Use of Fans, Motors, and Drives

Cooling tower fan motors must be properly selected for long life and trouble free operation. In addition, it is important to match the motor s enclosure to the application. Motors in draw through applications that are mounted directly in the air stream, for example, experience tough duty and must be of the totally enclosed type. Motors are categorized as follows  

In addition, the small motor fan attempts to blow air down over the motor and is no match for the much bigger cooling tower fan drawing air up and around the motor. The small fan in this case is useless- even counterproductive. This application needs a TEAO motor. Cooling tower manufacturers sometimes purchase special motors tailored for their application... special grease, seals, slingers, weep hole locations, epoxy coatings, etc.. Such features make an OEM replacement more desirable than an off the shelf replacement. 

Large towers often have the motor mounted horizontally connected to a right angle gear drive. The motor can be closely coupled- in the air stream- or connected with a drive shaft with the motor outside the air stream. Maintenance personnel typically prefer the external TEFC motor- when available- for its easier access. Cooling tower fans- like all fans- operate in accordance with the fan laws one of which states that the horsepower required to drive a fan increases to the cube of fan speed. 

As an example, consider the speed of a fan that is increased by 10%. Let s determined the revised horsepower requirement. 

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Pony motors are additional, small motors connected to the same fan shaft. They are typically about 1/4 the size of the full size motor. Pony motors do not lend themselves to gear box applications and are, therefore, primarily applied to belt drive applications. The big motor operates when full capacity is required and the small motor simply free wheels. At reduced capacity, the small motor operates and the big motor spins freely. 

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Until recently the majority of AC variable-speed drives have been applied to variable torque, piunp, and fan applications. Advances in drive technology have led to the use of induction motors in high-performance applications that exceed the capability of motors designed for operation on sine wave power. These applications, which have traditionally been served by DC systems, have created the need for definite ptupose AC induction motors designed specifically for operation on adjustable-ftequency controllers. This application study will highlight the limitations of standard motor designs. 

V-belt drive assemblies are generally used with fans 3 m (10 ft) and less in diameter and motors of 22.4 kW (30 hp) and less. 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

Use of wound-rotor induction motors has been largely in continuous-duty constant-speed supplications where particularly high starting torques and low starting currents are required simultaneously, such as in reciprocating pumps and compressors. These motors are also used where only alternating current is available to drive machines that require speed adjustment, such as types of fans and conveyors. 

The transmission motor or fan is operated by drive belts and pulleys. The support frame of the motor must be designed to obtain the tension of the belts without modification of the alignment of the pulley and to adjust this alignment. The motor class must be calculated with an over capacity of 30% of the used power and must be connected through a safety switch placed on the air handling unit. 

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