RELATIONSHIP BETWEEN RATED TORQUE AND SIZE OF PERMANENT-MAGNET MACHINES

The rated output of induction motors varies with frame size in a manne r which is well known to designers of these machines. However, in the case of permanent magnet excited machines, there is a shortage of comp arable information. The aim of the paper is to use both analysis and p ublished results to attack this problem. The analysis is presented in terms of torque and the principal dimensions of rotor diameter and len gth for cylindrical machines. Results are tabulated for a range of mac hines with ran-earth magnets in terms of available torque output. A cl ear relationship involving magnet volume and rotor diameter is shown. Consideration of cooling of the stator leads to the conclusion that th e observed relationship between torque and dimensions is important for maintaining a satisfactory frame temperature. The effect of the scali ng factors on the capability of field weakening for high speed operati on is also studied. The general scaling rule should prove useful in es tablishing the basic size of machine needed for a given output require ment.