The expanding role of PM machines in direct-drive applications

It is widely recognized that permanent magnet (PM) synchronous machines are attractive candidates for direct-drive applications that eliminate gearboxes or other mechanical transmission components between the machine and the load. But it raises the question of what is special about PM machines that makes them such suitable candidates for these applications? The physical basis of this appealing feature of PM machines is explored in comparison to induction machines and other familiar types of ac machines. Several applications are used to highlight both the strengths and the limitations of PM machines in direct-drive applications, illuminating key engineering tradeoffs that must be carefully evaluated when developing such designs. The major impact that direct-drive permanent magnet machines have already demonstrated in some application areas is highlighted, providing insight into technology changes that are currently under way in other fields.