THE OPERATION OF THE DYNAMO AS A MOTOR. 109 



not be realized, inasmuch as very low speeds cannot be reached 

 because of the saturation of the iron parts of the motor as ex- 

 plained above, and inasmuch as very high speeds are not prac- 

 ticable because of the trouble from sparking when the field exci- 

 tation of the motor is very small; the impressed voltage E x be- 

 ing constant in value. Now, on the other hand, a generator does 

 not tend to spark badly when run at low field excitation, because 

 when the field excitation is low, the voltage between commutator 

 segments is low also. Therefore a given generator may be sat- 

 isfactorily used to give a range of values of electromotive force 

 from zero, when its field excitation is zero, to a maximum value, 

 when its field magnets are saturated. The Ward Leonard 

 method of speed control makes use of this property of a gen- 

 erator as follows : An auxiliary constant speed motor takes cur- 

 rent from the supply mains and drives a generator, G, at con- 

 stant speed. The field winding of this generator, G, is provided 

 with a field rheostat which has a sufficient range of resistance 

 to change the field excitation of the generator from its full value 

 to nearly zero. The main motor, which is to be driven at varia- 

 ble speed, has its field connected to the supply mains so that its 

 field excitation is maintained at a constant value, while its arma- 

 ture is supplied with current from the generator G. With this 

 arrangement it is practicable to supply the main variable speed 

 motor with current at any desired voltage from zero up to the 

 greatest voltage obtainable from the generator G when the field 

 of the latter is fully excited. 



Comparison of methods of speed control. The armature rheo- 

 stat method of speed control is fairly satisfactory when the motor 

 load does not vary rapidly. The disadvantage of this method, 

 aside from the great fluctuation of speed with load, is that the use 

 of a rheostat in series with the armature involves a very consider- 

 able waste of power due to the PR loss in the rheostat. The 

 advantages of this method are : that it is simple and cheap, that 

 it can be arranged to give fine gradations of speed control by 

 having many resistance steps in the rheostat, and that it permits 



