Lightweight Electric/Hybrid Vehicle Design

66 Lightweight Electric/Hybrid Vehicle Design
inductance. The magnets in this application have been fitted with a sleeve on the rotor outside
diameter, for mechanical protection and to physically hold the magnets in place. A carbon-fibre
sleeve was chosen for this application; it offers at least twice the strength of the steel sleeve in
tension, so a much greater safety factor can be achieved. The sleeve on the rotor increases the
effective air gap but an unloaded air gap flux density of 0.6 tesla was achieved from this high
energy density rare earth magnet. The core loss in the stator, due to high frequency, is considered
and must be kept to an acceptable level. The grade of material considerable is radiometal 4550.
This alloy has a nominal 45% nickel content and combines excellent permeability with high
saturation flux density.
3.5.4 MAGNETIC CIRCUITS
The magnetic circuit for this design was calculated using the Nelco software. The most important
parameters in the design of the magnetic circuit were weight and to keep the core losses down to
a minimum whilst reducing the slot leakage to minimize the winding inductance. This is achieved
when a compromise has been reached in which the flux density in the teeth is 1.15 tesla, the
density in the core is 0.8363 tesla, and the yoke flux density is 0.78 tesla.
3.5.5 BRUSHLESS MACHINE DRIVE
The machine drive consists of a polyphase, rotating field stator, a permanent magnet rotor, a
rotor position sensor, and the electronic drive. During operation the electronic drive, according
to the signals received from the rotor position sensor, routes the current in the stator windings to
keep the stator field perpendicular to the rotor permanent field, and consequently generates a
steady torque. Conceptually, the drive operates as the commutator of a DC machine where the
brushes are eliminated. The main advantage here is that no current flow is needed in the rotor.
As a result, rotor losses and overheating are minimal, the input power factor approaches unity
and maximum efficiency is obtained. This is especially relevant in continuous duty applications,
where the limiting factor of traditional induction drives is invariably the difficulty of removing
rotor losses.
Fig. 3.9 Machine controller.