Chapter 20. -SHIPBOARD ELECTRICAL SYSTEMS 



SIMPLIFIED SCHEMATIC AND WAVE FORMS 



WYE CONNECTION 



DELTA CONNECTION 



27.244 

 Figure 20-20.— Three-phase a-c generator. 



each phase may be connected to form a common 

 junction. The stator is then called a wye - 

 connected or star-connected stator. The com- 

 mon lead may or may not be brought out of the 

 machine. If it is brought out, it is called the 

 neutral . The simplified schematic diagram (fig. 

 20-20) shows a wye-connected stator with the 

 common lead not brought out. Each load is con- 

 nected across two phases in series. Thus, R^)-, 

 is connected across phases A and B in series, 

 R^f. is connected across phases A and C in 

 series, and R]-,^, is connected across phases B 

 and C in series. Thus the voltage across each 

 load is larger than the voltage across a single 

 phase. The total voltage, or line voltage, across 

 any two phases is the vector sum of the indi- 

 vidual phase voltages. For balanced conditions, 

 the line voltage is 1.73 times the phase voltage. 

 Since there is only one path for current in a 

 line wire and the phase to which it is connected, 

 the line current is equal to the phase current. 

 A three-phase stator can also be connected 

 so that the phases are connected end to end, as 

 shown in figure 20-20. This arrangement is 

 called a delta connection . In the delta connec- 

 tion, the line voltages are equal to the phase 

 voltages. The line currents are equal to the 

 vector sum of the phase currents. The line cur- 

 rent is equal to 1.73 times the phase current, 

 when the loads are balanced. 



VOLTAGE REGULATION 



When the load on an a-c generator is changed, 

 the terminal voltage varies with the load. The 



amount of variation depends on the design of 

 the generator and on the amount of reactance 

 from the inductive or capacitive loads. Under 

 practical shipboard operating conditions, the 

 load varies widely with the starting and stopping 

 of motors. 



The only practicable way to regulate the 

 voltage output of an a-c generator is to control 

 the strength of the rotating magnetic field. The 

 strength of the electromagnetic field may be 

 varied by changing the amount of current flow- 

 ing through the coil, which is done by connecting 

 a rheostat in series with the coil. Thus, voltage 

 regulation in an a-c generator is accomplished 

 by varying the field current. This allows a rela- 

 tively large a-c voltage to be controlled by a 

 much smaller d-c voltage and current. 



Since manual adjustment of a-c voltage is 

 not practicable when the load fluctuates rapidly, 

 automatic voltage regulators are used. The con- 

 struction and operating principles of voltage 

 regulators varies; however, the essential func- 

 tion of any voltage regulator is to use the a-c 

 output voltage, which the regulator is designed 

 to control, as a sensing influence to control the 

 amount of current the exciter supplies to its 

 own control field. 



TRANSFORMERS 



A transformer (fig. 20-21) is an a-c device 

 that has no moving parts and that transfers 

 energy from one circuit to another by electro- 

 magnetic induction. The energy is always trans- 

 ferred without a change in frequency but usually 

 with changes in voltage and current. A step-up 

 transformer receives electrical energy at one 

 voltage and delivers it at a higher voltage, A 

 stepdown transformer receives electrical 

 energy at one voltage and delivers it at a lower 

 voltage. Transformers are not used on direct 

 current. 



The conventional constant -potential trans- 

 former is designed to operate with the primary 

 connected across a constant-potential source 

 and to provide a secondary voltage that is sub- 

 stantially constant from no load to full load. 



Various types of small single-phase trans- 

 formers are used on shipboard equipment. In 

 many installations, transformers are used on 

 switchboards to step down the voltage for indi- 

 cating lights. Low-voltage transformers are 

 included in some motor control panels to supply 

 control circuits or to operate overload relays. 

 Other common uses include low-voltage supply 



507 



