Document JJQvkknaE69a3M1M5zrEQyxDB

876 CHAPTER 40 1953 Guide X as near: rated load as possible. In addition, as shown in 'Fig.: 2, power factor''and'efficiency are better for higher speed motors. Wound Rotor motors are used for applications requiring high starting torque at low starting current, because a wound rotor motor with its con- , troller and resistance can develop full load torque when starting with about full load current. For comparison, a squirrel-cage motor1 would require froin 3-to 5.times as much current to develop full load'torque ;at starting.. .The wound rotor motor is also..used for varying speed service to drive fans, blowers, and other continuous duty, apparatus. .- The addition of resistance to the secondary winding of the,wound rotor motor changes the speed torque characteristics. The motor speed, with ^Motors and Motor Controls 877 ;. Synchronous, motors are used to drive fans, blowers, pumps; compressors and; other, applications. : Compressor applications having a'high' peak torque, require: the use of-flywheels to smooth .out power peaks, and should alwaysibe: referred to the electrical manufacturer for recommendations. ': Synchronous motors are provided with built-in damper windings on the rotor , and operate during the starting period similarly, to; squirrel-cage motors. - After the motor is nearly up to speed, field excitation, is applied and the motor draws into step at synchronous speed. After excitation, is applied, the. motor runs at exactly, constant speed and will remain at this .Fig. 1 Speed Torque Characteristics op Squirrel-Cage Motors ' the resistance added, is dependent on load, and consequently, the motor has very poor speed regulation when secondary resistance is added to reduce the speed to values below 50 percent. Synchronous motors are used for continuous duty applications at constant speed where efficiency and power factor are important. Another advantage of these motors is that of lower initial cost in large sizes and for low speeds when compared with squirrel-cage type motors. The outstanding advantage of the synchronous motor is .that its power faptor can be changed to compensate for the low power factor of other drives in the same location. Lagging power factor is an inherent charac teristic of all induction apparatus, such as induction motors and .neon signs. Unless synchronous motors or capacitors are installed, the plant power factor may be comparatively low. This does not necessarily mean that corrective equipment must always be installed, but in most cases it is desirable to determine what advantages may be gained by improving the power factor. With purchased power, if the rates include a elapse embodying a penalty for low power factor, or a bonus for high power factor, the saving in power costs may often make a very good return on the investment required for the corrective equipment. * Refer to Glossary at end of chapter. nunotrunrLn nai inu ,, Fig. 2. Efficiencies and Power Factors for Squirrel-Cage Induction ' ` Motors speed until a load approaching the pull-out load is . reached, whereupon the motor pulls out of synchronism and stops. In applying synchronous motors, consideration must be given to, the torque,the motor can develop on pull-in, that is, at the instant when field excitation is applied. Table.5 shows typical application requirements of synchronous motor drives, listing starting, pull-in, and pull-out . torques. Multi-Speed motors provide flexibility in many types of drives. Syn chronous motors can be furnished only with a 2 to 1 ratio in speed,, single winding. . Squirrel-cage induction motors may be 2, 3 ar 4 speed. Twospeed induction motors are usually, of single winding type, having a 2 to 1 speed ratio such as 600 rpm and 1200 rpm, or may be double-winding. Three-speed induction motors are always two winding, and four-speed