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832 CHAPTER 39 . 1950 Guide horsepower ratings. The constant horsepower type of motor is'considerably 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 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. Table 7. Typical Application Requirements of Synchronous Motor Drives Showing Starting, Pull-In and Pull-Out Torques Afflxcatioh Method or ComracnHO Moron to Load Tobques Cokoriohb Start Pull- Pulling in Out Remabes 1 Exhaust and venti Coupled at belted Usually loaded SO 60-125 150 FFfi> of tan must be lating considered Cycloidal positive Coupled or engine type 1 Blowing frngiimw fciproeating Turbo high speed Engine type Direct connected or step up gear Unloaded Unloaded 40-60 40-60 150 Two-speed motors sometimes used 40 -40-60 150 Unloaded (In SO 60 150 TFJP.of blower must take closed) be considered Air Engine type Unloaded 40 30 150 Flywheel effect im portant [ A mrannln and am High speed--belted Unloaded (by 40 30 150 Flywheel effect im monia booster Low speed--engine by-pass) portant type occasionally coupled 88 Freon High speed--belted Unloaded (by 45 50 150 Flywheel effect im Low speed--engine by-pass) portant Gaa reciprocating High speed--belted Unloaded (by 40 SO 150 Flywheel effect im Low speed--engine by-pass) portant ' Capacitor start motors develop high starting torque in fractional hp rat ings, 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 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 Motors and Motor Controls 833: suited for small fan drives. Operation, is similar to the capacitor start motor, except that the capacitor is not cut out when running. Repulsion-Induction motors develop high starting torque. The motors have two rotor windings--a squirrel cage for running and a wound rotor connected to a commutator for starting. No switching device is required to change from starting to running winding as this is accomplished by a gradual shift with speed in the magnetic flux path, so that near rated speed the motor operates completely on the squirrel cage winding. Repulsion Start-Induction Run motors are similar to the repulsioninduction motor, but they have only the commutator winding. They are supplied with a centrifugal short circuiting switch which shorts the com mutator bars when the motor comes up to speed to obtain a winding ap proximately like the squirrel cage in its function. . Fig. 10. Speed-Torque Characteristics of Single Phase Motors Repulsion-Induction and Repulsion-Start-Induction Run motors are suitable for applications such as industrial compressors requiring high break away 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 con ditions 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 Fig. 10.