THE OPERATION OF THE DYNAMO AS A MOTOR. 105 



(23^) and (25). The very considerable current which is neces- 

 sary to develop torque sufficient to enable the motor to carry a 

 load, demands a large value in the numerator of equation (23^) 

 when R a is large, and this large value of (E x QZ'n] necessi- 

 tates a great drop in speed in order that QZ'n may be con- 

 siderably less than E x . It is to be especially noted, however, that 

 the zero load speed of a shunt motor is not perceptibly affected 

 by a moderately large value of R a , inasmuch as I a is very small 

 at zero load. It is for this reason that the speed of a shunt 

 motor cannot be satisfactorily controlled by inserting resistance 

 in series with the armature. The difficulty is that, when the speed 

 is cut down in this way while the motor is loaded, the speed 

 increases greatly when more or less of the load is thrown off. 



58. Speed control of the shunt motor. The peculiar and valu- 

 able properties of the shunt (and compound) motor are : (a) that 

 it runs at a definite speed at zero load, that is to say, it has no 

 tendency to race when the load is thrown off; and (ft) that its 

 speed drops but little below zero load speed even when the motor 

 is fully loaded. For many purposes, for example, in the driving 

 of lathes by motors, it is desirable to run the motor fast or slow 

 at will. This speed control is accomplished in the case of the 

 shunt motor by five different methods, as follows : 



(1) Armature rheostat method. When the load on a motor is 

 constant, its speed may be controlled by inserting resistance in 

 series with its armature, as explained in the last article. 



(2) Field rheostat method. -- From equation (26) it is at once 

 evident that the zero load speed (and consequently the full load 

 speed to nearly the same extent) may be raised by decreasing <I>, 

 or lowered by increasing 4>. These changes of < may be accom- 

 plished by means of a rheostat in circuit with the field winding 

 of the motor. By means of this rheostat the resistance of the 

 shunt field circuit may be raised or lowered, thus decreasing or 

 increasing the field current, and thereby decreasing or increas- 

 ing ^>. 



