Why Small Permanent Magnet Machine Drives are Relegating the Induction Machine Drive to Niche Applications

The interest in adopting electric actuation in defense applications such as surface ships and submarines, as well as in a host of "more electric" applications, 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 discusses some fundamental principles as originally presented by Laithwaite, as well as some commonly held beliefs, or rules of thumb. These principles and rules of thumb indicate that the power and torque density (kW/m³ , Nm/m³), the specific power and the specific torque (kW/kg, Nm/kg), and the efficiency must all degrade dramatically as the machine rating goes down. The paper then presents some numbers for existing machines, representing standard, generally available technologies today. The paper then examines 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 are deduced that the small induction machine eventually lose its place as the drive of choice in small applications, and be relegated to "niche" applications. It is further concluded that this displacement can be justified on cost alone, with dramatic efficiency increases being simply fortuitous additional benefits