174 THE MAGNETIC CIRCUIT [ART. 54 



magnetic leakage. The leakage factor of the main poles is also 

 somewhat increased by the presence of the interpoles; this is one 

 of their disadvantages. Some other disadvantages are : the ven- 

 tilation of the field coils is more difficult, and a smaller ratio of 

 pole arc to pole pitch must be used. However, the advantages 

 gained by the use of commutating poles are such that their use 

 is rapidly becoming universal. 



The interpole winding removes the effect of the transverse 

 belts T 7 , T, in the commutating zone, but does not neutralize their 

 distorting effect under the main poles. Hence, the distortion and 

 its accompanying reduction of the main flux are practically the 

 same as without the interpoles. To remove this distortion a com- 

 pensating winding (Fig. 44), connected in series with the main cir- 

 cuit of the machine, is sometimes placed on the main poles. The 

 connections are such that the compensating winding opposes the 

 magnetizing action of the armature winding. By properly select- 

 ing the specific electric loading of the compensating winding the 

 transverse armature reaction under the poles can be removed, 

 either completely or in part. This winding was invented inde- 

 pendently by Deri in Europe and by Professor H. J. Ryan in this 

 country; on account of its expense, it is used in rare cases 

 only. 



When a compensating winding is used in addition to the inter- 

 poles, the number of ampere-turns on the interpoles is consider- 

 ably reduced, because the compensating winding can be made to 

 neutralize the larger portion or all of the armature reaction. In 

 such a machine a much higher specific loading can be allowed than 

 in an ordinary machine of the same dimensions. Therefore, such 

 compensated machines are particularly well adapted for rapidly 

 fluctuating loads, and for sudden overloads or reversals of rotation 

 in the case of a motor. 



Prob. 13. From the following data determine the ampere-turns 

 required on each commutating pole of a turbo-generator: The com- 

 mutating poles are made of cast steel; the average flux density on 

 the face of the interpole is 6000 maxwells per sq.cm.; the pole-face 

 area 250 sq.cm.; the pole cross-section 160 sq.cm.; the radial length 

 of the interpole 27 cm.; the leakage factor, 1.5. The air-gap reluctance 

 is 2.7 millirels, the true tooth density 20 kilolines per sq.cm., the height 

 of the tooth 4.5 cm., and the armature ampere-turns per pole 9500. 



Ans. About 15,300. 



Prob. 14. The rated current of the machine in the preceding problem 



