176 ALTERNATING CURRENTS 



the excessive mechanical vibration which is set up. This is due to 

 the powerful magnetic side-pull brought into play by the strengthening 

 of the field-poles corresponding to the motor section, and the weaken- 

 ing of those corresponding to the generator section of the armature 

 winding, by armature reaction. In order to adapt the method to 

 modern types of alternators, Behrend * divides the field coils into two 

 equal sections, and varies the exciting currents of these independently 

 so as to obtain magnetic balance. With this modification, it is 

 possible to overcome the vibration trouble to a considerable extent. 



101. Goldschmidt's Method 



The following method of carrying out the heat test of a large 

 alternator has been devised by Goldschmidt.* The alternator is run 

 at normal full-load excitation, so that the iron and field copper losses 

 are the same as those occurring at full-load. The armature copper 

 loss is supplied by sending a continuous current from a battery or 

 dynamo around the armature winding, the continuous current being 

 made equal to the r.m.s. value of the full-load armature current. It 

 need hardly be pointed out that the connections must be arranged so 

 as to prevent the high e.m.f. of the alternator armature from reaching 

 the source of continuous current. Various ways of securing this 

 result are available. In the case of a single-phase machine, the 

 armature coils may be divided into two equal groups, connected so as 

 to oppose each other ; the resultant alternating e.m.f. will then be 

 zero, and the armature circuit will form a simple resistance so far as 

 the source of continuous current is concerned. In a two-phase 

 machine, each phase may be similarly treated, a separate source of 

 continuous e.m.f. being used for each phase. With three-phase 

 machines having a A-connected armature, the A may be opened 

 at one corner to allow of the introduction of the continuous current. 

 If the windings are star-connected, they may be temporarily altered to 

 a A connection for the purposes of the test. Another method may 

 also, however, be used. A suitable star-connected three-phase 

 transformer is joined across the alternator terminals, and the con- 

 tinuous current led into the system at the neutral point of the 

 transformer windings, and out of it at the neutral point of the 

 armature winding each phase carrying in this case a current equal 

 to -- of the continuous current. In order to balance the magnetic 

 effect of the continuous current on the transformer core, a continuous 

 current producing an equal and opposite number of ampere-turns 

 must be sent through the secondary windings of the transformer. 



* Elektroiechnische Zeitschrift, vol. xxii. p. G82 (1901). 



