Document 8RmZZwvLGGOGnOMoe9OZaOBJa

830 CHAPTER 39 1950 Guide 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 motor would require from 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. Typical specifications or wound rotor motors are shown in Specification D at end of chapter. The addition of resistance to the secondary winding of the wound rotor motor changes the speed torque characteristics as indicated in Fig. 9. The motor speed, with the resistance added, is dependent on load and Motors'and Motor Controls 831 that corrective equipment must always be installed, but inmostcases it is desirable to determine what advantages may be gained, by improving the power factor. With purchased power,, if; the rates include a clause 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. 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 always be 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 squirrelTcage motors. After the motor is nearly up to speed, field excitation is applied consequently the motor has very poor speed regulation when secondary resistance is added to reduce the speed to values below 50 per cent. Synchronous motors are used for continuous duty applications at constant speed where'eflSciency 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 factor 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 Motob with External Resistance 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 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 be1 given to! the torque the motor can develop on pull-in, that is, at the instant when field excitation is applied. Table 7 shows typical application requirements of synchronous motor drives, listing starting, pull-in, and pull-out torques. Typical' specifications for synchronous motors are shown in Specifica tion E at end of chapter. 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 or 4 speed. TwcP speed induction motors are usually of single winding type, having a 2toT speed ratio such as 600 rpm and 1200 rpm, or may be double winding! Three-speed induction motors/ire always two winding, and four-speed motors are usually two winding with a 2 to 1 speed ratio.'in 'cach' winding! Motors can be provided in constant torque, varying torque,1 or- constant