CHAPTER 6 | Fan Motors

Cooling tower fan motors must be properly selected for long life and trouble free operation.

It is most 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, see very tough duty and must be of the totally enclosed type. Motors are categorized as follows:

-TEAO: Totally Enclosed Air Over where the motor has large cooling fins and depends on the cooling tower air stream for air movement. The motor shaft protrudes from the enclosure at one place only.

-TEFC: Totally Enclosed Fan Cooled where the shaft extends from both ends with a cooling fan attached to one with a shroud that directs air over strategically located cooling fins. This motor is more commonly used when the motor is outside the air stream as when driving a gear box with a drive shaft or on blow-thru towers.

-ODP: Open Drip Proof where there are openings to the windings through the enclosure and a cooling fan inside that causes air to flow through the motor.

ODP motors should never be installed in a discharge air stream. They can be placed in the inlet air when located in such a way as to be protected from splash out and rain. TEFC motors should be the minimum standard.

TEFC motors in the discharge air stream with the shaft pointing down- as with most belted applications- have an increased possibility of moisture entering the motor around the cooling fan. 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.

One advantage of having the motor in the air stream is the generous cooling that it receives. A motor rated at, say, 60hp in a ‘normal’ application may be capable of providing a continuous 72hp without any difficulty because of this cooling. This is why it is important to size conductors, fuses, starters, etc. based on actual motor amps (which the manufacturer should provide in his submittal) and not from standard application charts.

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. Or,

HP2 = HP1 (RPM2 / RPM1)3

Example: The speed of a fan is increased 10%. What is the revised horsepower requirement?

HP2 = HP1 (RPM2 / RPM1)3 = HP1 (1.1 / 1.0)3 = 1.33HP1

The horsepower increased 33% while the speed went up by only 10%. Similarly, slowing the fan by a small amount causes a marked reduction in the horsepower and amp draw.

It is apparent that speeding up a cooling tower to increase its airflow and capacity must be carefully approached. Aside from possibly exceeding the maximum safe speed for the fans or causing the increased airflow to ‘fling’ water past the eliminators - a phenomenon referred to as ‘spitting’- the required horsepower climbs very quickly requiring oversized motors with attendant energy costs for only modest capacity gains.

This fan law demonstrates another important fact... fan motors have small torque requirements at start up and don’t begin to really work until they get near top speed. Fan motors, therefore, do not need special starting schemes such as part winding or Y-start, Delta-Run. Fan motors are simply started across the line’. Occasionally, motors show optional starting features on their name plates causing people to seek out the special starters depicted for no reason. This is simply a case where the motor manufacturer stocks motors with a wide application range.

Cooling tower fan motors are often used to modulate air flow through a cooling tower to kill off excess capacity during periods of low load or winter operation. This capacity reduction can be achieved with fan cycling, multi-speed motors, extra motors, or variable speed drives as described in the following paragraphs.

Fan cycling (turning off fan motors) works good when a tower has numerous fan motors. If there are four fan motors, for example, turning one motor off reduces the capacity by about 1/4 etc.. This is an easy capacity control method but doesn’t work well when close temperature control is required resulting in frequent motor starts. Six starts per hour should be considered maximum. Excessive starting causes heat build up and insulation failure. Prolonged “pump on, fan off” operation is not good and should be avoided.

Two speed fan motors are available as either single or dual winding. The single winding motor has its entire winding active at low or high speed. The winding is simply reconfigured by the starter as either 8-pole or 4-pole (900rpm or 1800rpm). Low speed is always half of full speed. These motors are wound for a specific voltage- most often 460v. The two winding motor has two separate and distinct windings- one for low speed and the other for high speed. It is possible for one winding to fail leaving the other intact but such occurrences are rare. Typically, a faulty winding takes out its neighbor. And, since the motor has to be removed for repair anyway, there is no real standby advantage to such a motor. One advantage of a two winding motor is that the speed ratio is not necessarily 2:1. Common speeds are 1800/900 and 1800/1200. These motors are also wound for a specific voltage. In general, single winding motors cost less but their starters cost more. Conversely, two winding motors cost more and their starters are less expensive. In the end, there is little cost difference. Single winding motors are more likely to be stocked and are far more popular. Either should be specified as variable torque.

Pony motors are simply 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. The appropriate drive ratio is selected for each motor so that it is fully loaded when in operation- a distinct advantage over two speed, variable torque motors where the available horsepower is proportional to the square of fan speed while the required fan horsepower varies as to the cube of fan speed (from the fan law). As an example, a fan motor that can produce 40hp at high speed can produce 10hp at low speed while a fan that requires 40hp at high speed only requires 5hp at half speed. As a result, 1800/900rpm motors are always 100% oversized at low speed. And, since the motor usually operates at low speed most of the time, the inefficiency of the lightly loaded motor is noticeable.

