Page 152 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Battery/fuel-cell EV design packages CHAPTER 6.1

the torque for a given armature current is, of course, relatively slowly during ‘on’ periods and similarly falls
reduced, see (b). The efficiency of the motor is low at slowly during ‘off’ periods, provided it has a path
low speeds, in overcoming armature inertia, and again at through which to flow. The latter is provided by the
high speeds as heating of the windings absorbs input ‘flywheel’ diode FD, a rectifier placed across the motor
power. Motors can thus be more highly rated by the to oppose normal voltage. During chopper operation,
provision of cooling fans. Average power in service should current i b flows in pulses from battery to motor while
in general be arranged at 0.8 of the rated power and the current i m flows continuously through the motor. Elec-
transmission gear ratio be such that the motor is loaded tronic timing circuits control the switching of the thy-
to no more than its rated power for level-ground cruising. ristors, (a).
The motor characteristics shown at (c) are obtained by Single ratio drives from motor to driveline are not
replotting the conventional characteristics on a speed suitable for hilly terrain, despite the torque/speed char-
base. The wide range of speeds available (up to 2:1) are acteristic, as the motor would have to be geared too low
around rated power and show how full field can be used to avoid gradient overloading and thus be inefficient at
for uphill running while weak field is used on the level cruise. A 5:1 CVT drive is preferred so that the motor
enabling speed reduction to compensate for torque in- can be kept at its rated power under different operating
crease in limiting battery power requirements for nego- conditions. There is also a case for dispensing with the
tiating gradients. weight of a conventional final drive axle and differential
With little or no back-EMF to limit current at starting, gear by using two, say 3 kW, motors one for each driven
resistance is added to keep the current down to a safe wheel.
level, as at (d). The current is maintained at the required The behaviour of lead–acid batteries, (b), is such
accelerating value, perhaps 2–4 times rated current. The that in the discharged condition lead sulphate is the
starting resistance is reduced as the motor gains speed so active material for both cell-plates which stand in dilute
as to keep the accelerating current constant to the point sulphuric acid at 1.1 specific gravity. During charging
where the starting resistance is zero, at the ‘full voltage the positive plate material is converted to lead peroxide
point’. Thereafter a small increase in speed causes gradual while that of the negative plate is converted into lead,
reduction in current to the steady running value. As the as seen at (c). The sulphuric acid becomes more con-
current is supplied from the battery at constant voltage, centrated in the process and rises to SG ¼ 1.5 when
the current curve can be rescaled as a power curve to fully charged, the cells then developing over 2 volts. In
a common time base, as at (e). The shaded area then gives discharge the acid is diluted by the reverse process.
energy taken during controlled acceleration with the While thin plates with large surface area are intended
heavily shaded portion showing the energy wasted in re- for batteries with high discharge rates, such as starter
sistance. So rheostatic acceleration has an ideal efficiency batteries, the expansion process of the active material
of about 50% up to full voltage. This form of control is thus increases in volume by three times during discharge and
in order for vehicle operation involving, say, twice daily the active material of very thin plates becomes friable
regular runs under cruise conditions but unwise for in numerous charge/discharge cycles, and a short life
normal car applications. results. Normal cells, (b), comprise interleaved plates
with porous plastic separators; there is one more neg-
ative than positive plates, reducing the tendency to
6.1.3.2 Motor control alternatives buckle on rapid discharge. Expensive traction batteries
have tubular plates in some cases with strong plastic
Alternatives such as parallel/series (two-voltage) rheo- tubes as separators to keep the active material in place.
static control, or weak field control, can be better for Discharge rates of less than half the nominal battery
certain applications, but the more elaborate thyristor, capacity in amphours are necessary to preserve the
chopper, control of motor with respect to battery active material over a reasonable life-span, but short
(Fig. 6.1-10) is preferred for maintaining efficiencies bursts at up to twice the nominal rate are allowable.
with drivers less used to electric drive, particularly in The graphs at (d) permit more precise assessments of
city-centre conditions. It involves repetitive on–off range than the simple formula at the beginning of the
switching of the battery to the motor circuit and if the section which assumes heavy discharge causes battery
switch is on for a third of the time, the mean motor capacity to be reduced by 70–80% of normal, 25 kWh
voltage is a third of the supply voltage (16 V for a 48 V becoming 20.
battery), and so on, such that no starting resistance is When charging the gassing of plates must be consid-
needed. Effective chopper operation requires an in- ered, caused by the rise in cell voltage which causes part
ductive load and it may be necessary to add such load to of the current to electrolyse the water in the electrolyte
the inherent field inductance. Because an inductive cir- to hydrogen. Gassing commences at about 75% full
cuit opposes change in current then motor current rises charge. At this point, after 3–4 hours of charging at

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