PRINCIPLES OF NAVAL ENGINEERING 



41.19 



Figure 20-17.— Generation of sine-wave voltage. 



right of the figure represents successive values 

 of the a-c voltage induced in the conductor as 

 it moves at uniform speed through the 2-pole 

 field, because the instantaneous values of rota- 

 tionally induced voltage are proportional to the 

 sine of the angled that the rotating vector makes 

 with the horizontal. 



The sine wave in figure 20-17 represents one 

 complete revolution of the armature or one 

 voltage cycle. The frequency of a-c voltage is 

 measured in cycles per second (cps) and may 

 be determined by the following formula: 



f = 



P X rpm 

 120 



where 



rpm 

 P 



: frequency (in cps; according to the 

 National Bureau of Standards Special 

 Publication 304, frequency in cycles 

 per second in the International Systems 

 of Units is expressed as Hertz (H2). 

 One hertz equals one cycle per second.) 

 revolutions per minute 

 number of poles in the generator 



A generator made to deliver 60 cps, and 

 having two field poles, would need an armature 

 designed to rotate at 3600 rpm. 



PROPERTIES OF A-C CIRCUITS 



Resistance, the opposition to current flow, 

 has the same effect in an a-c circuit as it does 

 in a d-c circuit. However, in the application of 

 Ohm's law to a-c circuits, other properties 

 must be taken into consideration. 



Inductance is that property which opposes 

 any change in the current flow and capacitance 

 is that property which opposes any change in 

 voltage. Since a-c current is constantly chang- 

 ing in magnitude and direction, the properties 

 of inductance and capacitance are always pres- 

 ent. 



The amount of opposition to current flow in 

 an inductive circuit is referred to as its inductive 

 reactance, Xj^. The value of inductive reactance 



(in ohms) depends on the inductance of the circuit 

 and the frequency of the applied voltage. Ex- 

 pressed in equation form, 



X^ : 27rfL 



504 



