lo-C] SYNCHRONOUS CONVERTER. 329 



22. Inverted Converter. When a converter is driven by 

 direct current, its speed depends upon the field excitation, as in 

 the case of a D.C. motor. If the field is weakened through any 

 cause (by a decrease in field current or by the demagnetizing 

 effect of armature reaction), the speed increases; likewise, if the 

 field is strengthened (by an increase in field current or by the 

 magnetizing effect of armature reaction), the speed decreases. 



When alternating current is being delivered by the converter 

 at unity power factor, the magnetizing or demagnetizing effect 

 of the armature current is insignificant. 



At other power factors, however, a lagging current weakens 

 and a leading current strengthens the field, as in a generator, 

 and makes a corresponding change in speed and in frequency. 

 A machine designed for operation as an inverted converter 

 should, therefore, be designed with a magnetically weak arma- 

 ture ; or, some device should be provided for controlling the exci- 

 tation and maintaining the speed constant. This is sometimes 

 done by using an exciter mounted on the same shaft so that any 

 increase of speed of the converter is checked by the increase of 

 exciting current which it produces. 



23. Test. Operate an inverted converter with an inductive 

 load* and with a non-inductive load adjusted for the same value. 

 (CAUTION: Be careful to avoid excessive speed.) With constant 

 field current, compare the speeds in the two cases. (Complete 

 curves from no load to full load may be taken when desired.) 



24. Note the change in field current necessary to produce the 

 same speed in the two cases. 



25. Compounding with Series Reactance. With a given 

 series reactance, determine the number of series turns needed to 

 give the same D.C. voltage at full load as at no load (n); 

 proceed as with a D.C. generator (28, Exp. i-B). 



* An induction motor, locked, may be found convenient for this. 



