Document bdoK7pZX9668YVJQm5r3JQQD

644 CHAPTER 45 1959 Guide 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 clause embody ing 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 equip ment. 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 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' 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. Synchronous motors can be furnished.only with a 2 to 1 ratio in speed, single-winding.' Squirrel-cage induction mo tors may be 2-, 3-, or 4-speed. Two-speed 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 twowinding, and four-speed motors are usually two-winding with a 2 to 1 8peed 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-winding 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 twowinding 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 horsepower ratings, and moderate starring 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-capadtor motors develop high starring torque Table 5 .... Typical Application Requirements of Synchronous Motor Drives Showing Starting, Pull-In, and Pull-Out Torques Application Method of CaamdMf Motor to Load Starting Conditions Starting Torques Pud-fa Pud-Out Fans 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 Blowers Blowing eogines re Engine-type ciprocating Unloaded 40 40-60 150 Turbo high speed Direct connected or Unloaded (in 30 50 150 WR* of blower must step up gear take closed) be considered Air Engine-type Unloaded 40 30 160 Flywheel effect im portant Ammonia and am High speed--belted Unloaded (by 40 30 150 Flywheel effect im monia booster Low speed--engine- bypass) portant type occasionally Compressors coupled Freon Gas reciprocating ' High speed--belted Unloaded (by Low speed--engine bypass) High speed--belted - Unloaded Low speed--engine bypass) (by 45 40 .50 30 150 Flywheel effect im portant 150 Flywheel effect im portant Motors and Motor Controls employing a starting capacitor and a running capacitor. The starting capacitor gives high starring ability, but is suited for short time operation only, and is cut out for the running condition by a centrifugal switch. The running capacitor 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 Email 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 start ing 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 ob tain a winding approximately like the squirrel-cage in its function. Repulsion-start Induction-run motors are suitable for ap plications, such 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 starring, but is disconnected through the action of a centrifugal switch as the motor comes i up to speed. Under running conditions 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 adjust able 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. 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 light-load density 645 areas, power companies check carefully the starting currents in order to prevent objectionable voltage dips. This is par ticularly important for motors started frequently, such as those controlled by pressure or temperature-sensitive devices and applied to refrigerators, stokers, oil burners, and water pumps. The report of a joint committee of AIEC-EEI-NEMA recommended three application rules taking into account the greater annoyances resulting from frequent motor starting during lighting hours against infrequent starting at any time. Table 6 shows typical applications of these rules which are becoming widely accepted for full-voltage starting. Rules 1 and 2 apply to general use. Rule 1 limits the locked-rotor current to 20 amp at 115 volts, and 25 amp at 230 volts when automatically controlled (usually frequently started) motors are used. Rule 2 permits Table 6 .... Recommended Single-Phase Motor Ratings for Full-voltage Starting locfcsd-refar Current* of 25 C HP Rating Amperes Cansref Ute (fatal i A 2) Special Condition* (with Utility Permit- don) (Rule 3) 115 V 230 V 115 V 230 V 115 V 230 V Ho 20 .10 A-M A-M A-M A-M H 20 10 A-M A-M A-M A-M X 20 10 A-M A-M A-M A-M H 23 11.5 A-M A-M A-M A-M X 31 15.5 M A-M A-M A-M H 45 22.5 M A-M A-M A-M X 61 30.5 M A-M A-M 1 70 35 M A-M A-M ix 40 M A-M 2 60 M A-M 3 70 A-M 5 100 A-M A refers to automatically controlled device*. H refers to manually controlled devices. These values of Locked-rotor current are t^>* same as single phase Desicn H motors. hTEMAstandard far Rg. 4 .... Speed Torque Characteristics of Single-phase Motors twice these values when manually controlled (usually in frequent starting) motors are used. Rule 3 applies to special conditions where larger currents may be allowed above Rules 1 and 2 upon approval of the electric company. Hermetically-enclosed single-phase a-c motors are widely used in appliances such as domestic and commercial refriger ators, room air conditioners, beverage coolers, freezer chests, and water coolers. In such appliances, the motors are sealed inside the compressor, and the gas and oil circulate freely within and around the motor. Centrifugal starring switches are not used on such motors because any arcing in the gas and oil atmosphere would be detrimental, and also because the units are permanently sealed and cannot be serviced or repaired in the field. In place of a centrifugal switch, an external relay-is used to start hermetically-enclosed motors. Hermetically-enclosed motors for refrigeration compressors will, in the future, have the horsepower rating eliminated. The full-load current is to be the operating current in am peres, when the compressor is delivering rated output. The motors for hermetically-enclosed service are usually of the