Abstract
This article investigates the tradeoffs that result from constraining the back-emf at maximum speed for fractional-slot surface permanent magnet (SPM) machines. Two 55 kW (peak) SPM machines designed to meet requirements established in the US FreedomCar program are used as the basis for this investigation. The two machines are 36-slot/30-pole SPM fractional-slot concentrated-winding designs that use sintered NdFeB magnets. The first design has no back-emf constraint at maximum speed, allowing it to exceed the 600 V pk l-l limit, while the second design has been designed to meet this requirement. Results of this study show that the introduction of back-emf limits can lead to significant increases in the SPM machine's rated current, with only a modest impact on the machines mass and volume. A combination of closed-form analysis and finite element analysis is used to carry out this investigation.