CHAPTER III 

 STABILITY OF OPERATION OF ROTARY CONVERTERS 



WHEN in operation, rotary converters sometimes produce variations 

 of voltage above and below their normal voltage; and they may also, 

 at times spark, or even flash, at the commutator. These phenomena 

 might be attributed to changes of frequency due either to variations of 

 angular velocity of the prime mover driving the alternator which 

 supplies current to the converters, or else due to oscillations of its speed- 

 governor. 



To eliminate these two causes of irregular operation, the follow- 

 ing experiment was tried: the rotary converter was driven as a motor 

 by current taken from a storage battery, which also excited the shunt- 

 field winding. Three-phase currents taken from the A.C. side of the 

 machine were passed through a raising transformer to raise their voltage. 

 The voltage was then lowered by step-down transformers and the 

 energy transmitted through them was absorbed by means of three 

 identical rheostats. The arrangement was equivalent, practically, 

 to connecting a three-phase inductive resistance to the secondary of 

 the first transformer (Fig. 26). It was then observed that, on increasing 

 gradually the load on the converter, a critical point was reached where 

 its speed began to oscillate between limits which were all the farther 

 apart the more the load was increased. 



To explain this phenomenon, let us note that the converter was 

 sending reactive currents into the circuit, and that these reactive cur- 

 rents tended to weaken the magnetic field. Now in the case of a load 

 which is below a certain value(which we will term the limiting or critical 

 load) the strength of this reactive current should decrease when the 

 speed of the rotary converter increases, and reciprocally, it should 

 increase when the speed decreases. The contrary effect should be 

 produced when the load exceeds this critical load. 



To demonstrate this, let us refer to a transformer 'diagram of the 

 kind which takes magnetic leakage into consideration. It is known 

 (Fig. 27) that if OF is the axis of E.M.F.'s the end of the vector (OM) 



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