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690 CHAPTER 35 ' ,1949 .Guide Repulsion-Induction and Repulsion-Start-Induction Run motors are suitable for applications such as industrial compressors where high break away torque is required 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 Table 6. Typical Application Requirements of Synchronous Motor Drives Showing Starting, Pull-In and Pull-Out Torques Fans Application Method op Connecting Motor to Load Tobqoes Conditions Start Pull- Pulling in Out Remarks - Exhaust and venti Coupled or belted Usually loaded SO 60-125 150 WR* of fan must be lating considered Cycloidal positive Coupled or engine . type S Blowing engines re ciprocating Engine type Unloaded Unloaded" 40-60 40-60 150 Two-speed motors - sometimes used 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 30 150. Flywheel effect im portant a Ammonia and am High speed--belted Unloaded (by 40 30 150 Flywheel effect im 0 monia booster Low speed--engine by-pass) ' portant 1 type occasionally a coupled So Freon High speed--belted Unloaded (by 45 50 150 Flywheel effect im O Low speed--engine by-pass) portant Gas reciprocating High 8peed--belted Unloaded (by 40 30 150 ' Flywheel effect im Low speed--engine by-pass) portant 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. Typical specifications for single phase motors are shown in Table 8. CONTROL FOR ALTERNATING CURRENT MOTORS Squirrel-Cage motors are usually linestarted where power company limitations permit. In sizes up to 2 hp the motors are started.by means of manual switches with an overload current element for motor protection. In larger, ratings a linestarter is usually provided with either an addi tional safety switch or circuit breaker for disconnecting and short circuit protection. Reduced voltage starting may be either of manual or push button controlled, magnetic type. In specifying this.type of starter con sideration should be given to the fact that starting torque of squirrelcage-motors varies as the square of the applied voltage.; For example, a. motor developing ;100 lb-ft starting torque on full voltage would produce, only 25 lb-ft torque on starting on half rated voltage. Fig. .11 illustrates Motors'and Motor Controls 691 recommended control practice for squirrel-cage motors. - Typical speci fications for squirrel-cage motor controls are shown in Table 9. Wound Rotor motors require control of both primary and secondary circuits. The primary* control may be the same as for squirrel-cage motors, manual or magnetic, at full voltage. Secondary* control pro vides means of varying secondary resistance for starting and speed control. The secondary controller should be specified for starting duty only or for speed regulating duty. Fig. 12 illustrates recommended control practice for wound rotor motors. Typical specifications for wound rotor motor control are shown in Table 10. Synchronous motor starters should provide pull-out protection, auto matic synchronization or automatic stopping of the motor after pull-out, and insurance of complete starting sequence, as well as overload and low; voltage protection. The control may be either magnetic or semimagnetic at full or reduced voltage. Semi-magnetic starters provide automatic field control but require hand operation for closing the line contactors to' start and transfer to full voltage. , In...applying reduced voltage starters to synchronous motors it should be; remembered that, since these motors are started on damper windings and during the acceleration period function similarly to squirrel-cage, motors, the. starting torque varies as the square of the applied voltage. Consideration should be given to insure development of sufficient motor torque, to .accelerate the load. Typical specifications for synchronous motor control are shown in Table 11. '- ! Refer to Glossary at end of chapter.