Relationship between rated torque and size of permanent magnet machines

The rated output of induction motors varies with frame size in a manner which is well known to designers of these machines. However, in the case of permanent magnet excited machines, there is a shortage of comparable information. The aim of the paper is to use both analysis and published results to attack this problem. The analysis is presented in terms of torque and the principal dimensions of rotor diameter and length for cylindrical machines. Results are tabulated for a range of machines with rare-earth magnets in terms of available torque output. A clear relationship involving magnet volume and rotor diameter is shown, Consideration of cooling of the stator leads to the conclusion that the observed relationship between torque and dimensions is important for maintaining a satisfactory frame temperature. The effect of the scaling factors on the capability of field weakening for high speed operation is also studied. The general scaling rule should prove useful in establishing the basic size of machine needed for a given output requirement.