October 29, 



NATURE 



61 



SOME NOTES ON THE FRANKFORT INTER- 

 NA TIONAL ELECTRICAL EXHIBITIONS 



IV. 

 Alternate Current Motors. 

 A LTERNATE current motors constitute one of the 

 ■^*- most striking features at the Frankfort Exhibition, 

 and the commercial use of such motors will probably date 

 from this year, so that the one great objection to the 

 employment of alternating currents for the electric 

 transmission and distribution of power will soon dis- 

 appear. 



It is well known that the direction of rotation of an 

 ordinary series, or shunt, direct current motor is the same 

 whichever way the direct current passes round the motor, 

 in spite of a patent of Mr. Edison's to utilize the contrary 

 fact on electric railways ; hence it follows that if an alter- 

 nate current be sent round such a motor it will start 

 rotating and develop mechanical power. Only a com- 



FiG 10. — Alternate current synchronizing motor. <— Direct current. 

 < " ^> Alternating current. 



paratively small power and efficiency, however, will be 

 obtained ; first, because the large self-induction of the 

 field magnet of the motor will seriously diminish the 

 strength of the alternating current ; secondly, because, in 

 consequence of the rapid reversals of the magnetism, 

 much power will be wasted in heating the iron core of 

 the field magnet, even although this core be laminated 

 like that of the armature. 



If, on the other hand, a direct current be sent round 

 the field magnet, Mj, IVU, M3, of an alternate current 

 machine, and an alternating current round the arma- 

 ture, Aj, A2, A3 (Fig. 10), the armature will not move, 

 because at every two of the successive rapid reversals of 

 the current the armature receives an impulse in opposite 

 directions. To enable such a machine to work as a motor, 



it is necessary to first make the armature rapidly rotate 

 by mechanical means at such a speed that any armature 

 coil, A2, moves forward by the distance between two of 

 the poles IVl,, M of the field magnet in half the periodic 

 time of the alternation of the current. When this speed 

 has been once attained, the machine will go on running as a 

 powerful and efficient alternate current motor, at a per- 

 fectly definite speed, depending simply on the rate of 

 alternation of the current, and independent within 

 wide limits of the load put on the motor. 



So that when the armature of the motor is once " in 

 step" with that of the dynamo the two will continue "in 

 step," whatever be the amount, within wide limits, of the 

 power transmitted. 



When a considerable amount of power has to be 

 sent from a source to a distant town, and has there to 

 be distributed for light or for driving machinery, it will 

 certainly be best (as far as our present knowledge goes) 

 to use alternating currents in the transmission of the 

 power between the two distant places, because with 

 alternating currents the pressure can so easily be trans- 

 formed up at the source, and transformed down again 

 at the other end of the line. 



But in the c^stribution of the received power direct 

 currents are the more convenient, since they can be 

 utilized for light, for electroplating and electrotyping, 



NO. 



' Continued fro-n p. 546. 



48, VOL. 44] 



Fig. II. — Coupled alternate current mct.:)r an 1 c'irect current dynamo. 



as well as for small and large direct current electro- 

 motors, both of which have already reached a con- 

 siderable degree of perfection, and are of course self- 

 starting. Hence it is probable that there will be 

 employed a synchronizing alternate current motor, 

 coupled mechanically to a direct current dynamo, the 

 latter being used to supply current to the town and 

 excite the field magnets of the motor. .Such combina- 

 tions, seen in Fig. ii, are exhibited by Messrs. Siemens 

 and Halske in the Frankfort Exhibition, the alternate 

 current motor being to the left and the direct current 

 dynamo to the right in the figure. 



In the particular form of direct current dynamo shown 

 in Fig. II, and which represents a type much used now 

 on the Continent, the field magnets are inside the rotating 

 armature, and the wires on the outside of the Gramme 

 ring itself are bare, and act as the commutator. 



The impossibility of starting the simple synchronizing 

 motor with an alternating current will be of little conse- 

 quence when a large amount of power has to be trans- 

 mitted, seeing that in the receiving station there will be 

 several sets of geared alternate current motors and direct 

 current dynamos, some of which will be always running day 

 and night. Hence, to start any alternate current motor, all 

 that need be done will be to send round the direct current 



