CHAPTER IV 

 POLYPHASE SYSTEMS 



42. Reasons for the Use of Polyphase Currents. In many 

 industrial applications of alternating current, there are objections 

 to the use of single-phase power. 



In a single-phase circuit, the power delivered is pulsating. 

 Even when the current and voltage are in phase, the power is zero 

 twice in each cycle, as shown in Fig. 19, page 22. When the 

 power-factor is less than unity, the power is not only zero four 

 times in each cycle, but it is also negative twice in each cycle. 

 This means that the circuit returns power to the generator for a 

 part of the time. This is analogous to a single-cylinder gasoline 

 engine in which the fly-wheel returns energy to the cylinder dur- 

 ing the compression part of the cycle. Over the complete cycle, 

 both the single-phase circuit and the fly-wheel receive an excess 

 of energy over that which they return to the source. The pul- 

 sating nature of the power in single-phase circuits makes such 

 circuits objectionable in many instances. 



A polyphase circuit is somewhat like a multi-cylinder gasoline 

 engine. With the engine, the power delivered to the fly-wheel 

 is practically steady, as one or more cylinders are firing 

 when the others are compressing. This same condition exists in 

 polyphase electrical systems. Although the power of any one 

 phase may be negative at times, the total power is constant if the 

 loads are balanced. This makes polyphase systems highly 

 desirable for power purposes. 



The rating of a given motor, or generator, increases with the 

 number of phases, an important consideration. Below are the 

 approximate capacities of a given machine for different numbers 

 of phases, assuming the single-phase capacity as 100. 



Single-phase 100 



Two-phase 140 



Three-phase 148 



Six-phase 148 



Direct-current 154 



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