The pony motor also has the advantage of allowing one motor to be removed for servicing while the other remains on line. Plus, such simple, single speed motors are readily available in the event of a breakdown.

Electrically, the pony motor arrangement is equivalent to a two winding motor- It’s just that the two windings physically reside in separate motors.

Questions arise as to the ‘idle’ motor acting as a generator when rotated by the active motor. It does not. Aside from other differences between motors and generators, the fact that there is no excitation current means that there can be no output. The only losses seen by induction motors are from windage and belt flexure and are so small as to be virtually undetectable. This author has never encountered power factor correction capacitors in conjunction with pony motors but it seems logical that there could be some swapping of energy between the capacitors and windings for motors that are rotated by exterior means. It would seem a good idea to consider switching any unused capacitors out of their circuits.

Single phase motors definitely exhibit operational problems in pony motor applications. The capacitors in these motors store energy and the motors resist external attempts to rotate them. Single phase motors can be made to work on pony motor applications by splicing into the capacitor circuit and connecting it in series with an open auxiliary switch in the active motor starter. Examination of the drive ratios shows that if both motors have 1800rpm synchronous speeds that the pony motor is made to operate near 3600rpm when the big motor is operating. This is typically not a problem as to rotor balance or bearing duty because manufacturers make 3600rpm versions of these same motors. Nevertheless, the duty should be checked and if this is a problem, the small motor can be changed to a 900rpm model and the synchronous speeds of each motor will not be exceeded.

Two speed motors and pony motor arrangements both require a time delay that prevents low speed or pony motor operation until approximately 15 seconds after high speed operation. This insures that the low speed winding is not energized while the motor is rotating faster than its synchronous speed.

For example, an 1800 rpm pony motor rotates at about 3600rpm when the ‘big’ motor is operating. If the ‘big’ motor is switched off and the pony motor switched on immediately, it would attempt to operate at its 1800 rpm synchronous speed but would already be rotating closer to 3600rpm. The conflict will cause a deceleration of the rotating components so severe as to possibly cause damage. The 15 second time delay insures the motor is rotating under its synchronous speed when energized. It will then gently accelerate to full speed. Fifteen seconds is a starting point; The time delay can be reset to a lower value appropriate for each project.

Variable speed drives are the ultimate in capacity control but introduce a level of sophistication that may not be required. Projected fan motor energy savings make their use attractive but the designer must look at a bigger picture... Often the ‘excess’ energy consumed by a single speed motor is more than offset by increased system efficiency from the reduced water temperature provided by the cooling tower. When used, VFD’s and two speed motors should generally be set to provide the coldest temperature that the system will tolerate before reducing motor speed. VFD’s can, however, be very helpful in noise sensitive applications. Soft starting and gradual speed changes make cooling tower noise less noticeable to critical neighbors. Towers that operate at extremely light loads can also benefit from VFD’s. The VFD will keep a motor running with a positive (albeit small) air flow through the tower. This avoids excessive ‘motor off’ operation and attendant water ‘splash out’ problems. When applying VFD’s, make sure the fan motors are specified for VFD duty.

Also, make sure the cooling tower manufacturer is aware of your intentions and subjects the application to engineering review. Fans sometimes have natural frequencies in the range of normal operation. The manufacturer may advise speeds where the fan should not be allowed to dwell. The drive can be programmed accordingly. Finally, extremely low speeds can defeat the ‘sling’ lubrication employed in gear boxes requiring the drive to be programmed to avoid low speeds. Alternately, an electric oil pump can be applied to insure adequate lubrication at all speeds.

Windmilling can be another problem. This is when the cooling tower is baffled in such a way as to allow air to pass in a reverse direction past a supposedly idle fan when its neighbor is in operation. The ‘idle’ fan rotates backwards. This causes numerous problems not the least of which is tremendous stress on the drive components when they attempt to start while rotating in a reverse direction. Economic design is often responsible for the omission of baffles. The designer should not assume that just because there may be multiple motors that they can all be operated individually. Anti windmilling devices- essentially one way clutches- are available to some applications but are not the best solution. It is better to baffle the tower properly so that each fan can operate independently. When replacing fan motors, be sure to match all the nameplate characteristics such as hp, rpm, voltage, phase, frame size, enclosure, service factor, insulation class, group, etc.. Also check that the conduit box is located in the same place. (See chart at the end of this chapter.)

NEMA -the National Electrical Motor Association- provides standards for motor manufacture. Motor frame size is one important NEMA standard. A 284T frame motor from one manufacturer, for example, will have the same bolt pattern, shaft elevation, diameter, key size, etc...among all manufacturers.

UL -Underwriters Laboratories- examines components from the various motor manufacturers and publishes a list of UL recognized motors. This prevents UL from having to check each motor that they encounter when evaluating equipment in the lab. Most cooling towers are not UL Listed. This is because the smoke and debris that would result from a motor failure is not directed into occupied spaces. U.L. listing is therefore not required.

See the Glossary wiring diagrams.