222 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



The loss in the damping circuits was then gradually decreased from 

 3 per cent to 0.6 per cent only, without the converters falling out of 

 step. Experience shows that, as a general rule, the stability of the 

 converter is better when the supply-current is lagging than when it is 

 leading, probably because the induced E.M.F. is higher with leading 

 currents. Much better results will be obtained, both in respect to the 

 damping of speed-oscillations and in respect to ease of starting, by 

 using the Leblanc dampers in their most perfect form, namely, in the 

 form of grids consisting of copper bars or rods imbedded in the pole- 

 pieces (parallel to the shaft) and connected to copper plates at both 

 ends. (See part I, Fig. 62.) In most cases, however, to reduce the 

 cost, a simpler form of damper is used. 



USE OF ROTARY CONVERTERS FOR TRANSFORMING DIRECT 

 INTO ALTERNATING CURRENT 



Since rotary converters are reversible machines they can be 

 employed, in certain cases, for producing alternating currents from 

 direct current. 



There may be cases where a small portion of the output of a direct- 

 current generating plant must be transmitted to a considerable distance. 

 In such cases the power to be transmitted can be converted into high- 

 tension three-phase alternating currents which, after being transmitted 

 to the receiving end, can be utilized there as alternating currents or 

 else can be reconverted into direct currents. Again, in certain direct- 

 current installations, it may be desirable to convert a portion of the 

 power into three-phase alternating currents for operating certain 

 apparatus, such as, for instance, for supplying induction-motors, in 

 cases where commutator-motors would not be allowable. It is also 

 possible and it may be desirable in certain cases to produce poly- 

 phase alternating currents by means of a converter supplied with direct 

 current from a storage battery. 



In such cases the characteristics and the performance of the rotary con- 

 verter are materially different from what they are in the cases previously 

 considered, being similar in all respects to those of direct current motors. 



In the first place the speed is not constant, but it depends upon the 

 field-excitation, owing to the fact that the E.M.F. induced in the direct- 

 current armature-winding must always be substantially equal to the 

 E.M.F. of the source of (D.C.) current, less the ohmic drop. Hence, 

 when the field-excitation is made stronger the speed decreases, and 

 vice versa. 



