THE ALTERNATOR. 19 



sufficient torque to start under load, in the latter case the motor 

 is usually supplied by two phases of current by splitting the 

 single-phase supply as described in Chapter XII, and the starting 

 torque is very small. The single-phase motor of the Wagner 

 Electric Company is a single-phase induction motor which starts 

 as a repulsion motor. 



The single-phase induction motor is not suitable for variable 

 speed service. 



(d] The single-phase commutator motor has the same character- 

 istics as the series motor operated by direct current, and it is the 

 only alternating-current motor which operates satisfactorily 

 throughout a wide range of speed. The single-phase commu- 

 tator motor is beginning to be used extensively in railway ser- 

 vice where a large starting torque and fairly efficient operation 

 throughout a wide range of speed are required. 



5. The physical basis of the differences between direct-current 

 calculations and alternating-current calculations. Figure 17 

 represents a valveless pump, P t the piston of which oscillates up 

 and down causing a current of water to surge back and forth 

 through a circuit of pipe, and Fig. 1 8 represents an alternator A 

 of which the alternating electromotive force causes an electric 

 current to surge back and forth through a circuit of wire. 



The alternating pressure generated by the pump in Fig. 17 

 must not only overcome the resistance which the current of water 

 encounters in the pipe, but a large part of the pressure developed 

 by the pump must be used to overcome the inertia of the water 

 in the pipe, first in getting the current of water started and then 

 in stopping the current and starting it again in the opposite direc- 

 tion. Similarly, the alternating electromotive force generated by 

 the alternator in Fig. 1 8 must not only overcome the electrical re- 

 sistance of the wire, but a large part of the electromotive force de- 

 veloped by the alternator must be used to overcome the electrical 

 inertia, or inductance,* of the circuit of wire, first in starting the 



* It is assumed in this text that the student is thoroughly familiar with the ideas 

 of inductance and capacity as outlined in Appendix A. 



