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Fans can be driven by many different power sources such as gas engines and turbines; however, the source most frequently used is the electric motor. Like the fan and system, which are equilibrium with each other throughout their operating range, the fan and motor must also each operate satisfactorily under all conditions. This chapter outlines those items which must be considered when sizing a motor to drive a fan.

1) Motor Capability

Motors are manufactured in accordance with NEMA (National Electrical Manufacturers Association) standards and are categorized by frame size, type of enclosure, class of insulation, temperature rise, voltage, speed, frequency etc. When operated within the prescribed NEMA tolerances for voltage and frequency, a motor will operate satisfactorily and maintain its expected service life as long as the windings do not over heat. Winding overheating causes premature insulation failure and reduced motor service life. The winding temperature is the sum of the ambient temperature, the temperature rise due to the load on the motor, and a hot spot allowance which is a function of the type of enclosure selected. Some deviation from nameplate values is possible as long as the windings do not overheat.

The motor service factor is the ratio of power a motor can continuously produce without reducing its service life, provided it is operated at nameplate voltage and frequency, compared with its nameplate horsepower. A service factor greater than 1.0 is primarily intended to provide a margin of safety for variations in the operating parameters such as voltages, frequency, line unbalance, sine wave shape, etc., and is not intended to be used as a means of continuously overloading the motor by purposely under sizing the motor for the fan requirement. The operating conditions of fans can be variously changed due to the change of air density, air flow, etc. excepting the operating variations in the electric power sources. So, 1.15 service factor is quite required to the cooling tower application.

2) Aerodynamic Considerations

A fan's output and power requirement have a direct linear relationship to the density of air entering the fan inlet. If the air density drops, then the fan's output and power requirement also drop. The reverse is also true. Motors can overload when the air density increases, such as in cold weather. The power required versus the power available from the motor should be evaluated for the entire anticipated range of operation. Fans at operating conditions may require special caution during cold start-up conditions so as to not seriously over-load the motor.

Design margins that may be included in the initial system resistance calculations can cause the fan to operate at another location on the fan curve than the intended design point. Depending upon the slope and shape of the fan curve, the power required may be larger than expected and the motor could overload if not enough margin was left between the fan power required and the motor capability.

Fan ratings are based on tests conducted in a laboratory using AMCA Standard 210. This standard specifies test setup configurations so that accurate and repeatable test results are obtained. When a fan is installed in the field, the inlet or outlet connections can cause distorted velocity profiles which alter the expected fan performance. Test programs have demonstrated that depending upon the severity of the distortion and whether there is swirl in the air stream opposite to the direction of impeller rotation, fan power requirements may be larger than expected.

3) Mechanical Consideration

Several mechanical considerations involving physical aspects of a fan application must be evaluated when sizing a motor. Some of the most common ones are described as follows.

(1) Drive Train Losses: AMCA Standard 210 defines fan power input as the power required to drive the fan and any element (gear reducer & coupling shaft) in the drive train that is considered part of the fan. We don't need to consider the loss in the power for the shaft, but the loss for the gear reducer should be included in the evaluation of the fan shaft power.

(2) Motor Slip: Ship is a term which describes the percentage of difference between the synchronous speed and the actual speed of motor under load. Slip generally increases with an increase in motor torque; therefore, actual operating speed generally decreases with an increase in motor torque. Specify this full load speed when sizing the fans. If slip is not considered, the fan may not run at the selected speed.

Synchronous Speed of Motor = 120 x Frequency / Number of Pole (RPM)

Where,
120 = AC current flow changes 120 times every second. (2 RPM/Cycle)
Number of Pole: always pairs (couple of S-pole and N-pole) in the pole of motor. The rotation of motor shaft will occur because of the magnetic phenomenon that unlike magnetic poles attract each other and like poles repel. If we progressively change the polarity of the stator poles in such away that their combined magnetic filed rotates, then the rotor will follow and rotate with the magnetic field of the stator. Accordingly, the speed of the rotating magnetic field set up by the stator winding of an induction motor. In a synchronous motor the rotor locks into step with the rotating magnetic field, and the motor is said to run at synchronous speed. Pair pole rolls to restrict the motor speed.

Percent Motor Slip = Synchronous Speed - Full Load Speed / Synchronous Speed x 100 (%)

The smaller % motor slip is the higher power rating. The % motor slip of energy efficient motor is smaller then one of standard efficient motor.

(3) Motor Bearing Life: Specify a motor bearing life consistent with the specified gear reducer bearing life. Motor manufacturer can supply bearing lives when given the appropriate information. (Speed torque curve of load and Inertia moment of load, etc.)

(4) Torque: The full load torque is expressed as 5250 x Hp / Full Load Speed (lbs-ft). Since different loads present different torque requirement at starting (= breakaway), minimum (= pull up), breakdown (= pull-out) and full load, NEMA has defined four standard design classes A, B, C and D of squirrel cage poly-phase induction motors. These design classes are summarized in below table.

NEMA Design "B" of above is a standard general-purpose design. This has low starting current, normal torque, and normal slip. The field of application is very broad and includes fans, blowers, pumps, and machine tools. This design is a best choice as a driver of cooling tower fan.

The general torque/speed relationship with the four torque points defined is shown in below figure.

1. A. Breakaway or Starting Torque: This torque is normally defined by motor manufacturers as "locked-rotor" torque. It is the torque required to start a shaft turning. Methods of bearing lubrication and types of lubricants have a pronounced effect on this torque requirement. Some loads just harder to start turning than others. (In most NEMA Design B and one winding motors, this torque is equal to 160 to 180% to the full load torque.)
   
2. B. Minimum or Pull-Up Torque: This torque, usually expressed in percent of running torque, is required to accelerate the load from standstill to full speed. It is the torque required not only to overcome friction, windage, and product loading but also to overcome the inertia of the machine. The torque required by a machine may not be constant after the machine has started to turn. This type of load is particularly characteristic of fans and centrifugal pumps.
   
   In considering the torque required by a machine during acceleration, its maximum torque required is the most significant. The minimum accelerating torque capability of the driving motor must exceed the maximum accelerating torque required by the machine. Special consideration must be given to the selection of the motor to assure that it will have the necessary thermal capacity to bring the machine to full speed. During the period of acceleration one-half of the energy input to the motor is absorbed by the motor rotor circuit while the other half is stored in the driven machine. In larger cage motors of conventional design the temperature rise of the rotor may limit its ability to accelerate a high-inertia load to its full speed. (In most NEMA Design B and one winding motors, this torque is equivalent to 150 to 170% to the full load torque.)
   
3. C. Breakdown or Pull-Out Torque: This torque is generally called peak torque or maximum torque. The peak torque is the maximum momentary torque that a machine may require from its driving motor. The peak torque required by a load is directly related to the "breakdown" or "pull-out" torque for its driving motor. High peak torque requirements for brief periods of time are available from the break-down torque of an induction motor assisted by the inertia of the rotating system. If the peak-torque requirement is of any appreciable duration, however, it is necessary that the breakdown torque exceed the peak-load requirement. The term "pull-out" torque usually refers to the maximum running torque of a synchronous motor. Inertial energy cannot assist the pull-out torque in carrying a peak-torque requirement of the load unless the motor is of the non salient-pole or the reluctance type. It is therefore necessary in practically every instance to consider specifying a synchronous motor whose breakdown or pull-out torque exceeds the peak running torque of the load. (In most NEMA Design B and one winding motors, this torque is equivalent to 250 to 350% to the full load torque.)
   
4. D. Full Load Torque: This torque is a normal and continuous torque produced by motor at the full load speed.
   

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