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ELEMENTS OF ELECTRICAL ENGINEERING. 



they are short-circuited by the auxiliary brush, c, would have 

 excessive short-circuit currents produced in them by the large 

 induced electromotive forces in the sections. The sections must, 

 therefore, be out of the inducing field in the neutral spaces, pp, 

 when they are short-circuited by the auxiliary brush. When the 

 generator shown in Fig. 158^ is heavily loaded the armature cur- 

 rent tends to crowd the magnetic flux into the portions N' and 

 S r of the pole pieces thus causing the voltage between brushes, 

 a and c, to become greater and the voltage between brushes, 

 b and c, to become less. This tendency must be to some extent 

 counteracted, and it must be possible for the station attendant to 

 control the voltages, a to c and c to b, separately, in a practicable 

 three-wire generator. This is accomplished in the three-wire 

 generator of Dettmar * as follows : A four-pole field structure is 

 excited so as to be essentially a two-pole field magnet acting on 



Main r 



Fig. 1583. 



an armature properly wound for a two-pole field as shown in Fig. 

 158^. The magnetic flux out of N (and into S), Fig. 158^, pro- 

 duces the voltage, b to c, and this flux is produced by the excit- 

 ing coils, AA and AA, the current in which can be controlled by 



* Electrotechnische Zeitschrift, Vol. 1 8, pp. 55 and 230, 1897. 



