B-A] OPERATION AND LOAD TEST. 263 



eral some special provision for starting a single-phase motor must 

 be made. Whatever means are used for starting, the starting 

 torque is small and it is preferable to start the motor without 

 load. The load may be assumed after starting, by means of a 

 loose pulley or clutch. 



17. Shading Coils. One method for making small single- 

 phase motors self-starting depends upon the use of shading coils. 

 Each magnetic pole of the field of such a motor is divided into 

 two parts. Around the leading portion of each pole is wound a 

 short-circuit coil of low resistance, called a shading coil. When 

 the flux is changing in any pole, its increase or decrease in 

 the leading part is retarded by the short-circuited coil. In 

 consequence of this action, the leading part of each pole attains 

 its maximum magnetization after the other part, so that a revolv- 

 ing field is produced and the rotor is drawn around as in a poly- 

 phase motor. 



18. Repulsion Motor. A single-phase motor is often made 

 to start as a repulsion motor and is converted into an induction 

 motor when nearly full speed is attained. Such a motor has a 

 wound rotor provided with a commutator and brushes, as the 

 armature of a direct-current generator or motor. The brushes 

 are connected together by a heavy conductor. In a 2-pole model, 

 the brushes are opposite each other. 



In a 2-pole model, suppose the plane of the brushes (or rather 

 the plane of the coils to which they are connected through 

 the commutator) to be in the center line of the poles. There 

 would be a large flow of current through the rotor windings and 

 the brushes, but from symmetry there would be no torque ; half 

 the conductors would tend to turn in one direction and half in 

 the other. If the plane of the brushes were turned at right angles, 

 there would be no current flowing and therefore no torque. If, 

 however, the brushes are set obliquely, a current will flow and 

 there will be a resultant torque, for the current in the conductors 



