Design improvements in a permanent-magnet commutator machine including the use of rare earth magnets

This paper describes simple design procedures that can be implemented to decrease the material cost of a permanent-magnet commutator machine of the sort often found in domestic appliances and automotive applications. The main material components considered are the magnet size (reduced to an acceptable minimum) and magnetic steel cost, while certain design aspects (such as the armature stamping) are maintained. The cost of the material is displayed in a per-unit format for typical manufacturers´ costs at the time of writing. A design of a fictitious (but very typical) machine, which can be considered as reasonable, is studied. While the output power is maintained the speed is changed but it is assumed that the machine is used with a gear or chain reduction system so it is taken that the gearing can be altered. However, the machine becomes more susceptible to demagnetisation. Modern drives incorporate power electronic chopping circuits so that it is assumed that the current can be closely controlled. The motor design is then studied when the magnet material is changed from a ferrite to a rare-earth type. This produces further reduction in cost. The paper also looks at the transient demagnetisation of the machine in overcurrent up to the calculated demagnetisation current. This is cross-checked by use of finite element analysis.