[Proc. Roy. Soc. Victoria, 32 (N.S.), Pt. II., 19201. 



Art. XI. — The Design of an Induction Motor with large Aiv 

 Gap and Rotating Field Magnets. 



By J. K. ROBERTS, B.Sc. 



(Natural Philosophy Department, University of Melbourne). 



(With 2 Text Figures,) 



[Read 11th September, 1919.] 



It will easily be seen that the apparatus described in another' 

 paperi in these Proceedings is an induction motor, with the follow- 

 ing peculiarities : — • 



1. The field magnets are magnetised by direct current, and are- 

 rotated to produce the rotating magnetic field. This is necessary 

 in order that the axis of rotation of the magnetic field may be accu- 

 rately determined, and that there may be no movement of the mag- 

 netic field along the axis of rotation. 



2. The armature consists of a hollow cylinder of solid copper 

 with a soft iron core. 



3. The machine works at 100% slip, i.e., the armature remains 

 at rest. 



4. The air gap- is very much larger than usual to permit of the 

 insertion of the Dewar flask. 



The problem of designing the instrument was similar to that of 

 designing an induction motor. A first approximation to the- 

 behaviour of the instrument may be made by supposing the copper 

 armature replaced by another armature of the same size, consist- 

 ing of very narrow strips of conducting material insulated from 

 each other, the resistance of all the strips in parallel being equal 

 to that of the copper cylinder from end to end, and each strip- 

 being connected at either end with that diametrically opposite to- 

 it by a perfect conductor. 



If this armature be placed in a unifoTm magnetic field of 

 strengtli H, which rotates at the rate of N revolutions per second, 

 the usual theory of the induction motor gives the torque as : — 



where l.cm. is the length of the armature, c.cm. is the diameter 

 of armature. R. E.M. units is the resistance of each of the cir- 

 cuits (i.e., twice the resistance of one strip), n total number of 



1. Laby and Roberts. A New Method of Determining the Mechanical 

 Equivalent of Heat, pajre 148. 



