Lightweight Electric/Hybrid Vehicle Design

xxiv Lightweight Electric/Hybrid Vehicle Design
0.4 Electric-drive fundamentals
While battery-electric vehicles were almost as common as IC-engined ones, at the beginnings of
the commercialization of the powered road vehicle, it was not until the interwar years that serious
studies were taken into operating efficiency of such systems, as a precursor to their introduction in
industrial trucks and special purpose vehicles such as milk floats. Figure 0.4 illustrates some of
the fundamental EV traction considerations as the technology developed. For the Mercedes
Electromobile of the early 1920s, for example, seen at (a), more sophisticated wheel drives were
introduced, with motors formed in the wheels to eliminate transmission gear losses. An energy
diagram for this drive is seen at (b). The basic definitions and relationships of electromagnetism
are helpful in the appreciation of the efficiency factors involved.
0.4.1 ELECTROMAGNETIC BASICS
While the familiar magnetic line-of-force gives the direction of magnetic force at any point, its
field strength H is the force in dynes which would act on a unit pole when placed in the field. For
magnetic material such as soft iron placed in the field, the strength of field, or magnetic intensity
B, inside the iron is greater than H, such that B = μH, where μ is the permeability of the material
(which is unity for non-metallics). When the cross-section of the object, at right angles to the
magnetic field, is denoted by a, the magnetic flux φ is the product Ba in maxwells. Since it is taken
that at unity field strength there is one line of force per square centimetre, then magnetic induction
is measured in lines per cm 2 and flux is often spoken of as in ‘lines’.
Faraday’s law defined the induced EMF as rate of change of flux (-dφ/dt×10 -8 volts) and Lenz’s
law defined the direction of the induced EMF as such that the current set up by it tends to stop the
motion producing it. The field strength of windings having length l, with N turns, carrying current
I is
H = 4πIN/10l which can be rearranged as φ(l/ma) = 4πIN/10
where the flux corresponds to the current in an electrical circuit and the resistance in the magnetic
circuit becomes the reluctance, the term on the right of the equation being the magneto-motive
force. However, while in an electric circuit energy is expended as long as the current flows, in a
magnetic circuit energy is expended only in creating the flux, not maintaining it. And while electrical
resistance is independent of current strength, magnetic permeability is not independent of total
flux. If H is increased from zero to a high value, and B plotted against H for a magnetic material,
the relationship is initially linear but then falls off so there is very little increase in B for a large
increase in H. Here the material is said to be saturated. When H is reduced from its high value a
new BH curve lies above the original curve and when H is zero again the value of B is termed the
retentivity. Likewise when H is increased in the negative direction, its value when B is zero again
is the coercive force and as the procedure is repeated, (c), the familiar hysteresis loop is obtained.
In generating current electromagnetically, coils are rotated between the poles of a magnet, (d),
and the current depends on both the strength of the magnetic field and the rate at which the coils
rotate. Either AC or DC is obtained from the armature rotor on which the coils are mounted,
depending on the arrangement of the slip-ring commutator. A greater number of coils, wound
around an iron core, reduces DC current fluctuation. The magnetic field is produced by a number
of poles projecting inwards from the circular yoke of the electromagnet. Laminated armature
cores are used to prevent loss of energy by induced eddy currents. Armature coils may be lapwound,
with their ends connected to adjacent commutator segments, or wave-wound (series) when
their ends are connected to segments diametrically opposite one another. The total EMF produced