Document gaYMZvMng0O8yz1kGeROB9LeJ

878 CHAPTER 40 1953 Guide 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 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. 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- Pulling in Out Remarks s <3 Exhaust and venti lating Coupled or belted Usually loaded 50 60-125 150 WR* of fan must be considered Cycloidal positive Coupled or engine Unloaded v 40-60 40-60 150 Two-speed motors type sometimes used I Blowing engines re Engine type o ciprocating Unloaded 40 40-60 150 5 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 30 150 Flywheel effect im portant Compressors Ammonia and am High speed--belted' Unloaded (by 40 monia booster Low speed--engine by-pass) type occasionally coupled 30 - 150 Flywheel effect im portant Freon High speed--belted Unloaded (by 45 50 150 Flywheel effect im Low 8peed--engine by-pass) portant Gas reciprocating High speed--belted Unloaded (by 40 30 150 Flywheel effect im Low speed--engine by-pass) portant 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 pacitor gives high efficiency at full speed. These motors are used on Motors and Motor Controls 879 compressors, reciprocating pumps and similar equipment- which1 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-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 repulsiOn- Fig. 3. Speed-Torque Characteristics of Single Phase Motors induction 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. Repulsion-Induction and Repulsion-Start-Induction Run motors are suit able 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,