Document wgXJrmp6mg2b5b8ZQXNBoVVeQ

; 980 CHAPTER 39 1957 Gui(ie-^ winding. Squirrel-cage induction motors may be 2, 3 or 4 speed. Twct; speed induction motors are usually of single winding type, having a 2 to p 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; motors are usually two winding with a 2 to 1 speed ratio in each winding.-; Motors can be provided in constant torque, varying torque, or constant horsepower ratings. The constant horsepower type of motor is consider;, ably larger than the constant torque motor, due to the fact that the same horsepower must be developed at either reduced speed or high speed. In selecting two-speed motors for fan, pump, blower, or compressor applications, it is usually found that two winding motors are more expensive than the single winding type. The control cost for two-speed, two wind ing motors, however, is more economical, and therefore the combined Fio. 3. Speed-Tobque Speed-Current Characteristics of Wound Rotor Motors price of both motor and control for the two winding motor is only slightly higher. Because of the improved performance of the two winding motors, and because of the factor of safety provided by two independent windings, the increased cost is frequently worth the difference. Single Phase Motors Single phase induction motors have auxiliary windings or devices for starting, and are classified by the method used. Capacitor start motors develop high starting torque in fractional horse power ratings, and moderate starting torque in larger ratings. They are used for constant speed drive such as fans, blowers and centrifugal pumps. During the starting period, a winding with a capacitor in series is connected to the motor circuit and when the motor comes up to speed, a centrifugal switch cuts the capacitor and second winding out of the circuit. Two-value capacitor motors develop high starting torque employing a starting capacitor and a running capacitor. The starting capacitor gives high starting ability, but is suited for short time operation only, and is cut out for the running condition by a centrifugal switch. The running ca- Motors and Motor Controls 981 pacitor gives high efficiency at full speed. These motors are used on compressors, reciprocating pumps and similar equipment which may start under heavy load. Permanent split capacitor motors have low starting torque and are ideally suited for small fan drives. Operation is similar to the capacitor start motor, except that the capacitor is not cut out when running. Repulsion-Start Induction-Run motors develop high starting torque. The motors have a wound rotor connected to a commutator for starting. A centrifugal switch shorts the commutator bars when the motor comes up to speed to obtain a winding approximately like the squirrel cage in its function. Repulsion-Start Induction-Run motors are suitable for applications, such Table 5. Typical Application Requirements of Synchronous Motor Drives Showing Starting, Pull-In and Pull-Out Torques Application Method op Connecting Motor to Load Starting Conditions Torques Start Pull- Pull- ing in Out Remarks JF a n a Exhaust and venti Coupled or belted lating Usually loaded 50 60-125 150 WR* of fan must be considered Cycloidal positive Coupled or engine Unloaded type 40-60 40-60 150 Two-speed motors sometimes used 1 Blowing engines re Engine type so ciprocating Unloaded 40 Turbo high speed Direct connected or Unloaded (in 30 step up gear take closed) 40-60 150 50 150 WR1 of blower must be considered Air Engine type Unloaded 40 30 150 Flywheel effect im portant 1e Ammonia and am monia booster High 8peed--belted Low speed--engine Unloaded (by by-pass) 40 30 150 Flywheel effect im portant type occasionally a coupled o O Freon High speed--belted Unloaded (by 45 50 150 Flywheel effect im Low speed--engine by-pass) portant Gas reciprocating High speed--belted Unloaded (by 40 30 150 Flywheel effect im Low speed--engine by-pass) portant as industrial compressors, requiring high breakaway torque, and where commutator and brush noise are not factors. Split Phase motors have a high resistance auxiliary winding which is in the circuit during starting, but is disconnected through the action of a centrifugal switch as the motor comes up to speed. Under running condi tions it operates as a single phase induction motor with one winding in the circuit. These units are available for the small horsepower ratings, and when equipped with a high slip rotor, may be used for adjustable varying speeds through line voltage control. The motors are ideally suited for fan duty. Speed-torque characteristics of single phase motors are shown in Pig. 4. Full voltage starting of single phase motors is general practice, but since most of these motors are connected to secondary distribution systems located in fight load density areas, power companies check carefully the starting currents in order to prevent objectionable voltage dips. This is particularly important for motors started frequently, such as those con trolled by pressure or temperature sensitive devices and applied to rengerators, stokers, oil burners, and water pumps,