176 ELECTRICAL MACHINERY 



rapidly else the over-load release will blow; neither must 

 it be moved over too slowly else the resistance wire in the 

 rheostat is likely to be melted. The contact points on the 

 face of the rheostat should be kept clean, as must also the 

 sliding contact, or " shoe/' on the rheostat arm. 



37. Speed Control of Motors. It is often necessary to 

 vary the speed of a motor according to the changing require- 

 ments of the load. To make the electric motor suitable 

 for railway service, operation of machine tools, etc., it 

 must be possible to change quickly and easily its speed 

 through a wide range, and the scheme for obtaining this 

 variation in speed must be such that the motor operates 

 with a good efficiency at any one of the speeds required. 



Possibility of varying Speed. By inspection of equation 

 31, page 164, 



E-IRg 



it is seen that the speed may be varied by a change in the 

 impressed voltage E, the resistance of the armature circuit, 

 R a , or the value of the field flux, <. The loss in heat in 

 the armature circuit is equal to I 2 R a (where R a is the 

 resistance of the armature circuit), and if this quantity is 

 large the motor must necessarily be inefficient; speed con- 

 trol by the addition of resistance in the armature circuit is 

 therefore seldom used and will not be discussed here. 



Multiple Voltage Control. In the system using multiple 

 voltage control, the power line consists of several wires, 

 and the voltage between various pairs is different. In 

 one system four wires are used to distribute power to the 

 motors and the power supplied to these four wires is obtained 

 from a set of three generators. The various voltages 

 obtainable by using different pairs of supply wires are 

 shown in Fig. 107. The voltage impressed on the arma- 

 ture in this system may be varied in steps from 00 volts 



