208 ELECTRICAL ENGINEERING 



torque can be obtained without excessive current in the armature. 

 Second, resistance must be connected in series with the armature 

 to limit the current and this resistance must be cut out gradually 

 as the motor speeds up and generates a back voltage. If the re- 

 sistance is cut out too quickly the motor draws a very large cur- 

 rent and accelerates too rapidly. When all the resistance is out 



Starting Resistance 



FIG. 177. Starting a shunt motor. 



the motor runs at full speed and generates a back voltage almost 

 equal to the applied voltage. If full voltage is applied to the 

 armature at rest the current is limited by the armature resistance 

 only, which is very small. Assume that the armature resistance of 

 a 110-volt motor is 0.01 ohm. If 110 volts is applied the current 



tends to rise to a value ? = 7T7* = 11,000 amperes and will im- 

 mediately operate any protective devices on the system. 



129. Applications of Motors. There are three types of direct- 

 current motors, (1) shunt, (2) series and (3) compound. 



(1) The characteristics of a shunt motor are (a) constant speed 

 and (b) torque proportional to current. 



Shunt motors are used for lathes, boring mills and all constant- 

 speed machine tools, for driving line shafting when the starting 

 load is not too heavy, for fans, centrifugal pumps, etc. 



The starting torque is not large and when the motor has to start 

 under load it draws an excessive current. 



The speed of shunt motors can be increased by field weakening 

 about 70 per cent above normal without injurious sparking if the 

 output remains constant, or 100 per cent if the output is decreased 

 to 80 per cent of full load. 



For lathes speed ranges of 4 : 1 are necessary since the cutting 

 speed must remain approximately constant although the diameter 

 of the material changes. Field control must be used and in order 

 to get the required range a multiple-wire supply system is often 



