Environmental Engineering Reference
In-Depth Information
Polyvinyl chloride (PVC) is the most common insulation system for wire
harness in passenger cars. The higher temperature capable cross-linked PVC known
as XLPE is used in wet locations such as door interior harness because of its
moisture resistance and insulation creep integrity necessary to maintain environ-
mental seals. Wire gauge selection can be computed using the geometric series
relation of wire gauge number to diameter as described below. Note from Figure
4.44 that 20 AWG is the most common wire gauge in both the present 14 V elec-
trical system and in the proposed 42 V PowerNet system. We take the properties of
20 AWG wire as a baseline and from it compute the characteristics of any other
wire gauge in the EDS using the relations below where
R
20
(
x
,
z
,
T
) is the known wire
of AWG '
x
', length '
z
' in feet and at temperature
T
0
in degree Celsius. The refer-
ence temperature,
T
0
is 20
C in all wire tables:
R
20
ð
x
0
,
z
0
,
T
0
Þ¼
10
:
15
ð
m
W=
ft
Þ
ð
4
:
31
Þ
1
2
ð
x
y
Þ=
3
R
cable
ð
y
,
z
,
T
Þ¼
R
20
ð
x
,
z
0
,
T
0
Þ
z
ð
1
þgð
T
T
0
ÞÞ
ð
m
WÞ
ð
4
:
32
Þ
For example, the battery cable in a conventional 14 V automotive system is 2
AWG stranded copper wire of total length of approximately 7 ft when the battery is
located under-hood. For this cable, (4.32) predicts a cable resistance
R
cable
at the
under-hood temperature of 70
C of 1.327 m
W
. Each connector due to crimping and
material properties will have a resistance of 0.5 m
W
. A relay contact or switch
contact is on this same order of resistance.
To further illustrate the impact of harness and connector resistance on elec-
trical system performance consider now that the cable discussed in relation to
(4.32) has eight connections (terminations): one at the ground wire to the engine
block, two at the battery terminals, two at the starter motor solenoid contactor, two
at the starter motor brushes and one at the starter motor case ground to engine
block. In this complete circuit the battery will have an internal resistance that is a
function of its temperature, SOC and age. We can assume this to be a typical 70 Ah
lead-acid battery with internal resistance of 7 m
W
if new and at better than 80%
SOC at room temperature. To calculate the maximum current to the starter motor
under these conditions we assume that the starter motor armature has a resistance
matching the battery internal resistance at nominal conditions, but that it has the
same temperature dependence as the cable (both use copper) and that the starter
motor brushes develop a net voltage drop of 1.1 V. This yields a voltage drop of
0.55 V per brush, which is very typical of dc motor characteristics, and brush to
commutator properties. Since the battery is nearly fully charged, it will have an
internal potential of 6
2.1 V/cell = 12.6 V. We calculate the maximum current
delivered to the starter motor as
10
3
R
cable
ð
y
,
z
,
T
Þþ
N
c
R
conn
þ
R
int
þ
R
arm
ð
A
Þ
ð
V
batt
2
V
brush
Þ
I
starter
¼
ð
4
:
33
Þ
