Induction versus permanent magnet motors

It is clear that a premium will continue to be placed on energy efficiency in motors. The circumstances around global warming and the availability of future oil supplies only increase the focus on electric motors as key elements in the efficient utilization of energy resources. PMs offer an important tool in the quest for cost-effective ways to further increase motor efficiencies. The characteristics of PM motors are sufficiently distinct from that of induction motors that users need to understand their operation to ensure successful applications. The nonsalient PM rotor construction provides high efficiency and high-torque density, whereas the salient pole construction offers some distinct advantages, especially with regard to system issues. Full flexibility to use other than 50 or 60 Hz base frequencies to further optimize performance obviously requires application with an inverter. Once the decision is made to use an inverter, the variable-frequency degree of freedom, with regard to pole selection, works in favor of the PM motor design. The improving performance to cost relationship of PM materials is getting to the point where the increased power density of the PM motor can be large enough to offset the cost of the magnets. While the authors would not suggest that PM motors are going to replace the widespread usage of induction motors, the PM motors provide an interesting alternative, especially in the case where an inverter is to be used in the application. It is for those inverter-fed applications that the first usage is expected in the pulp and paper industry.