110 THE DESIGN OF STATIC TRANSFORMERS 



REGULATION OF TRANSFORMERS FOR THREE-WIRE 

 SECONDARY CIRCUITS 



The majority of core-type transformers used for lighting 

 purposes have windings of the concentric type, i.e., the primary 

 is wound over the secondary, or rice rerx<i, and primary and 

 secondary each consist of long cylindrical coils. The simple 

 concentric type (see Fig. 30 on p. 51) consists of two coils per 

 leg, the primary usually being wound over the secondary ; the 

 triple-concentric type (see Fig. 31 on p. 51) consists of three 

 coils per leg. Two of these three coils may constitute the 

 secondary, the primary coil heing wound in between these two 

 secondary coils. When using a transformer on a three-wire 

 secondary circuit, it is necessary to provide two separate 

 groups of secondary coils, each group of coils giving the same 

 pressure. At first sight, the most obvious way of connecting a 

 transformer of the core type to supply a three-wire secondary 

 circuit would appear to be to utilise the secondary coils on one 

 leg of the transformer for one side of the three-wire circuit, and 

 the secondary coils on the other leg for the other side of the 

 three-wire circuit. With these connections, equal pressures 

 would be obtained from both secondaries provided they were 

 equally loaded ; but if the two secondaries are not equally 

 loaded it is evident that the transformer will regulate badly, 

 the pressure of the side on which the greater load exists being 

 below normal, while the pressure of the lightly loaded side will 

 rise above normal, the sum of the pressures on the two sides 

 remaining constant, whatever the distribution of the load. 

 That this must be the case may be explained as follows : 



Consider one side of the three- wire system to be unloaded 

 and the other side to be fully loaded. If both the primary 

 windings are connected in series, the same primary current 

 must flow in all parts. One-half of the primary ampere turns 

 will be on that leg of the transformer whose secondary is 

 loaded, and the other half will be on the leg with the unloaded 



