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



Art. XL — Tlie Design of an Induction Motor with large Air- 

 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' 

 paf^eri in these Proceedings is an induction motor, with the follow- 

 ing peculiarities : — • 



1. The field niMgiiets 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 ma!g- 

 netic field along the axis of i-otation. 



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

 witli 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 tlie 

 insertion of the Dewar fiask. 



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 eipial 

 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 uniform magnetic field of 

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

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



,/.=7rNH-/VR7*(R-^ + 47r'^N^L-) ~ ' 



where ^cni. is the length of the armature, c.cni. is the diameter 

 of armature. 1\. 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 cf Determining the Mechanical 

 Equivalent of Heat, paste 148. 



