THE CONTINUOUS CURRENT MOTOR 



177 



Generators 



< ilO 



140 



190 



250 



to 250 volts and the speeds obtainable would vary in about 

 the same proportion so that the highest speed obtainable 

 would be about four times the lowest speed. This would 

 be called a 1 : 4 speed control. 



This scheme is used in the operation of motors for 

 driving machine tools. If a high speed is desired for a 

 certain operation on a lathe, the motor is connected to the 

 250-volt line, and, when lower 

 speeds are desired, the motor is 

 connected to one of the other 

 pairs of wires, having a suitable 

 voltage. Such a system of 

 speed control involves compli- 

 cated wiring and controllers for 

 the motors but its advantage 

 lies in the fact that the motors 

 operate at a comparatively 

 high efficiency at any one of 

 the speeds obtainable. 



Speed Control by Field 

 Variation. Another important 

 method for obtaining various 

 speeds consists in weakening 

 the field of the motor. This 

 is called field control. When 



first discussing the speed-load curves of motors it was 

 shown that, under any condition of operation, the counter 

 e.m.f. developed in the armature winding must be nearly 

 equal to the impressed voltage. If now the impressed 

 voltage is maintained constant and the field flux is varied, 

 it is evident that the above condition can be fulfilled only 

 if the speed of the motor follows the changes in field strength : 

 a high speed corresponding to a weak field and vice versa. 



The amount of variation which can thus be obtained 

 from a shunt motor of ordinary design is not very great; 

 the twisting of the main field by the armature reaction 



Dist 



ibuting Lines 



FIG. 107. Power Distribution 

 Scheme for Multiple Voltage- 

 speed Control. 



