Document Yrg0MvEZmvrnbLBvadoEEqn7D

American Society of Heating and Ventilating Engineers Guide, 1934 Table 2. Classification of Motors Gaot"nnj?" Ttpb Cub _ Speed *. ACTEBI&TZCS Starting Torque Current Applications A 1 Shunt Wound d-c Constant Medium High Fans 2 Squirrel Cage a-c Constant 3 Synchronous a-c Constant Medium Medium High -- About six times full load Starts as Sqiurrel Cage Motor Fans, Centrifugal Pumps Motor Generator Sets, Air Compressors, 4 Slip Ring or a-c Constant Wound Rotor Heavy Low Fans Vacuum Pumps, Air Compres- S Double Squir a-c Constant rel Cage Heavy Medium sors Frequent and Heavy Starting Loads, Pumps. 6 Low-Torque a-c Constant Capacitor 7 High-Torque a-c Constant Capacitor 8 High-Torque a-c Constant Capacitor Light Medium High Low Low Medium Compressors Direct-Con- nected Fans Belt Drive of Fans For Heavy Starting Load Such as Larger Fans, Pumps. 9 Repulsion a-c Constant High Induction Medium Compressors Fans, Pumas. Compressors B 1 Brush Shifting a-c Adjustable Medium 2 Cumulative d-c Adjustable Comp'd with Shunt Predominance 3 Squirrel Cage a-c Poles can be Regrouped MultiSpeed Heavy Medium Low High Stokers, Boiler Fans Pumps High Fans, Ice Ma chines C 1 Series d-c Variable 2 Cumulative d-c Variable Comp'd wit-h Series Predominance 3 Slip King-- a-c -Variable Jsing External Resistance in Secondary Heavy Heavy Heavy Low Low Low Fans Single Acting Reciprocating Pumps ?ans 238 Chapter 17--Fans quietness of operation when used for fan installations in buildings where quietness is a factor. In the construction of fan and motor foundations where the machinery is mounted on the floor or upon a concrete platform, it is a usual practice to install a layer of cork on top of which is laid or floated the base which carries the apparatus. It is essential that the bolts or lag screws which fasten the machines to this foundation shall not extend through to the floor. It is wise to fasten curbs to the floor, these presenting insulated surfaces to the machinery foundation and so preventing it from traveling. The general classification of motors used for heating, ventilation and air conditioning is shown in Table 2. Control for Electric Motors Very small direct current motors may be started by throwing them directly on the line through a suitable starting switch. The larger sizes require some type of starting rheostat. When speed adjustment is desired, the controller for adjusting the speeds of the motor usually functions also as a starting device. Alternating current motors of 5 hp and under usually may be thrown directly on the line. It is good practice to use a starting switch equipped with a thermal overload or inverse time limit overload device. This type of switch provides protection to the motor beyond the power of fuses to supply. Fuses when used necessarily must be large enough to take care of the inrush current which makes them inadequate for protecting the motor under operating conditions. The thermal overload device allows for this inrush and does not function until an overload has become persistent, the time element depending upon the percentage of overload over the rating of the element. This type of switch is available for manual operation and also is furnished in the magnetic type for remote operation by push button, or for operation by other types of pilots, such as pressure switches and thermostats. On the standard squirrel cage motors above 5 hp a starting compen sator usually is employed to keep the inrush current to within the limits specified by the local power companies. Compensators may be obtained in transformer types and primary resistor types, and usually are furnished for manual operation. They can be secured for remote control also, but are necessarily expensive. However, the new type of high reactance, self-starting motors, may usually be thrown across the line up to 30 hp in size, and still have their inrush current within the limits of the rules of the National Electric Light A ssociation. With this type of motor a magnetic contactor usually is used. This device may be operated from a remote point by push button, if desired. These magnetic contactors are furnished usually with thermal overload and no-voltage protection. For remote operation of motors through magnetic starters, the operat ing buttons may be located in the engineer's or manager's office, and tell-tale indicating lamps may be wired up with the circuit to indicate whether or not the unit is in operation. This type of control is very desirable in large buildings where the engineer is to have complete charge of the ventilating system. Remote or automatic control of the units may be affected also by 239