Page 151 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 6 .1 Battery/fuel-cell EV design packages

rolling resistance, such that a resistance of 100 kgf/tonne field and armature, the same current is carried by both
is equivalent to a gradient of 1:10. If the fraction of and as it increases in magnitude so does the magnetic flux
the total vehicle weight contributed by the battery and the torque increases more than proportionally with
is f b then range is given by {(14 3600)/(9.81 current. Rotation of the armature creates a back-EMF in
1000)}f b h. Pessimistically h is about 30 at 50 km/h and opposition to the applied voltage because the wires at the
if f b is 0.4 then cruising range would be about 60–65 km. edge of the armature are moving across the field flux.
This of course is reduced by frequent acceleration and Motors are designed to equalize applied and back-EMFs
braking. at operational speed. This will be low when field current
Series-wound DC motors, Fig. 6.1-9, have been is high and vice versa. The speed/current curve can be
chosen for low cost conversions because of their advan- made to move up the x-axis by reducing the field current
tageous torque/speed characteristics, seen at (a), given to a fixed fraction of the armature current (0.5–0.7),
relatively low expected road speeds. Series winding the with the help of the field diverter resistance shown but

Fig. 6.1-9 Series-wound DC motor: (a) motor characteristics; (b) field diverter resistance; (c) speed-base motor characteristics;
(d) rheostatic control; (e) parallel/series battery control.

152