SYNCHRONOUS MACHINES. [Kxp. 



In a synchronous converter, the armature revolves while the 

 field and brushes are stationary ; for mechanical reasons the re- 

 verse arrangement with revolving* field and brushes and with 

 stationary armature is undesirable. Connections for operating 

 a synchronous converter are shown in Fig. 2. 



3. Other devices for deriving direct from alternating cur- 

 rents are: synchronous commutators^ (which have proved short- 

 lived both as individuals and as a class) ; and rectifiers that depend 

 upon a valve effect, as the aluminum rectifier, mercury-arc recti- 

 fier, etc., the latter being the only one of these with high enough 

 efficiency to warrant extensive use. 



4. A synchronous converter is normally used to receive alter- 

 nating and to deliver direct current, but may be used as an in- 

 verted converter to receive direct and to deliver alternating cur- 

 rent or, as a double current generator driven by power and 

 delivering both direct and alternating currents. 



5. Voltage Ratios. Terminal voltages in any machine differ 

 somewhat from the induced or generated voltages on account of 

 drop in the windings which varies with the load. The ratio of 

 generated voltages in a converter may be computed as follows: 



Consider a converter driven as a generator, delivering direct 

 current and single-phase alternating current. When the brushes 

 are properly set, the D.C. voltage will be equal to the maximum 

 value of the A.C. voltage. The effective A.C. voltage will depend 

 upon wave form, being for a sine wave I/ V 2 times the maximum, 

 or direct current, value. 



* (2b). Permutators. The rotating field may be produced electrically, 

 in which case both field and armature windings are stationary, the brushes 

 being driven at synchronous speed by a light driving mechanism. A modi- 

 fication of this arrangement is the permutator, the introduction of which has 

 no doubt been prevented by the difficulties introduced by the revolving 

 brushes. See: Elektrotcch. Zeit. (Vienna), Aug. 28, 1898; L'Industrie 

 Electrique, Feb. 10, 1902, Nov. 25, 1905; Lond. Electrician, Dec. 9, 1905, 

 Dec. 10, 1906; Elect. Age, Nov., 1908; Siblcy Journal of Eng., June, 1909. 



t For a test of such a commutator, see paper by J. B. Whitehead and L. 

 O. Grondahl, Elec. World, pp. 896 and 914, April 15, 1909. 



