposure of the vertical-shaft motor to the cooling tower’s wet environment. So after 3,300 operating hours, the prototype was pulled, delivered to Baldor’s test lab in Greenville, S.C., disassembled and inspected. Reliability engineers found no evidence of water ingress or bearing degradation.

Baldor’s RPM AC synchronous permanent-magnet motor uses laminated, finned frame construction — the stator laminations themselves comprise the stator housing, which maximizes the amount of efficiency-producing lamination steel, minimizes weight and bulk, and provides a direct heat conduction path for motor cooling. For this application, it’s constructed as a totally enclosed air over (TEAO) motor with a slinger and an Inpro bearing isolator to keep water out, and equipped with foot mountings designed for drop-in replacement of common cooling-tower right-angle gearboxes.

The motor is paired with the company’s VS1CTD PM cooling tower drive, preprogrammed with the critical motor operational parameters to simplify installation and startup. When the fan is off, the specialized drive also provides a trickle current to warm the motor and prevent any internal condensation. With the permanent magnet rotor, the trickle current electrically locks the motor and prevents windmilling.

The drive system is now available in sizes from 9.5 hp through 118 hp, 150 rpm to 500 rpm, to drive cooling tower fans from 7 ft. to 18 ft. diameter.


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