216 



HANDBOOK OF MECHANICAL DESIGN 



Repulsion-start induction-run motors develop a continuous rotating effect on 

 the rotor because of induced currents in the rotor made continuously effective by- 

 commutation to produce torque during the starting period. 



Repulsion-start induction-run motors have high starting and accelerating torques 

 and when running as a single-phase induction motor with squirrel-cage rotor, or 

 its equivalent, are very efficient. These motors at starting are repulsion motors, 

 but on reaching a predetermined speed expanding governor weights push a device 

 under the commutator which short circuits the commutator bars through a common 

 ring; the same movement releases tension on the brushes with the result that the 

 armature is short-circuited and is the equivalent of a squirrel-cage rotor in a poly- 



Self starting, ^ 



single-phase moTor, 

 with phase coil cutout 



o 



Repulsion start and o 

 run, single-phase motor O 



Repulsion start, j-, 



induction run, o 



single-phase motor 



^ Phase -^ 



Capacitor start, 

 induction run, 

 single- phase motor 



o"o"OTroirv-i 



~:^ Phase dsdz 

 3 ,^s^-3x |V:: 



Capacitor start, Oq 

 capacitor run, -^O /° Q "Jj 

 single-phase motorg V> -°/ 

 O 



Fig. 445. — Wiring diagrams of winding schemes and starting devices used in typical alternating-current fractional 



horsepower motors. 



phase induction motor. When the motor stops, the governor and mechanism return 

 automatically to their original starting positions. 



Repulsion-start induction-run type motors are suited for loads requiring high 

 starting and accelerating torques. Repulsion-start induction-run type motors are 

 furnished only for single speed apphcations. 



The spht-phase start induction motor develops its magnetic rotating effect b}' 

 spUtting the magnetic field of the stator mnding into two separate windings displaced 

 in space and having different electrical characteristics. One winding is a starting or 

 phase winding, and the other is the main or running winding. When the motor starts, 

 both windings are on the line. After accelerating up to a predetermined speed, a 

 governor attached to the rotor acts to open a smtch and cuts out the starting winding. 

 The motor then continues to operate on the running winding as a single-phase induc- 

 tion motor. 



Split-phase motors can be designed with high starting torque but only by using 

 relatively high starting current. They are purposely designed with low starting 

 torque so that the current and consequently the heating in the starting winding will 

 be limited. 



Equipment driven with spht-phase motors should be easy to start. The inertia 

 of the load should be small so that the motor can accelerate rapidly to avoid "cook- 

 ing" the starting winding. Feed mres should have capacity great enough to carry 

 the high starting current without reducing the voltage at the motor terminals with 

 consequent reduction of the motor torque. 



