Document re9dk4B2bkX87jGBJ5q96g2Qr

American Society of Heating and Ventilating Engineers Guide, 1932 j I $ 1 Chapter 34--Fans and Motive Power ELECTRIC MOTORS Electric power is almost the universal solution for fan operation, as electric motor speeds are flexible for adapting to direct-connected fans. Electric motors are readily suited to various types of drives, such as belts, chains or gears. Each type of electric motor, and kind of electric current has its advan tages and disadvantages as applied to a fan application. Direct-connected electric motors usually are very efficient for fan driving because there is no slippage due to belts, and no wear or noise due to chains or gears. There is less maintenance and upkeep to a directconnected unit, and with an overhung fan wheel on the motor shaft, the usual fan bearings are eliminated. The disadvantage of a slow-speed direct-connected motor is that it may be unduly large and heavy as well as costly, but this may be offset by the compactness of the unit as a whole due to limited space for fan equipment. Should anything go wrong with a slow-speed direct-connected motor there may be a considerable delay in securing replacements, as these motors are not usually carried in stock, as is the case with moderately high-speed motors. If a change of speed is found necessary with a direct-connected motor, it will mean a change of motor, which may necessitate a change in the motor foundation usually built with the fan in such cases. On the other hand, non-direct-connected motors have transmissions subject to wear and slippage, and chains or gears may be noisy with this latter type. However, should a change in speed be necessary where the motor is not direct-connected, changes in speed ratio can easily be accomplished by changing pulleys, sprockets or gears on either the fan or the motor. In the case of a motor breakdown a standard stock motor may easily be substituted. A type of drive using-wedge-shaped rope-like belts, often in multiple, has become very popular recently as it enables the use of high speed motors with slow speed fans. These motors are less expensive and more efficient, and further allow, very short belt centers, saving floor space, thus making the fan`unit and the motor much more compact than the usual belt drive. The compactness secured by this equipment compares favorably with a direct connected layout. This type of drive is also very quiet in operation, being similar to a conventional belt drive in this respect. Alternating current motor designs are such that improved operating characteristics are obtained with the higher motor, speeds. Efficiencies and power factors are improved over those in effect with slower speed motors, thus showing a considerable saving in power consumption, where some effective speed reducing transmission device to the fan, is installed. Quietness of operation is more readily obtained with moderately high speed induction motors than with low speed motors, as any slight magnetic unbalance is not as easily heard. Magnetic unbalancing at times causes noises whose repetition and wave length is such as to cause vibrations and harmonics. Amplifications of the noises in other parts of a building 495 '. / 1