150 ALTERNATING CURRENTS 



X,, and in most cases it may be assumed, without appreciable 

 error, as being equal to it. 



67. Three-phase Application. The preceding discussion and 

 problems have all been applied to single-phase generators. This 

 has been done merely to illustrate methods. Very poor results 

 accompany the practical application of these methods to single- 

 phase alternators. In a single-phase alternator the armature 

 reaction is pulsating, even for a constant value of armature cur- 

 rent. The flux in the poles pulsates, because of the variation of 

 the current in the armature coils as they pass the poles. For 

 this reason the synchronous reactance is an indefinite quantity 

 and calculated results of regulation with single-phase machines 

 are far from satisfactory. 



In a polyphase machine, however, the armature reaction is 

 substantially constant if the load be constant and balanced. 

 When the current in one phase has decreased, the resultant cur- 

 rent of the other two phases has increased, etc. Therefore, the 

 magnetomotive force of the armature is practically constant in 

 value and is stationary in space with respect to the field poles. 

 That is, if the field poles rotate, the armature mmf . follows them 

 at rotor speed and is practically constant in magnitude for a 

 fixed value of armature current. This effect will be described 

 more in detail under the induction motor. 



Under these conditions the synchronous reactance becomes a 

 more definite quantity and more satisfactory results are therefore 

 obtainable with these various methods of testing. Figure 153 (a) 

 shows the connections for making the open-circuit test of a 

 three-phase alternator. This is substantially the same method 

 as is used with direct-current generators. The field is excited 

 from some direct-current source and the field current is measured 

 with an ammeter. The armature is driven at the rated or syn- 

 chronous speed and the open-circuit voltage measured for differ- 

 ent values of field current. The voltage of one phase only need 

 be measured as the phase voltages should all be equal. A fre- 

 quency indicator, F, may be used for determining the speed of 

 the machine. An additional resistance, Ri, in the field circuit 

 is often necessary for obtaining the points on the lower part of the 

 saturation curve. 



In the short-circuit tost all three phases must be short-cir- 

 OF ELECTRi 



FACULTY OF APPLIED SCIENCE. 



