The “Goodness” of Small Contemporary Permanent Magnet Electric Machines

The interest in adopting electric actuation in Navy ships, and in particular in submarines, usually founders on the issue of the mass and volume of electric actuators, which are perceived to be significantly worse than those of competing technologies. Certainly looking at the volume, mass, power and torque of a typical 3 phase induction machine, even in comparison with a typical automobile engine produces some rather disheartening numbers. This paper will discuss some fundamental principles as originally presented by Laithwaite, as well as some commonly held beliefs, or rules of thumb. The paper will then present some numbers for existing machines, representing standard, generally available technologies today. The paper will then examine some specific designs of special purpose permanent magnet (PM) machines, which produce numbers which appear to confound both the theory and the practice described. Conclusions will be deduced that the small induction machine will eventually lose its place as the drive of choice in small applications, and be relegated to “niche” applications. It will further be concluded that this displacement can be justified on cost alone, with dramatic efficiency increases being simply fortuitous additional benefits.