152 ELECTRICAL MACHINERY 



The term, compound motor, is therefore used to designate 

 that type in which the series and shunt fields assist one 

 another. 



The characteristics of the three types of motors are 

 given here and will be explained more fully in the later 

 paragraphs. 



The shunt motor has a fair starting torque and nearly 

 constant speed for all loads. It is used where the load 

 requires practically constant speed and the starting torque 

 demanded is not excessive. 



The series motor operates through a wide range of 

 speed as the load changes, and at very light loads the motor 

 is likely to " run away," i.e., reach dangerous speeds. It 

 gives very great starting torque and is therefore used where 

 a heavy starting torque is demanded and the motor may be 

 positively connected to its load. The principal application 

 of this motor is in railway service. 



The compound motor has a starting torque greater 

 than that of the shunt motor but less than that of thte series 

 motor. It has, however, a definite upper speed limit and 

 if all of its load is removed it will not run at dangerous 

 speeds. Its principal application is in elevator service, 

 machine tool drive, etc., where a fixed speed limit is neces- 

 sary and considerable starting torque is required. The 

 number of series turns used on the field coils varies some- 

 what, according to the service required of the motor, but, 

 in general, we m&y say that, at full load, the series ampere- 

 turns are from 10% to 50% of the shunt ampere-turns. 

 The decrease in speed from no load to ful] load may vary 

 from 12% to 50% in different motors. 



31. Torque of a Motor. A motor develops torque 

 because on the periphery of its armature are placed con- 

 ductors through which current flows and those conductors 

 lying under the pole faces are in a magnetic field. These 

 conductors are then acted upon by a force which tends to 

 move them in a direction perpendicular to the magnetic 



